Recognizing feeding injury to corn ears in the United States

Abstract

Corn is one of the major commodities in the United States, and is grown for fuel, feed, and food around the world. Much of the production is centered in the Midwest, but corn is grown throughout the country and has a national production value of $91.7 billion. Because of the substantial national economic impact of corn production, as well as the profitability of individual farming operations, crop protection from pests is critical. Corn is most vulnerable during ear and kernel formation, and pest infestations during this time can have a substantial impact on grain quality and yield. Detecting pests is one of the most important components of integrated pest management, and regular scouting can allow farmers to make timely management decisions for pests in corn. However, farmers and crop consultants sometimes do not notice ear-feeding pests of corn until the fall, or near harvest, when pests are nearly done feeding or have vacated the ear. When this happens, it can be difficult to diagnose the problem, which can be important for assessing management tactics that were used in the field during the current growing season or making decisions for the following growing season based on pest activity in the field. This article provides profiles of common ear-feeding pests, with written descriptions and photographs of typical injury to corn ears for those pests.

sampling and detection, agriculture, IPM, crop protection

Introduction

The domestication and hybridization of corn, Zea mays L. (Poales: Poaceae), produced dramatic genetic improvements to develop high-quality hybrids. In 2022, 35.9 million hectares (88.6 million acres) of corn were planted in the United States with an average grain yield of 10,858.8 kg/ha (173 bu/acre) and a production value of $91.7 billion (USDA-NASS 2023). The majority of corn production is concentrated in the Midwest, sometimes referred to as the “Corn Belt,” with 30.4 million ha (75.2 million acres) planted in 2022 (USDA-NASS 2023). Yields are variable but have consistently exceeded 10,042.8 kg/ha (160 bu/acre) nationally in the past decade.

Corn ear and kernel formation are the most vulnerable stages to protect from pests and represent a relatively short window of time compared to the entire growing season. Abendroth et al. (2011) provide a detailed publication about corn growth and development. Table 1 and Fig. 1 highlight the key ear structures. The corn kernel has many components with many uses, including starch, fiber, protein, and oil. Most of the harvested grain is used for animal feed, but food-grade products and other uses exist (e.g., oil, sweeteners, cardboard packaging, bioplastics, and fabrics). According to Iowa Corn Growers Association (https://www.iowacorn.org/corn-uses), US corn is used for feed for beef, dairy, swine, and poultry (39%); ethanol and fuel (27%); exports (16%); food and industrial (9%); and residual (9%). Because of the economic value of corn in the United States, it is critical to scout for pests and practice integrated pest management (IPM) to ensure profitability.

Table 1.

Corn growth and development stages with approximate duration

	Stage	Abbreviation	Durationa,b	Description
Vegetative growth	Emergence	VE	84 GDD per leaf from VE-V10 (new leaf develops every 4–6 days)	Shoot (coleoptile) has emerged from the soil
	First leaf	V1		Lowest leaf has a visible collar; this leaf has a rounded tip
	Second leaf	V2		Two of the lowest leaves have a visible collar, the second and subsequent leaves have pointed tips
	Nth leaf	V(n)	56 GDD from V11–Vn (new leaf develops every 2–3 days)	‘n’ leaf collars present, most corn hybrids produce between 18 and 21 leaves
	Tassel	VT	180 GDD (about 10 days)	Lowest branch of the tassel is visible
Reproductive growth	Silk	R1		One or more silks extends outside of husk leaves
	Blister	R2	220 GDD (about 8 days)	Kernels resemble ‘blisters’ with clear liquid
	Milk	R3	150 GDD (about 6 days)	Kernels filled with ‘milky’ fluid
	Dough	R4	100 GDD (about 7 days)	Inside the kernels are a ‘doughy’ consistency
	Dent	R5	550 GDD (about 33 days)	Dent forms on kernel and milk line progresses toward kernel tip
	Maturity	R6		Kernels at maximum dry matter accumulation; a ‘black layer’ will form at kernel base (2–3 days after physiological maturity)

^aProviding an example of development duration in growing degree days (GDD) for a 110-day corn hybrid used in central Iowa, based on Abendroth et al. (2011).

^bdos Santos et al. (2022) provide additional notes about phenological development and duration: mainly driven by temperature (growing degree days) followed by environment, genetics, and management; high solar radiation will slow plant development.

Fig. 1.

Corn components, including a) ear and b) kernel with pericarp and tip cap (bran), starchy endosperm, and germ (embryo). Illustrations by Eric Hoile, Iowa State University.

For the purpose of this article, we define IPM as the comprehensive approach to managing host stress by pests that is economically and ecologically sustainable (Peterson et al. 2018). A successful IPM program in corn includes prediction, prevention, detection, and remediation (Wright and Van Duyn 1999). Prediction involves the anticipation of pest activity that might occur and potentially build to economically damaging levels. Historical pest experiences can be helpful to predict future occurrences. Prevention tactics enable farmers to avoid pests or reduce damage to the crop. Examples of prevention include date of planting, hybrid selection, and crop rotation. One of the most important IPM strategies (and the focus of this publication) is detection. Proper scouting and identification can aid in determining pest dynamics and whether densities warrant further action. Often, economic thresholds, or action thresholds, have been developed to provide management guidelines for yield protection. Thresholds are based on pest biology and injury potential, management costs, and crop market values (Pedigo et al. 2021). Lastly, remediation attempts to reduce the negative impacts from insect injury.

