Abstract
A 3-yr study was conducted in California’s southern San Joaquin Valley to determine the overwintering survival of the corn leafhopper, Dalbulus maidis (DeLong and Wolcott), and the mollicute, Spiroplasma kunkelii, the causal agent of corn stunt disease. Corn leafhopper populations were sampled from November to March using yellow sticky cards, D-vac suction samples, and inspection of volunteer corn plants and spring planted corn. S. kunkelii presence was determined by sampling sentinel plants placed in the field during the winter, leafhoppers collected throughout the winter, and evaluation of volunteer plants over the winter and spring planted corn. Leafhoppers were collected on yellow sticky cards throughout the winter during all 3 yr. They were also regularly recovered from alfalfa, Medicago sativa L., winter forage (wheat, Triticum aestivum L., and triticale, Triticale hexaploide Lart. and riparian areas by D-vac suction sampling. Females constituted the majority of leafhoppers (>80%) recovered on both sticky cards and from D-vac samples. S. kunkelii was recovered from leafhoppers throughout the winter, from sentinel plants, and in spring planted corn. Volunteer plants were determined to be a critical key in leafhopper overwintering, and consequently, the survival of S. kunkelii. Volunteers extended the season by as much as 2 mo, thus shortening the period of time the leafhoppers were forced to go without a food source. The possible reasons for a shift in the leafhopper from cyclic pest to persistent pest in the region are also discussed.
introduction
THE CORN LEAFHOPPER, Dalbulus maidis (DeLong and Wolcott), is found on corn (Zea mays L.) from Mexico to South America and throughout much of the southeastern and southwestern United States (Nault and Madden 1985). In addition to yield losses caused by feeding injury (Bushing and Burton 1974), corn leafhopper is a vector of corn stunt spiroplasma (CSS), Spiroplasma kunkelii (Whitcomb et al. 1986), the causal agent of corn stunt disease. Corn stunt disease can result in even more significant yield losses than those attributable to direct leafhopper feeding (Nault 1985, Moya-Raygoza and Nault 1998). In Latin America, three pathogens, corn stunt spiroplasma (CSS), maize bushy stunt phytoplasma (MBSP), and maize rayado fino marafivirus (MRFM), are involved in the corn stunt complex (Nault and Bradfute 1979), collectively referred to as "achaparramiento" (Hruska and Gomez Peralta 1997). Currently in California, of the three entities, only S. kunkelii (CSS) has been isolated (Purcell and Suslow 1984). Although plants with MBSP-like symptoms have been observed, recent attempts to recover this pathogen have been unsuccessful (C.G.S. and D.C.O., unpublished data).
The corn leafhopper was first reported causing injury to field corn in Fresno and Tulare counties in 1942 (Frazier 1945). Since that outbreak, corn leafhopper was not reported again until the late 1960s (Bushing and Burton 1974, Bushing et al. 1975). Apparently, damage was due solely to leafhopper feeding, because no evidence of corn stunt disease was reported. Corn leafhopper reappeared in 1981, and corn stunt was apparently present in at least some fields (Kloepper et al. 1982, Purcell and Suslow 1984). The report by Kloepper et al. (1982) is the first report of corn stunt in California. In 1996, corn leafhopper populations reached extremely high levels on late-maturing field corn (silage and grain) and sweet corn in Fresno, Tulare, and Kings counties (C.G.S., unpublished observations). In addition, many fields had a high incidence of corn stunt disease, and S. kunkelii was identified from plants exhibiting characteristic symptoms. The combination of leafhopper feeding and disease resulted in substantial yield and quality losses. Since then, the presence of leafhopper populations and the incidence of corn stunt disease have continued yearly in the southern San Joaquin Valley (SJV).
Ability of the corn leafhopper to overwinter in California has been the subject of considerable speculation. Corn and teosinte, Zea spp., the wild progenitors of corn, are the only hosts (Larsen et al. 1992, Nault 1998). Teosintes do not occur in California, and corn does not usually survive throughout the winter here. Because there are no overwintering hosts for D. maidis in California, it was suggested that it migrated north from overwintering sites in Mexico (Purcell 1988). In other areas of the United States, investigators have speculated that infestations came from elsewhere as well. Pitre et al. (1967) reported that the corn leafhopper did not overwinter in Mississippi and hypothesized that it probably arrived there from elsewhere in the southeast (location not specified) or from Mexico. Nault (1998) noted that the appearance of D. maidis and outbreaks of corn stunt disease in Florida were likely the consequence of the vector being introduced from the Caribbean Islands and transported to the United States mainland by hurricanes.
