Abstract
The sugarcane borer, Diatraea saccharalis (F.) (Lepidoptera: Crambidae), is the primary pest of sugarcane, Saccharum spp., in Louisiana. Spring populations are not considered economically damaging, but quantifying infestations can provide an indication of the spatial and temporal character of the damaging summer populations. Statewide surveys quantified the density of sugarcane tillers killed by D. saccharalis (deadhearts) from sugarcane fields across the state in spring from 2003 to 2020. Deadheart density varied greatly among years with a high of 1,318/ha in 2003 to a low of 0/ha in 2018. Linear regressions of the 3-yr rolling average showed declines in spring D. saccharalis populations and the percentage of acreage treated with insecticides over 17 yr. Weather factors including minimum winter temperatures and average spring temperatures were poor predictors of D. saccharalis populations. Only total precipitation in the month of April was positively correlated with numbers of deadhearts per hectare. Results suggest overwintering mortality is not a key factor influencing populations of the first generation of D. saccharalis in Louisiana. Total precipitation in the month of July was positively associated with percentage of treated acreage. Spring deadheart density was directly related to percentage of acreage treated with insecticides during the summer. Quantifying first-generation D. saccharalis populations by recording deadheart density can aid in predicting pest pressure later in the growing season.
The sugarcane borer, Diatraea saccharalis (F.) (Lepidoptera: Crambidae), has been the primary economic pest of sugarcane (Saccharum spp.) in Louisiana for more than 100 yr (Hensley 1971). Management of D. saccharalis is achieved through an integrated pest management (IPM) program relying on chemical control, cultivar resistance, and conservation biological control (Reagan 2001, Reagan and Mulcahy 2019). Nearly all of the state’s sugarcane acreage is monitored by crop consultants for D. saccharalis infestations which are managed with insecticide applications guided by economic thresholds (Hensley 1971, Reagan 2001, Wilson et al. 2017a). One of the initial advances was the discovery that the first spring generation does not impact yields thus does not require chemical control (Hensley et al. 1963, Hensley 1971). The population size of this first generation, however, is thought to be an indicator of overwintering survival and summer infestation levels. The spring generation is quantified by recording the density of sugarcane tillers with dead growing points (deadhearts) from D. saccharalis boring that occur prior to the formation of internodes (Bessin and Reagan 1993).
Diatraea saccharalis overwinter as late-stage larvae within the stems and stubble of cultivated grasses including sugarcane, corn (Zea mays), and rice (Oryza sativa) as well as wild grasses (Katiyar and Long 1961, Kirst 1973, Beuzelin et al. 2011a). As temperatures warm in the spring, overwintering larvae pupate, and moths emerge to begin the first of up to five generations produced annually (Hensley et al. 1963, Rodriguez-del-Bosque 1995). Overwinter survival is reported at <10%, and researchers have sought to reduce numbers of this generation to decrease intensity of subsequent generations (Holloway 1928, Hensley 1971, Kirst 1973). Freezing temperatures and wet conditions are thought to be the primary factors in reducing overwintering survival (Rodriguez-del-Bosque 1995). Previous studies showed the number of days below freezing was more important than minimum temperature in terms of influencing larval mortality (Kirst 1973). While it is presumed that high winter mortality reduces subsequent pest pressure during the summer, no studies have documented the influence of overwintering survival on populations of spring and summer D. saccharalis populations.
Objectives of this study were to 1) examine trends in D. saccharalis populations in Louisiana sugarcane production regions, 2) determine environmental factors that influence density of spring and summer populations, and 3) assess the ability of spring deadheart surveys to predict summer pest pressure. Results from deadheart surveys and estimations of percentage of acreage treated with insecticides from 2003 to 2020 are presented.