Farmers, crop consultants, and others involved in agriculture are encouraged to understand and implement IPM. Scouting for corn pests is a particularly essential skill, with many economically important species present in every growing season. Multiple pests can be actively feeding at the same time. In some cases, pests migrate north in the spring, and the timing and severity of infestation can be unpredictable. In addition, population fluctuations are common due to weather, crop health, and natural enemies. Regular field visits can help make timely treatment decisions and minimize plant injury. Scouting regularly is especially important because the timing of pest infestations is variable annually and ear-feeding pests need to be identified and managed prior to entering the ear. The corn husk provides protection from foliar insecticide applications once the pest is inside. Once the pest is in the ear and begins feeding, rescue treatments are not effective.

Two terms that are often incorrectly interchanged in agriculture are “injury” and “damage.” Here, we use the definitions provided by Pedigo et al. (2021). Injury is the effect of the pest activities on the host that is usually deleterious. Damage is the measurable loss of host utility, most often relating to quantity, quality, or aesthetics. In other words, injury is centered on the pest and damage addresses the host. This article is focused on the injury created by pests. The impact on plant growth and development depends on the amount of feeding and timing of injury. Pests are capable of inflicting direct and indirect injury to plants, and it is important to understand the difference (Pedigo et al. 2021). Indirect injury is the result of an insect feeding on any nonyield-bearing tissue of a plant (e.g., leaves, stems, or roots). Conversely, direct injury is the result of an insect feeding on the yield-bearing tissue of a plant (e.g., seeds, flowers, or fruit) and can result in poor pollination and kernel development (see examples in Figs. 2–12).

Fig. 2.

a) Corn rootworm beetles feed on the silks and the ear tip. There are three species, with two subspecies: b) Mexican corn rootworm, c) northern corn rootworm, d) southern corn rootworm, and e) western corn rootworm.

Fig. 3.

Japanese beetle adults a) typically aggregate on ears to feed on silks b) near the field edge.

Fig. 4.

a–c) Sap beetle feeding injury to corn kernels. d) Adult dusky sap beetles are typically smaller than northern corn rootworms, while e) Glischrochilus quadrisignatus are roughly the same size. f) The larvae also feed on the ear and can be distinguished from other larvae by dark projections on the abdomen.

Fig. 5.

a, b) Cornsilk fly feeding injury to corn kernels.

Fig. 6.

a) Large infestations can indirectly reduce ear size b) when colonies of bird-cherry oat aphids or c) corn leaf aphids d) feed on leaves. These infestations can spill over onto the tassel, husk, and stalk throughout the season.

Fig. 7.

a) Stink bugs can feed on corn kernels by piercing through the husks. b) Toxic saliva is injected during feeding and causes discoloration and ear development abnormalities. c, d) Ear molds can develop on injured kernels. e) The usual suspects are the brown stink bug, f) onespotted stink bug, g) green stink bug, and h) brown marmorated stink bug.

Fig. 8.

a) European corn borers and b) southwestern corn borers c) leave behind a trail of white frass when they feed on kernels. d) Feeding injury is usually on the side or butt of the ear. e) Corn borers typically enter the shank before tunneling into the stem.

Fig. 9.

a) Corn earworm larvae are variable in color. b) They enter ears through the silks and can feed down the silk channel, c) but typically corn earworm larvae are found near the ear tip. d) They injure and consume kernels, typically leaving behind oatmeal-like frass.

Fig. 10.

a, c, e) Fall armyworm larvae may feed on leaves or tunnel into the whorl if ears are not yet developed. b) Multiple larvae can be present on an ear, and d, f) extensive feeding by fully grown larvae can result in severely damaged ears that allow for g) pathogen entry and ear molds to develop.

Fig. 11.

a) Young western bean cutworm larvae may create small entrance holes in the c) husk, b) while mature western bean cutworm larvae c) create large exit holes. d) Feeding injury may occur anywhere on the ear and e) result in pathogen entry and the development of ear molds.

Fig. 12.

a) Severe grasshopper infestations could lead to ears being completely consumed, except the cob, or b, c) plants stripped of their leaves. Several grasshopper species feed on corn, including, c) differential, d) redlegged, and e) 2-striped.

