Comparative susceptibility of European corn borer, southwestern corn borer, and sugarcane borer (Lepidoptera: Crambidae) to Cry1Ab protein in a commercial Bacillus thuringiensis corn hybrid

One field strain each of the European corn borer, Ostrinia nubilalis (Hübner); southwestern corn borer, Diatraea grandiosella Dyar; and sugarcane borer, Diatraea saccharalis (F.); were collected from cornfields in northeastern Louisiana. Susceptibilities of the field strain and a corresponding laboratory strain of the three borer species to Cry1Ab protein in DK69-70 Bacillus thuringiensis (Bt) corn hybrid were determined by exposing neonates to intact leaf tissues from whorl stage plants or by feeding neonates or third instars on a meridic diet treated with different concentrations of Cry1lAb protein extracted from Bt corn leaves. Mortality and growth of larvae were evaluated after 2 and 4 d posttreatment in the bioassays by using intact leaf tissues or after 7 d in the bioassays by using diet incorporating Cry1Ab protein. D. saccharalis was the least susceptible species to Cry1Ab protein among the three species, followed by D. grandiosella, whereas O. nubilalis was most susceptible. The 2-d mortality of D. saccharalis neonates on intact Bt leaf tissues was lower than that of O. nubilalis and D. grandiosella. All neonates of O. nubilalis were killed on the diet treated with Cry1Ab protein at 0.5 and 1 mg/kg. The mortality of D. grandiosella was > 75% at 1 mg/kg, but it was < 6% for D. saccharalis at 1 mg/kg. The LC50 values of D. saccharalis were 3- and 11-fold higher than those of D. grandiosella and O. nubilalis, respectively. The LC90 values of D. saccharalis were 8- and 32-fold higher than those of D. grandiosella and O. nubilalis, respectively. Larval growth of the three species on Cry1Ab-treated diet was inhibited, but the inhibition was greater for O. nubilalis and D. grandiosella than for D. saccharalis. The lower susceptibility of D. saccharalis to Cry1Ab protein suggests that it is necessary to verify if a high-dose Bt corn for O. nubilalis and D. grandiosella is also a high dose for D. saccharalis.     

Dispersal behavior of neonate European corn borer (Lepidoptera: Crambidae) on Bt corn

European corn borer, Ostrinia nubilalis (Hübner) (Lepidoptera: Crambidae), has historically been a significant economically important insect pest of corn (Zea mays L.) in the United States and Canada. The development in the 1990s of genetically modified corn expressing genes derived from Bacillus thuringiensis (Bt) that encodes insecticidal crystalline (Cry) proteins has proven to be effective in controlling this insect as well as other corn pests. The purpose of this study was to assess the movement and dispersal behavior of neonate European corn borer on Bt corn. We examined differences in neonate European corn borer dispersal behavior for the first 4 h after eclosion in the field among a stacked pyramid (Cry1F X Cry1Ab X Cry34/35Ab1) Bt corn, a Cry1F Bt corn, and a non-Bt sweet corn; and in the laboratory among a Bt corn hybrid containing Cry1F, a hybrid containing Cry1Ab, a pyramid combining these two hybrids (Cry1F X Cry1Ab), and a non-Bt near isoline corn. In field experiments, we found that dispersal was significantly higher on Bt corn compared with sweet corn. In laboratory experiments, dispersal was significantly higher on Cry1Ab Bt corn and Cry1F X Cry1Ab Bt corn than on non-Bt near isoline corn. Results indicated that neonate dispersal may be significantly greater in Bt cornfields compared with non-Bt cornfields. The findings on dispersal behavior in this study will be useful in evaluating the efficacy of a blended seed refuge system for managing European corn borer resistance in Bt corn.     