In California, before the 1996 outbreak, leafhoppers appeared only late in the season (August and September), thus giving credence to the theory that they migrated northward from Mexico, arriving late in the growing season. Corn leafhopper overwintering biology was never investigated. After the 1996 leafhopper outbreaks, D. maidis appeared earlier each year, and by 2000, was found in March and April. This observation suggested to us that the leafhopper was overwintering in the area rather than migrating here from Mexico. Also, D. maidis has never been found in Imperial County (E. Natwick, personal communication), despite that county's large acreage of sweet corn, which is just across the border from Mexico. It would seem reasonable that leafhoppers migrating north from Mexico would infest corn planted in its path as it progressed northward. This paper reports the results of studies conducted to determine the overwintering capabilities of D. maidis and the mollicute, S. kunkelii, in the southern SJV.
Materials and Methods
Yellow Sticky Cards.
Double-sided yellow sticky cards, 7.6 by 12.7 cm (Gempler's, Madison, WI), were affixed to the top of 1.2-m lath stakes with binder clips (Stationers Supply, Des Plaines, IL). Beginning in October or November, the sticky cards were placed adjacent to corn fields in Fresno, Kings, and Tulare Counties that had high numbers of corn leafhopper during the previous summer, next to alfalfa fields, and in riparian areas next to fields previously planted to corn. The traps were replaced weekly or biweekly, depending on weather conditions. Traps were returned to the laboratory, and the number and sex of leafhoppers on each card was determined.
D-vac Samples.
Areas adjacent to heavily infested corn fields were sampled beginning in October or November using a D-vac suction machine (Rincon-Vitova, Ventura, CA) with a 0.093-m2 sampling cone. Areas suspected of harboring leafhoppers, including weedy fields, ditch and fence lines, alfalfa (Medicago sativa L.), winter forage (wheat, Triticum aestivum L.; triticale, Triticale hexaploide Lart.) and riparian areas were sampled. Sample bags were returned to the laboratory and chilled (≈7°C) in the refrigerator to slow leafhopper activity, and the number of D. maidis males and females was counted using a dissecting microscope.
Volunteer Plants.
During the winter of 2002-2003 and 2003-2004, volunteer plants were examined during January, February, and March for the presence of surviving leafhoppers. Plants were cut at ground level, transported to the laboratory where they were chilled to slow leafhopper movement, and examined for the presence of adults and nymphs.
Spring Planted Corn.
Silage corn, 'Asgrow RX913', was planted at the UC Kearney Research and Extension Center (Parlier, CA) in early March of 2003 and 2004, the normal planting time in the southern SJV. Both plantings were adjacent to a corn field that had high populations of corn leafhopper the previous fall. Beginning ≈1 wk after seedling emergence (mid-March), the plants were examined carefully over the course of several days, and the number of adult leafhoppers in the whorls was counted. Plants containing leafhoppers were marked with surveyor's flags and tested 6 wk later for the presence of S. kunkelii using enzyme-linked immunoabsorbant assay (ELISA). Commercially planted corn in grower fields was also examined for the presence of leafhoppers and tested for the occurrence of the spiroplasma.
Sentinel Plants.
Sentinel corn plants 'Asgrow RX913' were grown in an insect free greenhouse in 10-cm pots, three plants per pot. When the plants were ≈10 cm tall, six pots were placed in a metal pan filled with water and transported to the field. Twelve to twenty-four plants were placed at randomly chosen field locations for ≈2 wk. Plants were watered as necessary. At the end of 2 wk, plants were transported to the laboratory and placed in the greenhouse, and new potted corn plants were placed in the field. Each returned pot was treated with ≈0.5 g aldicarb (15 G) to kill any arthropods present. Corn plants were held in the greenhouse for 6-8 wk at 27°C before ELISA testing for the presence of S. kunkelii.
Detection of S. kunkelii.
The presence of S. kunkelii within corn plants and leafhoppers was determined using ELISA (Agdia 2004) and polymerase chain reaction (PCR) techniques (Barros et al. 2001). When testing immature corn, a portion of the mid-rib, found to contain a high titer of spiroplasma, was removed from symptomatic leaves. A portion of the tassel rachis was used in mature plants. Approximately 0.5 g of plant tissue was placed in a small self-sealing bag (7.5 by 10 cm). The tissue was macerated on a laboratory bench using a large-faced brass mallet, a small amount of general extract buffer, pH 7.4, (Agdia, Elkhart, IN) was added to dilute the macerate ≈5-10 times, and further maceration was completed. The tissue macerates were stored in a refrigerator and used for ELISA testing within 1-4 h.