Most often, farmers and their consultants discover ear-feeding when they are estimating yield prior to harvest in the fall. At this time, direct injury is noticeable, but the pest has likely vacated the ear. It is difficult to identify pests that are not present, but some differences in ear injury may occur and be diagnostic. Determining which pest is present during a growing season, even if it is after a pest has vacated the ear, is important for several reasons. Identifying the pest or injury can help determine whether a pest colonized a transgenic corn hybrid that should have provided protection. Similarly, if an insecticide was applied during the growing season, identification of ear injury and associated pests can help assess the efficacy of that application. This estimate would be most accurate if used in conjunction with season-long scouting, because knowing what pests were present prior to the application would be important. Lastly, knowing what pests are present late in the season could aid in decision-making for the following growing season. If pests overwinter in the field or in proximity, they could be persistent problems if corn is planted annually. This publication will review the most common corn ear feeders (Table 2) in the United States and provide visual examples of indirect and direct injury.

Table 2.

Examples of pests that cause injury to corn ears

Insects
Order	Scientific name	Common name	Key injury descriptions
Coleoptera	Diabrotica barberi Diabrotica undecimpunctata howardi Diabrotica virgifera zeae Diabrotica virgifera virgifera	Northern corn rootworm Southern corn rootworm Mexican corn rootworm Western corn rootworm	Direct: silk clipping, feeding on ear tip; feeding on exposed kernels Indirect: poor pollination from lodging
	Popillia japonica	Japanese beetle	Direct: silk clipping, feeding on ear tip
	Carpophilus dimidiatus Carpophilus lugubris Glischrochilus quadrisignatus	Corn sap beetle Dusky sap beetle -----	Direct: hollowed out kernels (R3, R4); circular scars from feeding on pericarp (R4 and beyond)
Diptera	Chaetopsis massyla Euxesta annonae Euxesta eluta Euxesta stigmatias	----- ----- ----- -----	Direct: brown or decomposing silks; endosperm destroyed
Hemiptera	Rhopalosiphum padi Rhopalosiphum maidis	Bird cherry-oat aphid Corn leaf aphid	Indirect: stunted ears; honeydew or sooty mold growth
	Halyomorpha halys Euschistus servus Chinavia hilaris Euschistus variolarius Nezara viridula	Brown marmorated stink bug Brown stink bug green stink bug 1-spotted stink bug Southern stink bug	Direct: small, discolored spots on kernels or husks Indirect: curved ears or poor pollination
Lepidoptera	Ostrinia nubilalis Diatraea grandiosella	European corn borer Southwestern corn borer	Direct: feeding at ear tip, along top of kernels, or along a row of kernels; may tunnel into cob Indirect: poor ear development
	Helicoverpa zea	Corn earworm	Direct: feeding usually confined to ear tip
	Spodoptera frugiperda	Fall armyworm	Direct: feeding usually confined to ear tip Indirect: poor ear development
	Striacosta albicosta	Western bean cutworm	Direct: feeding can occur anywhere on ear; entrance/exit holes likely visible on husk
Orthoptera	Melanoplus differentialis Melanoplus femurrubrum Melanoplus bivittatus	Differential grasshopper Redlegged grasshopper 2-striped grasshopper	Direct: silk clipping; kernels consumed or partially consumed; no frass left behind
Vertebrates
Order	Scientific name	Common name	Key injury descriptions
Icteridae	Quiscalus quiscula Agelaius phoeniceus	Common grackle red-winged blackbird	Direct: ragged edges on empty kernels
Ursidae	Ursus americanus	Black bear	Direct: husk leaves removed and kernels eaten
Procyonidae	Procyon lotor	Raccoon	Direct: kernels eaten, but husk leaves intact
Cervidae	Odocoileus virginianus	White-tailed deer	Direct: husk leaves intact and tip bitten off

Corn Rootworms (Coleoptera: Chrysomelidae)

Scientific and Common Names

Diabrotica barberi Smith & Lawrence, northern corn rootworm.

Diabrotica undecimpunctata howardi Barber, southern corn rootworm or spotted cucumber beetle.

Diabrotica virgifera virgifera LeConte, western corn rootworm.

Diabrotica virgifera zeae Krysan & Smith, Mexican corn rootworm.

Injurious Stage(s)

Larvae and adults.

Injury Type

Indirect (larvae and adults) and direct (adults).

Injury to Ears

Adult corn rootworms may cluster on the tip of an ear, initially feeding on fresh green silks, where they attempt to burrow deeper and reach the developing kernels (Fig. 2a). An exposed ear not completely covered by the husk may attract dozens of beetles during the R2 stage. Adult corn rootworms cause direct injury by feeding on R2 (blister), R3 (milk), and R4 (dough) stage kernels. Adults feed only in the ear tip or on kernels that are exposed from incomplete husk coverage, or on exposed kernels created by other pests, for example, grasshoppers.

Larvae of corn rootworms also cause indirect injury to ears by feeding on the roots and affecting plant architecture. Corn rootworm larvae could reduce grain yield up to 45% (Tinsley et al. 2013) from root feeding alone. If plants fall over, or lodge, the ear is positioned closer to the ground with the ear tip (R1; silks) possibly obstructed by tangled leaves. This diminishes the amount of pollen reaching the silks resulting in poor fertilization and kernel set.