Bt抗、感种群亚洲玉米螟幼虫取食Bt玉米后Cry1Ab杀虫蛋白在其体内的组织分布与含量

采用ELISA方法检测了实验室汰选的对Cry1Ab产生107倍抗性的亚洲玉米螟Ostrinia furnacalis (Guenée)种群与敏感种群3龄幼虫取食表达Cry1Ab杀虫蛋白的Bt玉米心叶后,杀虫蛋白在幼虫体内的分布情况。结果表明：Cry1Ab杀虫蛋白在抗性种群幼虫中的组织分布情况与敏感种群相近,主要存在于中肠组织和血淋巴中。抗、感种群中均以含有内含物的中肠组织中含量最高,分别为277.2 ng/g 和104.9 ng/g;其次为血淋巴,分别为93.7 ng/g 和69.5 ng/g;不含内含物的中肠组织中52.7 ng/g 和40.1 ng/g;在丝腺和马氏管组织的含量很低,丝腺中分别为8.5 ng/g和11.7ng/g,而马氏管中分别为6.7 ng/g和6.5 ng/g。脂肪体、生殖器官中未检测到杀虫蛋白。抗性种群中肠组织(含有内含物和不含内含物)中Cry1Ab的含量显著高于敏感种群。幼虫期取食过Bt玉米的亚洲玉米螟发育的蛹、成虫及其卵中均不含杀虫蛋白,说明Bt杀虫蛋白不会通过幼虫取食向蛹、成虫及卵传递。     

Monitoring the Cry1Ab susceptibility of European corn borer in Germany

The European corn borer, Ostrinia nubilalis (Hübner), is one of the most important insect pests in corn, Zea mays L. Transgenic corn cultivars expressing Bacillus thuringiensis (Bt) toxin provide a promising crop protection strategy against European corn borer; however, management is needed to avoid resistance development of the target pest species. The aim of this work was to establish the baseline susceptibility of different European corn borer populations in Germany to be able to forecast a possible development of resistance at an early stage. To standardize test procedures for future resistance management, the efficiency of Cry1Ab toxins from different suppliers and different production was assessed. Furthermore, two different test methods, surface treatment and the incorporation method, were compared with regard to their practicability and efficiency. Neither method provided significant differences in the baseline susceptibility of populations from different German regions. Overall, the data suggested little differentiation among German populations in terms of their susceptibility to Bt toxin and their genetic background. Future monitoring could therefore use a single European corn borer population as a representative for southwestern Germany. However, toxins from different suppliers and different production batches produced a vast range of LC50 values. Changes because of different toxin batches may be mistaken as a change in baseline susceptibility or even as the start of a resistance development. Thus, it is important throughout insect resistance management that the same toxin batches will be available for baseline susceptibility bioassays and for future tests.     

Development of European corn borer larvae on Event 176 Bt corn: influence on survival and fitness

European corn borer larvae, Ostrinia nubilalis (Hübner) (Lepidoptera: Crambidae) that have completed development on Event 176 Bt corn hybrids have survived exposure to sublethal doses of the Cry1Ab Bt toxin or are exploiting plant tissues that do not express the toxin. To evaluate the impact of such exposure, diapausing larvae were collected from Event 176 and conventional hybrids and compared for rates of pupation, parasitism, fitness (pupal weight, longevity, and fecundity) and susceptibility to the Cry1Ab toxin. Larvae completing development on Event 176 corn exhibited approximately 10% higher survival rates and correspondingly lower parasitism rates than larvae completing development on conventional hybrids. No significant differences were detected in pupal weight, fecundity, longevity or susceptibility to the Cry1Ab Bt toxin. These results indicate that survival on Event 176 corn are not adversely affect fitness and does not cause increased tolerance to the Cry1Ab toxin in subsequent generations.     

Growth, development, and survival of Nosema pyrausta-infected European corn borers (Lepidoptera: Crambidae) reared on meridic diet and Cry1Ab

Transgenic corn, Zea mays L., hybrids expressing crystal protein endotoxin genes from Bacillus thuringiensis Berliner are an increasingly popular tactic for managing the European corn borer, Ostrinia nubilalis (Hübner), in North America. O. nubilalis populations also are often vulnerable to the ubiquitous entomopathogenic microsporidium Nosema pyrausta (Paillot). We examined the effect of feeding meridic diet incorporated with purified Cry1Ab on growth, development, and survival of Nosema-infected and uninfected neonate O. nubilalis. Infected larvae developed more slowly than uninfected larvae. Increasing the concentration of Cry1Ab in diet reduced larval development, and this effect was amplified by microsporidiosis. Infected larvae weighed significantly less than uninfected larvae. The relationship among Nosema infection, Cry1Ab concentration, and larval weight was fitted to an exponential function. The LC50 of infected larvae was one-third that of uninfected larvae, indicating that infected larvae are more vulnerable to toxin. This work has implications for resistance management of O. nubilalis and demonstrates that it is important to determine whether N. pyrausta is present when testing susceptibility of larvae to transgenic corn hybrids.     