The ELISA method used was a DAS (double antibody sandwich) assay developed by Agdia. Antibodies were made in rabbits using a CSS (corn stunt spiroplasma)-purified antigen and a standard alkaline phosphatase conjugate enzyme system. Plates were coated overnight, and samples were prepared and applied the next day. After incubation overnight at 4°C, the samples were washed out, and conjugate was applied, followed by a 2-h incubation at room temperature on the laboratory bench. Substrate was added at room temperature under moist conditions and incubated for 30-60 min depending on the rapidity of response. The plate wells were initially scored visually as they developed, and later, after fixing, the enzyme reaction was read with a plate reader (E-max 38161; Molecular Devices, Sunnyvale, CA) at 405 nm. Positives were at least 2.5 times background values.
Leafhoppers captured on the sticky cards and those collected in D-vac samples were examined for the presence of S. kunkelii using PCR (Barros et al. 2001). Leafhoppers collected on sticky cards were cleaned in xylene, followed by a wash in 70% ethanol. DNA from samples was extracted and prepared using a FastDNA Kit and the FastPrep Instrument from BIO 101 (Qbiogene, Carlsbad, CA). The amounts of reagents used in the preparation were determined by the number of leafhoppers in the sample. Primers used for the specific identification of S. kunkelii were CSSF1/R1 (Barros et al. 2001). As few as two adult leafhoppers were able to produce a CSS positive, but in most cases, samples from which DNA was extracted contained five insects. The PCR products were run on a 1.5% agarose gel for 3 h at 80 volts and stained with ethidium bromide to visualize the positive 600-bp band. In the early season, the PCR product of the reaction was used in a second reaction, and distinct bands were observed. In two separate situations, suspected S. kunkelii DNA was amplified using PCR, and the product was purified using QIAquick (Qiagen, Valencia, CA). This product was sequenced at Davis Sequencing (Davis, CA), and a BLAST search was performed to compare our product with the known sequences of S. kunkelii.
Plant samples were processed within 24-48 h of collection and were stored at 5°C before processing. Leafhoppers were stored at -20°C before processing.
Corn Leafhopper Voucher Specimens.
Voucher specimens were deposited in the Bohart Museum of Entomology, Department of Entomology, University of California (Davis, CA), lot no. 204002.
Results
Yellow Sticky Cards.
Adult leafhoppers were captured on yellow sticky cards throughout the winter in all 3 yr (Fig. 1). During 2001-2002, adults were captured each month (Fig. 1A). In 2002-2003, captures on yellow sticky cards were less frequent (Fig. 1B). The gaps in collections represent periods of particularly cold weather, when temperatures were below the flight threshold. The sites denoted as Dover, Dairy, 18th East, and Eighth (Kings County) all represent locations adjacent to corn fields, whereas the River site was a riparian area in Kings County, consisting mainly of grassy weeds located along the Kings River. Crop rotations required new trapping locations in 2003-2004 (Fig. 1C). The zeros in trap catches from December to February corresponded with cold weather and problems in maintaining the trap line because of localized flooding. However, collections beginning in early March were similar to those of previous years. Those collections (11 March) occurred before spring planted corn emerged. Leafhoppers trapped after 20 March of all 3 yr overlapped with newly planted corn.
Collections of D. maidis on yellow sticky cards placed in corn fields and riparian areas, November through March. (A) 2001–2002. (B) 2002–2003. (C) 2003–2004.
D-vac Samples.
D-vac suctions proved a reliable technique for recovering leafhoppers. Adults were consistently collected from both alfalfa and weedy areas throughout the winter (Fig. 2A-C). As spring approached, we began to routinely collect leafhoppers from winter forage. As with the sticky card captures, collections using the D-vac overlapped in time with the spring planted corn crop, again indicating that the adults had survived the period between the last corn in the fall and newly planted corn in the spring.
Collections of D. maidis by D-vac suction samples from selected overwintering habitats, November through March. (A) 2001–2002. (B) 2002–2003. (C) 2003–2004.
Percentage of Males and Females Overwintering.
Females surviving the winter outnumber males by a wide margin (Table 1). There was a general trend for an increased percentage of males surviving the winter over the 3 yr.
Percentage of overwinteringa male and femaleD. maidis from D-vac and sticky card samples
a Period from November to March.