Materials and Methods
Intensity of spring populations of D. saccharalis were quantified from deadheart surveys conducted in sugarcane fields across the Louisiana sugarcane production region annually from 2003 to 2020 (2008 and 2015 are absent). The survey dates and numbers of fields surveyed each year varied (Table 1). Regions surveyed primarily focused on the Bayou (Lafourche and Terrebonne Parishes) and River (Ascension, Assumption, Iberville, St. Charles, St. James, and St. John the Baptist Parishes) regions with fields in the Teche region (Iberia, St. Martin, and St. Mary Parishes) included in some years. Fields were selected randomly to reflect the composition of cultivars and crop age (ratoon year) prevalent throughout the state. At each survey field, the number of tillers and deadhearts were recorded from a randomly selected 22-m row length (0.004 ha) at least 50 m from field margins. Deadhearts were identified by the characteristic D. saccharalis injury (larval entry hole and frass) and dissected to collect the larvae. In 2003 and 2013, only deadheart data (not total tillers) were collected from 16 and 4 fields, respectively.
Summary of deadheart surveys conducted in commercial sugarcane fields in Louisiana, 2003–2020.
| Survey Year | Dates | No. fields sampled | Regions |
|---|---|---|---|
| 2003 | 20 May | 66 | Bayou, River, Teche |
| 2004 | 11–12 May | 100 | Bayou, River, Teche |
| 2005 | 7 June | 50 | Bayou, River |
| 2006 | 25 May | 50 | Bayou, River |
| 2007 | 24 May | 50 | Bayou, River |
| 2009 | 20 May | 50 | Bayou, River |
| 2010 | 7 May | 50 | Bayou, River |
| 2011 | 3 May | 50 | Bayou, River |
| 2012 | 8 May | 45 | Bayou, River |
| 2013 | 6 June | 46 | Bayou, River |
| 2014 | 8 May | 25 | Bayou, River |
| 2016 | 11 May | 63 | Bayou, River |
| 2017 | 8 May | 32 | Bayou, River |
| 2018 | 8 May | 35 | Bayou, River |
| 2019 | 7 May | 57 | Bayou, River, Teche |
| 2020 | 12–13 May | 44 | Bayou, River, Teche |
| Survey Year | Dates | No. fields sampled | Regions |
|---|---|---|---|
| 2003 | 20 May | 66 | Bayou, River, Teche |
| 2004 | 11–12 May | 100 | Bayou, River, Teche |
| 2005 | 7 June | 50 | Bayou, River |
| 2006 | 25 May | 50 | Bayou, River |
| 2007 | 24 May | 50 | Bayou, River |
| 2009 | 20 May | 50 | Bayou, River |
| 2010 | 7 May | 50 | Bayou, River |
| 2011 | 3 May | 50 | Bayou, River |
| 2012 | 8 May | 45 | Bayou, River |
| 2013 | 6 June | 46 | Bayou, River |
| 2014 | 8 May | 25 | Bayou, River |
| 2016 | 11 May | 63 | Bayou, River |
| 2017 | 8 May | 32 | Bayou, River |
| 2018 | 8 May | 35 | Bayou, River |
| 2019 | 7 May | 57 | Bayou, River, Teche |
| 2020 | 12–13 May | 44 | Bayou, River, Teche |
Summary of deadheart surveys conducted in commercial sugarcane fields in Louisiana, 2003–2020.
| Survey Year | Dates | No. fields sampled | Regions |
|---|---|---|---|
| 2003 | 20 May | 66 | Bayou, River, Teche |
| 2004 | 11–12 May | 100 | Bayou, River, Teche |
| 2005 | 7 June | 50 | Bayou, River |
| 2006 | 25 May | 50 | Bayou, River |
| 2007 | 24 May | 50 | Bayou, River |
| 2009 | 20 May | 50 | Bayou, River |
| 2010 | 7 May | 50 | Bayou, River |
| 2011 | 3 May | 50 | Bayou, River |
| 2012 | 8 May | 45 | Bayou, River |
| 2013 | 6 June | 46 | Bayou, River |
| 2014 | 8 May | 25 | Bayou, River |
| 2016 | 11 May | 63 | Bayou, River |
| 2017 | 8 May | 32 | Bayou, River |
| 2018 | 8 May | 35 | Bayou, River |
| 2019 | 7 May | 57 | Bayou, River, Teche |
| 2020 | 12–13 May | 44 | Bayou, River, Teche |
| Survey Year | Dates | No. fields sampled | Regions |
|---|---|---|---|
| 2003 | 20 May | 66 | Bayou, River, Teche |
| 2004 | 11–12 May | 100 | Bayou, River, Teche |
| 2005 | 7 June | 50 | Bayou, River |
| 2006 | 25 May | 50 | Bayou, River |
| 2007 | 24 May | 50 | Bayou, River |
| 2009 | 20 May | 50 | Bayou, River |
| 2010 | 7 May | 50 | Bayou, River |
| 2011 | 3 May | 50 | Bayou, River |
| 2012 | 8 May | 45 | Bayou, River |
| 2013 | 6 June | 46 | Bayou, River |
| 2014 | 8 May | 25 | Bayou, River |
| 2016 | 11 May | 63 | Bayou, River |
| 2017 | 8 May | 32 | Bayou, River |
| 2018 | 8 May | 35 | Bayou, River |
| 2019 | 7 May | 57 | Bayou, River, Teche |
| 2020 | 12–13 May | 44 | Bayou, River, Teche |
Percentage of treated acreage was estimated for 2003–2019 using annual sales volume records of key insecticides used in Louisiana sugarcane provided by industry cooperators. Volumes were converted to treated acreage based on labeled rates for D. saccharalis in sugarcane. Annual percentages of planted acres sprayed were then calculated using yearly total production acreage (American Sugarcane League 2020).