Development of diagnostic concentrations for monitoring Bacillus thuringiensis resistance in European corn borer (Lepidoptera: Crambidae)

Two candidate diagnostic concentrations of the Cry1Ab and Cry1Ac toxins from Bacillus thuringiensis corresponding to the LC99 and EC99 (effective concentration that causes 99% growth inhibition) for European corn borer, Ostrinia nubilalis (Hübner), were determined based on previously obtained baseline data. Validation experiments using field-collected European corn borer populations from across North America showed that for Cry1Ab, a concentration corresponding to the upper limit of the 95% confidence interval of the LC99, produced mortality > 99% for all populations tested. However, for Cry1Ac, adjustments and further validation are probably necessary. Development of B. thuringiensis resistance monitoring programs that rely on diagnostic techniques are discussed.     

Evaluation of transgenic Bt corn for resistance to the Asian corn borer (Lepidoptera: Pyralidae)

The Asian corn borer, Ostrinia furnacalis (Guenée), is the most important insect pest on corn in China. Bt transgenic corn provides a new tool for Asian corn borer control. Monsanto's YieldGard Bt transgenic corn expressing Cry1Ab protein, and a non-Bt control, were evaluated in Beijing. Laboratory bioassays were carried out by exposing neonates to an agar-free diet containing Bt corn whorl leaves, tassels, and anthers, or by exposing neonates directly to fresh silk and pollen. These are the tissues initially attacked by neonates in the field. All of these tissues, with the exception of pollen, contained sufficient insecticidal protein to kill > or = 95% of larvae within 7 d. Surviving larvae had also not grown beyond first instar and weighed < or = 0.1 mg. Although larvae feeding on Bt corn pollen were significantly smaller than those on non-Bt corn pollen, there was no significant difference in mortality. Field trials were also conducted with artificial infestations of Asian corn borer at mid whorl, late whorl, and silking stages. Damage ratings and number of larvae surviving per plant indicated that Bt corn was highly resistant to Asian corn borer. Therefore, YieldGard offers the potential for season-long protection against first- and second-generation Asian corn borer.     

Determination of baseline susceptibility to Cry1Ab protein for Asian corn borer (Lep., Crambidae)

Abstract: Although transgenic Bacillus thuringiensis (Bt) corn can provide a new tool for control of the Asian corn borer (ACB), Ostrinia furnacalis (Guenée), concern has been raised regarding the possibility of the target insect evolving resistance to the Bt protein under intensive selection pressure from Bt corn. Therefore, it is necessary to establish baseline data to enable detection of changes in susceptibility in field populations after prolonged exposure to Bt corn. Susceptibility to purified Cry1Ab protein from Bt was determined for 10 populations of ACB from the major corn‐growing regions of China, ranging geographically from Heilongjiang Province in the northeast to Shaanxi Province in the east‐central part. Neonate ACB were exposed to semi‐artificial diet incorporated with increasing Cry1Ab protein concentrations, and mortality and growth inhibition were evaluated after 7 days. The range of LC₅₀ (50% lethal concentration) among the populations was 0.10 to 0.81 μg/g (Cry1Ab protein/diet). Differences (P < 0.05) in susceptibility among the populations were significant. LC₅₀s generated from the Huanghuaihai Summer Corn Region were higher than those from the Spring Corn Regions. Bt was one of the significant natural biomortality factors of overwintering generation ACB. There was a significant correlation between percentage of the larvae infected with Bt and their LC₅₀ values to Cry1Ab protein in geographic distinct populations (r = 0.7350*, d.f. = 8, _r0.05 = 0.632). Based on the background of Bt formulations used for corn insect pests control in these areas, these differences were not caused by prior exposure to Bt insecticides. Instead, the small differences likely reflect natural Bt selection pressure. Because the variation in susceptibility to Cry1Ab was small (<10‐fold), the ACB apparently is susceptible to Cry1Ab across its range within China.     