Percentage of overwinteringa male and femaleD. maidis from D-vac and sticky card samples
a Period from November to March.
Volunteer Plants.
The winter of 2002-2003 was exceptionally mild, with the first freezing temperature coming on the morning of 25 December 2002, and a total of only 8 d =0°C (National Weather Service 2004). The coldest morning reached only -1°C. Although the outer leaves of the volunteer corn plants froze, the whorl leaves remained viable throughout the winter and supported populations of both adult and immature leafhoppers (Fig. 3A). Both stages were found throughout the winter into March, when leafhopper populations from volunteer plants (Fig. 3A) overlapped with those found in spring planted corn. Weather conditions during 2003-2004 were considerably different, with 23 d =0°C (National Weather Service 2004). The earliest subzero day came on 4 November 2003, and the coldest morning reached -4°C. Even though the corn was completely frozen by late November, leafhopper adults and nymphs continue to survive within the frozen plants until well into February (Fig. 3B). Nymphs failed to survive the entire winter of 2003-2004 and were not recovered in March when new corn was being planted. Adults continued to survive the entire winter as determined by the D-vac samples.
Collection of D. maidis adults and nymphs from volunteer corn throughout the winter of (A) 2002–2003 and (B) 2003–2004.
The mild winter of 2002-2003 resulted in the survival of large numbers of volunteer corn plants throughout the entire winter. S. kunkelii was found in these overwintered volunteer corn in the spring of 2003 (Table 2). These volunteers can serve as an early host for leafhoppers who become infective after feeding.
Results of ELISA and PCR evaluations of sentinel plants, spring planted corn, volunteer corn, and leafhoppers for the presence ofS. kunkelii
a Date plants were removed from the field or leafhoppers were collected.
b Number of plants producing positives/total no. plants tested.
c Leafhoppers were tested in groups of two to five and therefore cannot be evaluated in terms of the no. positives over the no. tested. x/y indicates no. positives over no. groups tested; numbers in parenthesis indicate the no. leafhoppers tested in each group.
d Corn planted adjacent to a field with heavy leafhopper populations in the previous fall in which plants were found to contain adult leafhoppers in the whorl. Kearney Research and Extension Center (Fresno County, CA).
e Commercial corn planted in the spring.
Results of ELISA and PCR evaluations of sentinel plants, spring planted corn, volunteer corn, and leafhoppers for the presence ofS. kunkelii
a Date plants were removed from the field or leafhoppers were collected.
b Number of plants producing positives/total no. plants tested.
c Leafhoppers were tested in groups of two to five and therefore cannot be evaluated in terms of the no. positives over the no. tested. x/y indicates no. positives over no. groups tested; numbers in parenthesis indicate the no. leafhoppers tested in each group.
d Corn planted adjacent to a field with heavy leafhopper populations in the previous fall in which plants were found to contain adult leafhoppers in the whorl. Kearney Research and Extension Center (Fresno County, CA).
e Commercial corn planted in the spring.
Spring Planted Corn.
In both years, corn planted adjoining a corn field that had high leafhopper population the previous season was colonized almost immediately after seedling emergence. Based on five separate observations, 20 and 16 adult leafhoppers were found in 2003 and 2004, respectively. Based on these numbers, we calculated an average of 1,902 and 1,252 adult leafhoppers per hectare in 2003 and 2004, respectively. In 2003, 4 of 35 plants observed to contain an adult leafhopper were positive for the presence of S. kunkelii, and in 2004, 5 of 15 plants were positive for S. kunkelii(Table 2). Five weeks after adults were first observed in the whorls in 2004, 46% of those plants contained nymphs.
Overwintering of S. kunkelii Determined by Sentinel Plants.
Sentinel plants, placed in the field to detect the presence of S. kunkelii, were found to harbor the mollicute in all 3 yr: 17, 33, and 17%, of tested plants in 2002, 2003, and 2004, respectively (Table 2). Infected plants were found among the sentinel plants placed in the field in January through April.
Detection of S. kunkelii in Leafhoppers.
Infected leafhoppers were recovered throughout the winter and spring in all 3 yr. The presence of S. kunkelii in leafhoppers collected in December through March was confirmed by PCR. These leafhoppers came from several locations throughout Kings, Tulare, and Fresno counties (Table 2). Infected leafhoppers recovered in March were capable of transmitting the spiroplasma to newly planted corn, which overlapped with the time frame in which the infected leafhoppers were recovered.