Comments

There are 4 species of rootworms: Mexican, northern, southern, and western (Fig. 2b–e). The injury, to either the roots by larvae or kernels by adults, is identical across all 4 rootworms and cannot be separated by species. It is a common misconception that silk clipping by adult corn rootworms always results in poor fertilization of kernels. If at least ½ inch (1¼ cm) of green silk protrudes from the husk, then silks can effectively capture pollen and beetle feeding does not reduce kernel set (Drees et al. 1999).

Distribution

Northern corn rootworm is found mostly in the upper Midwest, but also west to the Rocky Mountains, east to the mid-Atlantic coast, and south into Tennessee (Tollefson and Levine 1999). Western corn rootworm predominantly occurs throughout the corn-growing regions east of the Rocky Mountains, but can also be found in northern Alabama, Georgia, and South Carolina. It is replaced by the Mexican corn rootworm in Oklahoma and Texas (Stewart 1999, Sutter 1999). Western corn rootworm also has established populations in eastern Washington, eastern Oregon, and southern Idaho (Murphy et al. 2014). Southern corn rootworm is widely distributed in all states east of the Rocky Mountains plus the southwestern United States (Drees 1999). Revised distribution maps of all 3 species are available (Birchmore 2022).

Japanese Beetle (Coleoptera: Scarabaeidae)

Scientific and Common Name

Popillia japonica Newman, Japanese beetle.

Injurious Stage(s)

Adults.

Injury Type

Direct.

Injury to Ears

The adult Japanese beetle (Fig. 3a) is a minor and sporadic pest of corn. Adults will cluster on the green silks of pollinating corn (Fig. 3b), and this is typically in border rows, especially if the cornfield is adjacent to pasture where grubs may have completed development. When the beetles aggregate on corn silks it is often for mating, and careful observation may reveal that many of the beetles are paired for this purpose. Adult beetles will feed on and clip green corn silks, but silks longer than 12.5 mm (½ inch) will be able to capture pollen without any reduction in kernel set (Drees et al. 1999).

Comments

The adult Japanese beetle is a ubiquitous pest of over 300 ornamental and agricultural plant species (Fleming 1972), and an occasional pest of corn and soybean (Glycine max (L.)) (Shanovich et al. 2019). The grubs feed on the roots of many grasses, especially lawns, golf courses and pastures. They can be found within cornfields but are generally not an economic concern. Virgin adult females emit a volatile sex pheromone that is highly attractive to males (Potter and Held 2002), and this may cause large concentrations of beetles, especially males, to aggregate on corn silks in an attempt to mate with a female.

Distribution

Japanese beetle is an invasive species in the United States. It was first found in New Jersey in 1916. It has since spread westward and populations have been identified in every state east of the Mississippi River, except Florida and Louisiana, every state that borders the west side of the Mississippi River, and all the Great Plains states from North Dakota south to Texas, although the populations are often small and frequently scattered in the Great Plains (Bishop et al. 2020).

Sap Beetles (Coleoptera: Nitidulidae)

Scientific and Common Names

Carpophilus dimidiatus (F.), corn sap beetle.

Carpophilus lugubris Murray, dusky sap beetle.

Glischrochilus quadrisignatus Say.

Injurious Stage(s)

Larvae and adults.

Injury Type

Direct (larvae and adults).

Injury to Ears

Adult sap beetles will feed on exposed ear tips and can move further down into the ear. They will damage kernels in the R3 (milk) and R4 (dough) stages by hollowing out the kernel and leaving the outer pericarp, which shrivels as it dries down and turns brown (Fig. 4a). Adults also chew into R4 (dough) kernels, chewing through the endosperm and down into the germ near the base of the seed (Fig. 4b). When kernels reach the R5 stage, adults will feed and scar the outer pericarp, creating white circular scars that may turn black over time (Fig. 4c).

Comments

Sap beetles are typically secondary invaders of corn ears, entering the kernels only after they have previously been injured by birds, other beetles, or ear-feeding caterpillars, for example, corn earworm or fall armyworm. Glischrochilus quadrisignatus is about the same size as the adult northern corn rootworm, and the dusky sap beetle is slightly smaller (Fig. 4d, e). The larvae feed within the ear and superficially resemble corn rootworm larvae, which never feed in corn ears. They can be distinguished from other larvae in the ear, for example, early-instar European corn borers, by the pair of dark, pointed projections on the dorsal side at the end of the abdomen (Fig. 4f). The larvae will leave the ear and pupate in the soil.

Distribution

The corn sap beetle can be found in Florida and other southern United States (Myers 2022) Glischrochilus quadrisignatus occurs from the Great Plains states and Texas east to the Atlantic seaboard. The dusky sap beetle occurs throughout the United States (Von Kaster 1999).

Cornsilk Flies (Diptera: Ulidiidae)

Scientific Names

Chaetopsis massyla Walker.

Euxesta annonae Fabricius.

Euxesta eluta Loew.

Euxesta stigmatias Loew.

Injurious Stage(s)

Larvae.

Injury Type

Direct.