Statistical Analysis
Weather data were obtained from the National Weather Service (USNOAA 2020). Minimum winter temperature and total number of days below freezing included monthly data from January, February, and March of the survey year and November and December of the preceding year. Spring variables examined included monthly and in aggregate average temperature and total precipitation data for January, February, March, and April of the survey year.
Deadheart density, stand counts, and percentage deadheart data were analyzed with generalized linear-mixed models (Proc Glimmix, SAS Institute 2013) with year as a fixed effect and site(year) as a random effect. Deadheart density models utilized a Poisson distribution and log-link function. Means and standard errors were calculated with Proc Means (SAS institute 2013). Stand counts and percentage deadheart models utilized Gaussian distributions with identity link functions.
Long-term population trends were examined by determining relationships between deadheart density and insecticide usage over time. To examine long-term trends over the 17-yr study period, a 3-yr moving average was calculated for each survey year that was the mean of the survey year, the previous year, and the following year (2003–2020) (Brownlee 2016). A similar 3-yr rolling average was calculated for percentage of treated acreage (2003–2019). Linear regressions examined the relationships between the rolling averages for deadheart density and percentage treated acreage and time (survey year).
Relationships between weather variables and mean deadheart densities from each year (2003–2020) were analyzed with linear regression (Proc Reg, SAS Institute 2013). The same models were also run including the 3-yr rolling average as a covariate to account for trends over time. Similarly, regression analyses (Proc Reg, SAS Institute 2013) examined the influence of weather variables on percentage treated acreage from 2002 to 2019. Variables included monthly total precipitation and average temperature for April–August as well as total precipitation and average temperature for the spring (April–June) and summer (July and August). All models were analyzed with and without the rolling average covariate.
The feasibility of conducting deadheart surveys to predict summer infestation levels was assessed by linear regression that examined the relationship between deadheart density and percentage of treated acreage. The analyses were done with and without the previously described rolling average as a covariate.
Results
Mean number of deadhearts per hectare varied among years (F = 10.89; df = 15, 801; P < 0.001). Deadheart density was greater in 2003 and 2007 than in other years, and was >2-fold greater than the next highest density (598/ha in 2012). Density was lowest in 2018 when no deadhearts were observed in survey fields (Fig. 1A). Similarly, year influenced stand density (F = 26.88; df = 15, 781; P < 0.001) and percentage deadhearts (F = 5.47; df = 15, 781; P < 0.001). Number of tillers per ha ranged from 69,234 in 2019 to 175,221 in 2005 (Fig. 1B). Percentage deadhearts followed much the same pattern as deadheart density (Fig. 1C).
Spring D. saccharalis deadheart density (A), tiller density (B), and percentage deadhearts (C) in Louisiana sugarcane 2003–2020.
A negative relationship was detected (F = 24.30; df = 1,14; P < 0.001) between the 3-yr rolling average deadheart density and time (survey year). This relationship suggests deadheart densities have declined consistently over the 17-yr survey period (Fig. 2A). A similar relationship was detected in percentage treated acreage over time (F = 25.24; df = 1,13; P < 0.001) demonstrating thattreated acreage has declined by nearly 2% annually since 2003 (Fig. 2B).