Discussion
From the 1960s to the 1980s, serious outbreaks of D. maidis in the SJV lasted only 1-2 yr, and then the leafhopper disappeared or was present at very low levels for several years. Bushing et al. (1975) speculated that biological control agents exerted a cyclic regulating effect on the leafhoppers, although no data were presented to support this hypothesis. Previous field studies on the leafhopper in California were restricted to chemical control evaluations on late season corn (Bushing and Burton 1974, Bushing et al. 1975), and no biology or long-term population studies of either the leafhopper of the causal agent of corn stunt, S. kunkelii, were conducted. Until this study, nothing was known regarding the overwintering of either the leafhopper or the spiroplasma. Purcell (1988) concluded that regular spring migrations from Mexico might explain the reappearance of D. maidis in California. Similar conclusions were reached by Petri et al. (1967) for the appearance of D. maidis in several southeastern states. Beginning with the 1996 outbreak, D. maidis failed to conform to previous cyclic patterns, and populations continued to increase in subsequent years. Leafhoppers appeared earlier each year, and by 2000, they commonly were found in March and April. In addition, the incidence of corn stunt continued to increase annually as well. At this point, we suspected that D. maidis was overwintering locally. Immigration from Mexico seemed to be an unlikely because the distance from D. maidis overwintering sites in Mexico (Taylor et al. 1993) to the SJV is 1,700-1,900 km, and there are no prevailing winds that move from Mexico into central California across these latitudes. The summer monsoons of the southwestern United States move across southern California, Arizona, and New Mexico but rarely into central California (Ellis et al. 2004). Although D. maidis was reported from Los Angles and Riverside counties in the 1950s (Bushing et al. 1975), it has not been found in these southern California counties in recent years. Also, the absence of the leafhopper in Imperial County (E. Natwick, personal communication) indicates that it is not migrating into the states from Mexico.
This study encompassed three very different winter conditions. The winter (November through March) of 2001-2002 (designated average) was fairly normal for the region; mean temperature was 0.9°C above average, and precipitation was 0.03 cm below average. During 2002-2003 (designated warm and dry), warmer conditions prevailed, with temperatures 2.3°C above average, and precipitation was 0.67 cm below average. The warmer conditions may have contributed to an increase in the number of infected sentinel plants. In 2003-2004 (designated cold and wet), temperature averaged 2.7°C below normal, and precipitation was 3.2 cm above normal (National Weather Service 2004). The coldest temperature encountered during the 3-yr period was -4°C. Generally, subfreezing temperatures last only a few hours. Larsen et al. (1993) found that D. maidis could survive exposure of -5°C for up to 8.7 h (LT50), provided they had been preconditioned to cold temperatures. In our study, adult leafhoppers survived equally well during all three winters, as evidenced by collections on sticky cards and by D-vac samples. Temperatures in the SJV generally begin a downward trend during September through November, well in advance of freezing conditions, allowing ample time for adults to become preconditioned to colder temperatures. We noted that leafhoppers collected during the summer months rarely survived >24 h in the refrigerator (≈5°C), but those collected in October or November survive several days at the same temperature.
In the SJV, harvesting is normally completed by mid-November, and spring planting occurs in early March, with seedling emergence by mid-March. This time frame would normally present an ~3.5-mo host-free period. In actuality, the host-free period is much shorter in most years because volunteer corn plants, arising from seeds remaining in the field after harvest, appear after the first fall rains or an irrigation. These volunteer plants are present until the first killing frost, which, in some years, may not occur until late December or not at all. As soon as the soil begins to warm in the spring, usually by early February, germination begins again, and seedling corn may be present in mid-February. In 2002-2003, the volunteer plants never completely froze, tissue within the whorl remained viable, and both adults and nymphs thrived throughout the winter. Thus, the period of time when corn is not available may be as little as 2 mo. Larsen et al. (1992) found that D. maidis could survive periods of up to 9 wk in the absence of corn as long as it had access to water. They concluded that D. maidis can indeed survive the Mexican winter and reinfest corn the following spring, although Mexican winters are cool and dry in contrast to the cooler and wetter conditions in California's SJV. The common denominator between the two areas is the lack of corn for some period of time. In addition, infection with S. kunkelii or other mollicutes may increase survival of D. maidis compared with noninfected individuals (Nault 1990, Ebbert and Nault 1994, Hogenhout and Ozbek 2002). Purcell (1988) noted that D. maidis feeding on aster yellows-infected plants (nonhost plants for D. maidis) lived longer than did those feeding on healthy plants.