Injury to Ears

Larvae feeding on corn silks results in poor kernel set, and unequal kernel size and rows on the cob. Injured silks will turn brown along the larval feeding trail and could be completely destroyed with large infestations. An ear that is heavily infested will develop wet, decomposing corn silk within the silk channel. As larvae move down into the developing ear, they will feed on developing kernels and destroy most of the endosperm. The larvae may feed along the entire length of the ear. Field corn kernels may be damaged through the R3 (milk) stage. The cumulative feeding injury may result in ears with no marketable kernels (Fig. 5a, b).

Comments

An excellent summary of cornsilk fly biology and management options is given by Nuessly and Capinera (2013). Females are capable of laying hundreds of eggs on the same ear and can deposit eggs on green and brown silks.

Distribution

Cornsilk flies are primarily distributed in tropical and semitropical regions. The several species that have been recorded as pests of corn can be found from Puerto Rico, and north through Florida into Alabama, Georgia, Louisiana, and South Carolina. There are historical records of injury to sweet corn in southern Texas and California (Nuessly and Capinera 2013).

Aphids (Hemiptera: Aphididae)

Scientific and Common Names

Rhopalosiphum padi (Linnaeus), bird cherry-oat aphid.

Rhopalosiphum maidis (Fitch), corn leaf aphid.

Injurious Stage(s)

Nymphs and adults.

Injury Type

Indirect (nymphs and adults).

Injury to Ears

Large infestations of aphids can indirectly affect ear size, resulting in stunted ears (Fig. 6a). Large colonies of bird cherry-oat aphid (Fig. 6b) and corn leaf aphid (Fig. 6c) can develop within the whorl where they remove large amounts of water and nutrients from the phloem, and at VT (tassel) the colony can expand onto this new growth (Fig. 6d) (Bing 1999, Kieckhefer 1999). Other species can utilize corn but do not generally form dense populations after pollination. Colonies can be found on the stalks, leaves, and ear husks after pollination. After removing the nutrients, aphids excrete excess moisture and sugar, which creates a sticky residue on the leaves known as honeydew. On heavily infested plants, aphid feeding can kill the upper corn leaves. Thick layers of honeydew on the husk and leaves can promote a black sooty mold that likely interferes with photosynthesis. Both species are capable of briefly transmitting maize dwarf mosaic virusto corn, and can result in arrested ear formation and development (Signoret et al. 2008).

Comments

Ear size is defined before flowering, so any stress (biotic or abiotic) conditions affecting the crop during the mid- to late-vegetative stages could influence the size of the ear (Ciampitti 2018). A common misconception is that heavily infested tassels reduce pollen shed and kernel fertilization, but that would have to occur on every plant within a field. Otherwise, sufficient pollen is produced by nearby adjacent plants.

Distribution

Both species are worldwide in distribution and can be found anywhere that corn is grown.

Stink Bugs (Hemiptera: Pentatomidae)

Scientific and Common Names

Halyomorpha halys (Stål), brown marmorated stink bug.

Euschistus servus (Say), brown stink bug.

Chinavia hilaris (Say), green stink bug.

Euschistus variolarius (Palisot de Beauvois), onespotted stink bug.

Nezara viridula (Linnaeus), southern stink bug.

Injurious Stage(s)

Nymphs and adults.

Injury Type

Indirect (nymphs and adults) and direct (nymphs and adults).

Injury to Ears

Adults and nymphs insert their piercing-sucking stylets into the tender tissues and inject digestive enzymes that can be phytotoxic and cause plant growth abnormalities (Bergman 1999). Stink bug feeding during late-vegetative corn can interfere with developing ear, causing deformities or missing kernels (Reisig and Reay-Jones 2023). Piercing through the husk at VT (tassle), R3 (milk), and R4 (dough) stage can cause the ear to bend into a ‘banana’ (Fig. 7a–c). The injured kernel may have a small, darkened spot where the mouthparts punctured the tissue, and may become completely discolored. Feeding also allows the entry of pathogens, which then further reduce grain quality (Fig. 7c, d).

Adult and immature stink bugs will feed on seedling and early vegetative corn, too. Symptoms include severely stunted plants, bushy plants with excessive tillering, twisted leaves or stalks, and leaves with series of 4–6 holes, each encircled by a ring of yellow or necrotic tissue. These holes are caused by stink bugs piercing the whorl from the outside of the plant and puncturing the immature, developing leaves inside the whorl. Each of these injury types can cause subsequent yield loss in the corn ear. Excessive plant injury causes a delay in ear development, often to the point that many kernels do not get pollinated thereby resulting in curved and unfilled ears.

Comments

All 5 stink bug species are similar in appearance (nymphs have round bodies, while adults are shield-shaped) but differ in color and size (Fig. 7e–h), and injury to corn is similar regardless of species. Injury to corn is most likely to occur in border rows, especially near broad-leaf weeds, or in fields heavily infested with weeds that are killed with a postemergent herbicide. In areas with winter small grains grown nearby, harvest of these crops can induce stink bugs to move to corn.