Long-term trends in D. saccharalis populations in Louisiana sugarcane as measured by deadheart density (A) and percentage of acreage treated with insecticides (B).
No relationships were detected (P > 0.05) between deadheart density and any weather variable except total precipitation in the month of April. April rainfall was positively correlated with deadheart density (F = 7.18; df = 1,14; P = 0.018) (Fig. 3A). Of the weather variables examined for percentage treated acreage, only total precipitation for the month of July was significant (F = 4.90; df = 1,15; P = 0.043) with a direct linear relationship (Fig. 3B). Inclusion of the rolling average as a covariate did not improve fit for any weather parameters and either deadheart density or percentage treated acreage.
Relationships between D. saccharalis populations and weather variables. April precipitation and deadheart density (A), July precipitation and percentage of acreage treated with insecticides (B).
A direct linear relationship (F = 13.84; df = 1,13; P = 0.003) was detected between deadheart density and percentage treated acreage (Fig. 4).
Relationship between spring deadheart density and percentage of acreage treated with insecticides.
Discussion
This research is a unique example of long-term (>10 yr) studies examining insect pest populations, which are rare in the literature. The decline in spring populations of D. saccharalis over the 17-yr period demonstrated by this study is most likely the result of changes in sugarcane production practices and successful IPM rather than environmental factors. Other long-term studies have noted declining populations of agricultural pest species over time can result from improved pest management. Carriére et al. (2002) demonstrated declining pink bollworm, Pectinophora gossypiella (Saunders) (Lepidoptera: Gelechiidae), populations over a 10-yr period were associated with increased adoption of Bt cotton. Yamamura et al. (2006) attributed declining populations of the rice stem borer, Chilo suppressalis (Walker), over a 50-yr period in Japan to greater field sanitation from increasing use of mechanical combine harvesters. Similarly, the adoption of the combine harvester in the Louisiana sugarcane industry in the 1990s (Salassi and Champagne 1996) may have initiated the decline D. saccharalis populations because it leaves fewer stalks in the field than the whole stalk harvester (Gravois 2001). Thus, the combine harvester may more effectively remove overwintering D. saccharalis larvae from sugarcane fields, thereby reducing areawide populations over the following two decades. Areawide populations may have been further reduced by the adoption of effective insect growth regulators in the late 1990s that provide superior control compared to pyrethroids that were widely used at the time. Efficacy was again improved in more recent years with the increasing use of diamides, particularly, chlorantraniliprole (Wilson et al. 2017a). The adoption of these insecticides over pyrethroids not only improved efficacy, but also enhanced biological control through preservation of natural enemy populations (Butler et al. 1997; Reagan and Posey 2001; Beuzelin et al. 2010; Wilson et al. 2012, 2017a). Lastly, D. saccharalis resistant cultivars likely further aided in long-term pest control. The highly susceptible cultivar LCP 85–384 was the leading variety in 1997–2007 before being replaced with HoCP 96–540 (moderately susceptible). The most widely planted cultivar in Louisiana since 2015, L 01-299, has a high level of resistance to stem borers including D. saccharalis (Gravois et al. 2011; Wilson et al. 2015a, 2021). This variety was released in 2009 and is now planted on more than 60% of the state’s acreage (Gravois et al. 2019). The potential for resistant cultivars to reduce areawide populations is well-documented (Bessin et al. 1990, 1991). Furthermore, models developed by Bessin et al. (1991) indicate the influence of susceptible cultivars on D. saccharalis populations is reduced if these are dispersed among resistant cultivars, a common occurrence in the Louisiana sugarcane industry. Collectively, the consistent reduction of D. saccharalis demonstrated by this study verifies the success of IPM in Louisiana sugarcane.
The lack of influence of many weather variables on D. saccharalis populations reported herein suggests biotic factors may be more important than weather conditions in impacting densities of this pest. The similar levels of deadheart density and percentage deadhearts observed across years despite variation in tiller density suggests host availability does not limit D. saccharalis populations. However, other biotic factors such as predation and host plant physiology were not assessed.