The question of D. maidis using plants other than corn as feeding hosts during the absence of their primary host remains open. Reports indicate that oats, Avena fatua L.; johnsongrass, Sorghum halepense L. Pers.; barley, Hordeum vulgare L.; Rottboelia spp.; and aster, Callistephus chinensis Nees (Purcell 1988), can serve as feeding hosts and perhaps reproductive hosts as well (de Agudelo et al. 2001). D. maidis has also been collected from wheat, celery, Apium graveolens L., grasses (species not identified), Solanum marginatum L.,(Tsai and DeLong 1989). and potato, S. tuberosum L. (Stoner 1965). These plants were not specified as feeding hosts, and their role in the overwintering of D. maidis is unclear. We found overwintering D. maidis associated with alfalfa, triticale, johnsongrass, a number of winter annual weeds, and litter present both within and adjacent to corn fields. We have no evidence that any of these plants serve as feeding hosts for the leafhopper. They do, however, provide a sheltered habitat for overwintering adults. Our findings that females overwinter in substantially higher numbers than do males agree with those of Larsen et al. (1992) and Virla et al. (2003).
Spiroplasma kunkelii is both circulative and propagative within the leafhopper vector (Alivizatos and Markham 1986, Hogenhout and Ozbek 2002, Ozbek et al. 2003). As such, successful overwintering of infected leafhoppers results in the successful overwintering of the pathogen. Although the corn stunt spiroplasma has been found in mustard, Sinapis alba L.; pea, Pisum sativum L.; radish, Raphanus sativus L.; and spinach, Spinacia oleracea L. (Alivizatos 1984), it is not likely that these plants contribute to the overwintering of the mollicute. Our finding of spiroplasma-infected leafhoppers throughout the winter combined with findings of infected sentinel plants during the winter months and infected early spring planted seedlings (Table 2) indicates that the spiroplasma successfully overwintered within the leafhopper itself and was successfully transmitted to early spring planted corn. Ebbert and Nault (1994) also found that S. kunkelii survived the Mexican winter within the leafhopper. During warm winters such as 2002-2003, the spiroplasma is capable of surviving the winter in volunteer plants that do not completely freeze. Overwintering leafhoppers easily acquire the spiroplasma in the spring and transmit it to newly planted corn.
Dalbulus maidis does not enter diapause or another state of dormancy during the Mexican winter (Larsen et al. 1992), but survives as active adults. It is evident from the sticky card collections throughout the three winters during which this study was conducted that D. maidis survives as an active adult in the SJV. Flight occurred whenever temperatures exceeded the flight threshold of 22°C (Taylor et al. 1993). In a Mediterranean climate such as California, diapause or hibernation is not essential for surviving the mild winters. The absence of food is rarely a cue for diapause induction (Denlinger 1986).
The reason for the change in leafhopper dynamics, from the cyclic pest observed from the 1940s to the 1980s to its current persistent pest status, is not fully understood. The most likely explanation lies in a combination of genetic changes in the leafhopper population caused by selection for increased cold hardiness together with changes in corn culture in the region. During the 1980s, there was a substantial migration of the dairy industry from southern California to the southern SJV, especially Tulare and Kings counties. This led to an increased need for feed, most notably silage corn. The number of hectares of corn grown for silage in the two counties increased from 18,000 in 1988 to 76,000 in 2003, more than a 4-fold increase (Kings and Tulare County Agricultural Crop and Livestock Reports 1988-2003). To produce an increased amount of silage, plantings are established earlier (March) and later (August or September) than before. August and September plantings may not be harvested until November. This results in corn being grown for up to 9 mo. Add to this the presence of volunteers for an additional 1-2 mo, and corn becomes almost a year-round crop. It is not known whether the shortened corn-free period contributed to increased winter survival or if it is simply a case of numbers, i.e., the increase in corn acreage plus the proximity of fields to each other results in more individuals entering the winter, thus increasing the probability that more will survive the period and be available to recolonize corn the following spring.
We thank R. Smith, C. Frate and C. Collar for assistance in conducting various aspects of the field studies. D. Haines and J. Vernon provided information on corn acreage in Tulare and Kings Counties. Numerous growers gave unlimited access to fields, without which this study could not have been completed. We thank R. Randhawa for assistance with the ELISA and PCR. Portions of this research were funded by the UC integrated pest management Statewide Project, Pioneer Seeds, Eureka Seeds and Western Farm Services, and the California Department of Food and Agriculture. We thank E. Backus and U. Kodira for a critical review, which greatly improved the manuscript.
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