Distribution

All 4 species of stink bugs can be found in most of the lower 48 states, but they are more common in the Corn Belt and Gulf Coast states.

Corn Borers (Lepidoptera: Crambidae)

Scientific and Common Names

Ostrinia nubilalis (Hübner), European corn borer.

Diatraea grandiosella (Dyar), southwestern corn borer.

Injurious Stage(s)

Larvae.

Injury Type

Indirect and direct.

Injury to Ears

European corn borer (Fig. 8a) and southwestern corn borer (Fig. 8b) larvae will feed on any above-ground part of a corn plant (whorl, stalk, leaf midrib, and tassel). It is the larvae present later in the growing season that tend to feed on the corn ear. Larvae typically enter the ear through silks and feed on the developing kernels in the ear tip. As the kernels reach the R4 (dough) and R5 (dent) stages, the feeding damage mostly occurs along the top of the kernels, or more frequently in the groove between 2 rows of kernels where the larvae also pack white frass behind them as they feed (Fig. 8c). A single larva may damage 15–20 kernels in this manner, and several larvae may occur in the same ear. Larvae also tunnel into the center of the corncob (Fig. 8d) and ear shank (Fig. 8e).

Larval feeding during the vegetative stages, including leaf feeding, tunneling into the midrib, or tunneling into the stalks, may result in poor ear development, broken leaves or stalks, or plant death. This indirect feeding, especially during late-vegetative stages (V6–V16), could reduce grain yield up to 6% per larva per plant for European corn borer (Rice and Hodgson 2017).

Comments

The European corn borer has 4 generations per year in the southern United States, 2–3 generations in the Midwest, and 1 generation in the northern part of its range. The southwestern corn borer has 2 or 3 generations per year, depending on latitude and summer temperatures. Transgenic Bt corn has dramatically reduced populations of both corn borer species, and presently both insects are rarely considered economically damaging in field corn (Mason et al. 2018).

Distribution

The European corn borer is a moth native to Europe. It was accidentally introduced into North America in the early 1900s in the eastern United States. It can now be found in corn from the Atlantic coast states to the Rocky Mountains except for most of Florida and southwestern and southern Texas. It also occurs north into southern Canada, especially Ontario and Quebec. The first records of southwestern corn borer in the United States are from New Mexico in 1913, but the insect was not named until 1911 and could likely have been present prior to 1913. Southwestern corn borer likely originated in Mexico and is now known to occur throughout the southern United States, ranging from Arizona to Georgia, with southern Kansas being the northernmost record (Chippendale and Sorenson 1997).

Corn Earworm (Lepidoptera: Noctuidae)

Scientific and Common Name

Helicoverpa zea (Boddie), corn earworm.

Injurious Stage(s)

Larvae.

Injury Type

Direct.

Injury to Ears

Corn earworm larvae, which occur in a variety of colors (Fig. 9a), are very destructive to corn ears. Young larvae typically begin feeding on silks, where the female has laid her eggs. As the larvae grow, they move down the silk channels toward the developing kernels (Fig. 9b). Feeding may be concentrated near the ear tip throughout the growth of the larva (Fig. 9c), or the insect may extend its injury half the length of the ear. There are normally 6 instars, but 5–8 stages can occur, and it is the later stages when larvae do a large amount of injury. They may injure and partially consume 2 dozen kernels, filling the injured area with fecal material (Fig. 9d). Injured ears may develop molds after the larva finishes feeding. Upon completion of feeding, mature larvae chew a hole through the husk leaves, drop to the ground, and pupate in the soil. Larvae also will feed in the whorl of vegetative-stage corn if corn ears are not yet available. The economic impact of corn earworm is variable depending on planting date and geographic region (Reay-Jones 2019).

Comments

Females may lay multiple eggs on corn silks and husks, and larvae will feed near each other in the silks after hatching. As larvae reach the later stages, they become aggressive toward one another, and larger larvae will cannibalize smaller siblings. Therefore, typically only 1 mature larva is found in a corn ear but occasionally 2, or rarely 3, may occur in an ear. Additionally, the corn earworm can also feed on other crops and has several common names; for example, bollworm and tomato fruitworm.

Distribution

The corn earworm occurs in all lower 48 states, Hawaii, and southern Canada. Overwintering success is largely dependent on winter temperatures, so infestations that occur in the Corn Belt and Canada are largely due to moths migrating from southern states each summer.

Fall Armyworm (Lepidoptera: Noctuidae)

Scientific and Common Name

Spodoptera frugiperda (J.E. Smith), fall armyworm.

Injurious Stage(s)

Larvae.

Injury Type

Indirect and direct.