Prior studies have focused on minimizing overwintering survival as a critical component of D. saccharalis IPM (Kirst 1973, Fuchs and Harding 1978, Beuzelin et al. 2011b). Our findings indicate that environmental factors influencing winter mortality including minimum winter temperature and number of days below freezing did not affect spring deadheart densities. These findings are consistent with results from Beuzelin et al. (2011b) who found no differences in spring deadheart densities despite having 13-fold greater D, saccharalis infestations in the fall. Similarly, Bessin and Reagan (1993) found that fall infestation levels did not influence spring deadheart densities. Thus, disrupting overwintering on a local or regional scale may not reliably reduce D. saccharalis populations the following spring.
While removal of host plant material from soil surface has often been suggested as a management strategy, larvae overwintering in stubble below the soil surface are thought to have greater survival into the spring (Hensley 1971, Kirst 1973). These larvae would be less susceptible to cold temperatures. The lack of influence of winter temperature in our study could result from temperatures in most years not being sufficiently cold to kill subsoil larvae. The only year in which no deadhearts were recorded, 2018, followed a winter with a minimum temperature of −10°C, the coldest temperature recorded in the Louisiana sugarcane production region since 1989 (NOAA 2020). The average minimum temperature across the survey period, excluding 2018, was −4.2°C. This suggests a freeze in which temperatures drop well below average may be required to increase overwintering mortality to levels that reduce the size of the first spring generation.
The positive relationships detected between April and July precipitation and spring deadhearts and percentage of treated acreage, respectively, reported herein may be related to predation by the red imported fire ant, Solenopsis invicta (Hymenoptera: Formicidae). S. invicta is known to be a key predator of D. saccharalis larvae in sugarcane throughout the growing season (Showler and Reagan 1991, Bessin et al. 1991, Bessin and Reagan 1993, Beuzelin et al. 2009). Further, predation from S. invicta on spring populations of D. saccharalis has been shown to reduce deadheart densities by more than 70% (Bessin and Reagan 1993). Foraging by S. invicta is reduced by 40% during and after rainfall (Porter and Tschinkel 1987). Thus, frequent rainfall during April and July could be suppressing foraging by S. invicta allowing for greater survival of young D. saccharalis larvae. Alternatively, improved host quality, such as increased availability of soft vegetative tissue, may be a factor as conditions conducive for rapid sugarcane growth have been reported to increase D. saccharalis infestation (Hensley 1971).
Previous studies demonstrated spring infestations were poor indicators of summer populations in individual fields (Hensley et al. 1963); however, our data suggest that this does not hold true when deadheart densities are assessed across large regions. The correlation between spring deadheart densities and percentage of acreage treated reported in this study indicates deadheart surveys can be useful in predicting pest pressure later in the growing season. This relationship should be viewed with caution, as both deadheart density and treated acreage have declined over time. The utility of predicting future pest pressure in improving pest management may be limited. The relationship is not reliable enough to allow pest management implementers including crop consultants to rely on deadheart density to determine summer scouting frequency. However, it may assist farmers by informing them to budget for increased insecticide inputs in years when spring populations are above average.
While the current state of D. saccharalis management in Louisiana sugarcane has improved over the past two decades, establishment of the invasive Mexican rice borer, Eoreuma loftini (Dyar) (Lepidoptera: Crambidae), in the region threatens to alter the stem borer IPM program. The species is the primary pest of sugarcane in Texas (Wilson et al. 2012, Showler and Reagan 2017) and has now expanded its range into western Louisiana sugarcane production regions (Wilson et al. 2015b, 2017b). Fortunately, ongoing research suggests IPM tactics similar to those used for D. saccharalis can be applied successfully to E. loftini (Wilson et al. 2015a, 2017a). Future studies should examine efficacy of IPM strategies in controlling a complex of both stem-boring species.
Acknowledgments
The authors express gratitude to Katie Richard, Michael Duet, and Lawrence Lovell, Jr. ARS, USDA, Sugarcane Research Unit, Houma, LA for assisting in data collection. We thank chemical industry cooperators for providing insecticide sales records. This work was supported in part by USDA Hatch funds and the American Sugarcane League. This manuscript was approved for publication by the LSU AgCenter (manuscript #2020-270-34793).