Injury to Ears

Young larvae (Fig. 10a) typically begin feeding on leaf tissue within the whorl, particularly if developing ears are not available. In reproductive stage corn, injury to tassels, silks and kernels is identical to and indistinguishable from corn earworm injury. As larvae grow, they move down the silk channels toward the developing kernels. Unlike corn earworm, fall armyworm will chew through the husk leaves to enter the ear. Feeding is typically concentrated near the tip but can occur in the middle or butt of the ear. There are 6 instars, and like corn earworm, it is the last 2 stages (Fig. 10c, e) that create the most injury (Fig. 10c, d). They may injure and partially consume two dozen kernels (Fig. 10d, f), filling the injured area with fecal material (Fig. 10b). Larvae can also tunnel into the center of the corncob and ear shank similar to European corn borer. Upon completion of feeding, mature larvae chew a hole through the husk leaves, drop to the ground, and pupate in the soil. Injured ears may develop molds after the larva finishes feeding (Fig. 10g).

Comments

Females can lay egg masses that contain 100–200 eggs, and these are typically laid on corn leaves. Compared to corn earworm, fall armyworm larvae are much more tolerant of 1 another, and it is not uncommon to find several mature larvae feeding in the same ear, often separated by only a few kernels. Indirect injury to the ear is caused during vegetative stages of corn growth when larvae feed inside the whorl leaves. Occasionally larvae tunnel deep enough to damage the growing point, but more commonly their feeding cuts large holes in leaves, which sometimes fall from the plant after emerging from the whorl. This injury can reduce subsequent ear size on the infested plant.

Distribution

The fall armyworm is native to North and South America but is now distributed worldwide. In the United States, it overwinters in southern Texas and southern Florida, but the adult moth is a strong flier, and each summer migrates north to most states in the Great Plains and eastern United States. However, economically damaging populations usually only occur in the southeastern United States.

Western Bean Cutworm (Lepidoptera: Noctuidae)

Scientific and Common Name

Striacosta albicosta (formerly Richia albicosta) (Smith), western bean cutworm.

Injurious Stage(s)

Larvae.

Injury type

Direct.

Injury to Ears

Depending on the corn development stage, western bean cutworm first instars (Fig. 11a) may feed in different locations. In vegetative-stage corn, larvae will enter the whorl, penetrate the flag leaf and feed on the developing pollen and tassel. In VT (tassel) and R1 (silk) stages, larvae feed on shed pollen, leaf collar tissue, and silks (Rice and Dorhout 2006). Extensive silk feeding during pollination can cause incomplete kernel set.

The corn ear is the primary feeding site for larvae, and they enter the ear by chewing through the husk or silks (Hagen 1962). The husk leaves may show signs of entry with small holes created by early-stage larvae or exit with large holes (Fig. 11c) created by mature larvae (Fig. 11b) as they leave the ear to pupate in the soil. Unlike corn earworms, which are cannibalistic and restrict most of their feeding to the ear tip, multiple western bean cutworm larvae may occur in a single ear and will feed on developing kernels in the ear tip, middle of the ear, and ear butt (Fig. 11d). In addition to the loss of grain, ear molds may develop on the kernels, which can further reduce the quality at harvest (Fig. 11e).

Comments

Although the western bean cutworm occurs throughout the Corn Belt, annual damage to corn is most likely to occur in the drier regions of Colorado, Kansas, and Nebraska.

Distribution

The western bean cutworm was historically restricted to corn in the southwestern United States, particularly Arizona. In the early part of the 20th century, it moved into Kansas and Nebraska and became a significant pest of dry beans, in addition to corn. Then in 2004, moths were trapped in Missouri and Illinois (Dorhout and Rice 2004), and a couple of years later it moved further east into Ohio (Rice and Dorhout 2006). Eventually it reached Pennsylvania (Tooker and Fleischer 2010) and other states on the Atlantic seaboard.

Grasshoppers (Orthoptera: Acrididae)

Scientific and Common Names

Melanoplus differentialis (Thomas), differential grasshopper.

Melanoplus femurrubrum (De Geer), redlegged grasshopper.

Melanoplus bivittatus (Say), twostriped grasshopper.

Injurious Stage(s)

Late-stage nymphs and adults.

Injury Type

Indirect (late-stage nymphs and adults) and direct (late-stage nymphs and adults).

Injury to Ears

Grasshopper feeding begins at the ear tip and progresses toward the ear shank. Late-stage nymphs and adults will consume silks, apical portions of green husk leaves, and entire kernels, completely eating all except for the cob. Injury is typically confined to the apical 1/3 of the ear but may extend further down the ear if several grasshoppers feed on the same ear. Injury may occur on only one side of the ear or completely around the cob. Injury to kernels begins during the R2 (blister) stage and continues through the R5 (dent) stage of kernel development. Grasshoppers feeding on blister-stage kernels may leave some of the pericarp uneaten (Fig. 12a). Grasshoppers feeding on dent-stage kernels typically eat down to the tip cap, or they may bite into the side of kernels along the edges of the feeding area leaving some kernels partially eaten. Grasshoppers do not leave frass in the injured ear but typically kick it away with their hind legs.

Comments

Injury mostly occurs on corn plants in border rows when grasshoppers move out of ditches, waterways, and conservation strips. Dense populations of grasshoppers can nearly denude plants in border rows (Fig. 12b). Fields with substantial early season weed problems may support grasshopper populations throughout the field, which then contributes to scattered amounts of injury across the field but to a much lesser degree than the field edge.

Distribution

Differential grasshoppers (Fig. 12c) are found across most of the United States, especially in the Midwest from Indiana to Colorado, but are absent from the southeastern, northeastern, and extreme northwestern states (Capinera et al. 2004). Redlegged grasshoppers (Fig. 12d) occur throughout the continental United States, and the 2-striped grasshopper (Fig. 12e) occurs throughout in most of the US region except for the region along the Gulf of Mexico (Weiss 1999).

Vertebrate Pests

Scientific and Common Names

Agelaius phoeniceus (Linnaeus), red-winged blackbird.

Quiscalus quiscula (Linnaeus), common grackle.

Ursus americanus Pallas, black bear.

Odocoileus virginianus (Zimmermann), white-tailed deer.

Procyon lotor (Linnaeus), raccoon.

Injury Type

Direct.

Injury to Ears

Birds, especially common grackles and red-winged blackbirds, will attack the exposed tips of corn ears. They prefer kernels in the R4 (dough) stage. Birds will tear open a kernel, which leaves a ragged edge, and then peck out the softer endosperm and embryo (Fig. 13a–c). Birds rarely consume the entire kernel, which is typical feeding behavior of fall armyworm and corn earworm. It is common for insects, such as adult rootworms and sap beetles, to then enter these bird-damaged kernels to feed. It is important not to attribute bird activity to insects that simply are feeding on the already damaged kernels.

Fig. 13.

a–c) Birds can injure corn ears by tearing open kernels and eating the soft endosperm and embryo.

Black bears can cause 2 types of injury to corn ears. They bite off the ends of young ears, and this creates a stubby ear that in some corn-growing regions is called a “beer can ear” because of its similarity in size to a beer can. The other type of injury is to tear off the husk leaves and then chew the kernels, like the way a child eats an ear of sweetcorn (Fig. 14a).

Fig. 14.

a) Bears typically tear the husks off of the ear and eat it like sweet corn, b) while raccoons typically leave the husks intact.

Raccoons tear the husk leaves away to expose the ear, and they can chew off all the kernels (Fig. 14b). The damage looks identical to that caused by black bears except that the husk leaves typically are still attached.

White-tailed deer will bite off the tips of young ears through the R4 (dough) stage (Fig. 15a). They do not remove the husk leaves, but with these leaves removed the damage to the tip of the ear is more easily observed (Fig. 15b).

Fig. 15.

a, b) Deer injure corn ears by biting the tip of the ear off.

Scouting Tips

• Corn growth and development is based on genetics and accumulated degree days. The number of kernels is established around V12 and the plant is about 10–12 days away from pollination at V15 (Table 1; Abendroth et al. 2011). Forecasting and prioritizing scouting efforts is critical for making timely treatment decisions, especially during pollination.

• Scout cornfields early and often and use Table 2 as a quick reference to distinguish injury to corn ears. At R1 (silk), the ear is fully formed and particularly sensitive to stresses, like drought, hail, nutrient deficiencies, and high temperatures (Abendroth et al. 2011). Evaluating plant health and the presence/absence of insects from V15 through R5 (dent) is a general scouting timeline; inspecting developing ears every 5–7 days is recommended.

• Examine the entire plant for pests. Some pests, for example, beetles, stink bugs, and aphids, may be present all over the plant and not solely on or in the ear. Some caterpillars move from the leaves or tassels to the ear, and seeing this movement or their eggs on other plant parts can provide a warning and time to make a treatment decision.

• Check the outside of the ear and into the leaf axis next to the shank. Pests, for example, aphids, sometimes feed in the outer layers of the corn husk. Beetles and stink bugs may be resting in the leaf axis, and some caterpillars enter this area prior to boring into the shank or stalk. Some insects can chew through the husk and enter/exit the ear from anywhere.

• Inspect the silks prior to opening the husk and inspecting the ear. Beetles that feed on the silks are often hidden, and young caterpillars may be traveling through the silks to enter the ear.

• Once the rest of the plant has been inspected, gently grab the top of the husk near the silks and pull them apart to expose the ear. Do this slowly as pests may fly away or fall off the ear, making it difficult to identify them. Look at the entire ear, including the shank, for evidence of injury. Consider splitting the ear and shank to see if any insects are feeding inside.

Acknowledgments

We acknowledge the USDA-NIFA Multistate Research Project NC246 for support. All photos were provided by Marlin E. Rice except for Fig. 6d (Adam Sisson, Iowa State University); Fig. 8b, d (Scott Stewart, University of Tennessee); and Fig. 9d (Ashley Dean, Iowa State University).

Author Contributions

Erin Hodgson (Conceptualization [equal]), Ashley Dean (Writing—review & editing [equal]), and Marlin Rice (Conceptualization [equal], Writing—original draft [equal], Writing—review & editing [equal])