Southwestern corn borer (Lepidoptera: Crambidae) damage and aflatoxin accumulation in maize

Aflatoxin, a potent carcinogen, is produced by the fungus Aspergillus flavus Link: Fr. Drought, high temperatures, and insect damage contribute to increased levels of aflatoxin contamination in corn, Zea mays L. Plant resistance is widely considered a desirable method of reducing aflatoxin contamination. Germplasm lines with aflatoxin resistance have been developed. This investigation was undertaken to determine whether crosses among these lines exhibited resistance to southwestern corn borer, Diatraea grandiosella Dyar, and to assess the effects of southwestern corn borer feeding on aflatoxin accumulation. Differences in ear damage among southwestern corn borer infested hybrids were significant. Estimates of general combining ability effects indicated that the lines Mp80:04, Mp420, and Mp488 contributed to reduced ear damage, and SC213 and T165 contributed to greater damage when used in hybrids. Mean aflatoxin levels were 254 ng/g for hybrids infested with southwestern corn borer larvae and 164 ng/g for noninfested hybrids in 2000 when environmental conditions were conducive to aflatoxin production. In contrast, the overall mean aflatoxin level for southwestern corn borer infested hybrids was only 5 ng/g in 1999 when environmental conditions did not favor aflatoxin accumulation. Crosses that included lines selected for aflatoxin resistance as parents (Mp80:04 and Mp313E) exhibited lower levels of aflatoxin contamination both with and without southwestern corn borer infestation in 2000. Only the experimental line Mp80:04 contributed significantly to both reduced southwestern corn borer damage and reduced aflatoxin contamination.     

Interactions between Nosema pyrausta (Microsporidia: Nosematidae) and Bacillus thuringiensis subsp. kurstaki in the European corn borer (Lepidoptera: Pyralidae).

Larval susceptibility to Bacillus thuringiensis was determined for Nosema pyrausta-infected and uninfected European corn borers, Ostrinia nubilalis (Hübner), in bioassays using a commercial formulation of B. thuringiensis subsp. kurstaki, Dipel ES, incorporated into diet. LC50 values for N. pyrausta-infected larvae were significantly lower (P<0.0001) than for uninfected larvae and declined with increasing levels of infection. LC50 values for a 15-d bioassay using field-colony first instars were 0.006 and 0.027 mg of Dipel ES/kg of diet for larvae moderately infected by N. pyrausta and uninfected larvae, respectively. Nosema pyrausta-infected larvae reared on Dipel ES-amended diets produced 70-fold fewer spores (P<0.0001) than larvae reared on standard diet. For example, 15 d after placement as first instars on standard diet, infected field-colony larvae produced 7.6-8.7 million N. pyrausta spores per larva; similar larvae placed on diet containing 0.09 mg of Dipel ES/kg of diet produced 85-103 thousand spores per larva. Infected larvae also weighed less and failed to mature on Dipel ES-amended diets. Increased susceptibility of N. pyrausta-infected larvae to Dipel ES and reduced N. pyrausta spore production in larvae feeding on diet containing Dipel ES suggest that Bt corn will have a direct adverse effect on the survival and continual impact of N, pyrausta as a regulating factor on European corn borer populations.     

Economic analysis of planting dates to manage European corn borer (Lepidoptera: Crambidae) with Bt corn

Planting dates of transgenic Bacillus thuringiensis Berliner (Bt) corn were adjusted to determine the utility in managing European corn borer, Ostrinia nubilalis (Hübner). Transgenic Bt (events 176 and Bt11) corn and non-Bt corn were planted at three different times to use the early- and late- planted corn as a potential trap crop for ovipositing European corn borer moths. Grain moisture and yields were recorded to determine the economic benefits of Bt corn planted on the different dates, based on European corn borer populations and corn damage data collected before harvest. Data were recorded from three locations in southwestern, central, and northeastern Iowa for three summers (1996-1998). Economic benefits are discussed in relation to EILs and yield results. Adjusting the planting dates of Bt and non-Bt corn provided variable economic differences among planting dates in northern Iowa; however, greater economic benefits were realized when Bt corn was planted late during the planting sequence in central and southwestern Iowa. These results suggest that planting corn should be conducted in a timely manner and, if delayed or required to plant late, planting Bt corn would likely provide greater economic benefits. Although yield and economic variability were high, using Bt corn in combination with planting date adjustments may be a viable option for managing European corn borer.     

Identification, cloning and expression of a Cry1Ab cadherin receptor from European corn borer, Ostrinia nubilalis (Hubner) (Lepidoptera: Crambidae)

Transgenic corn expressing the Cry1Ab toxin from Bacillus thuringiensis is highly toxic to European corn borer, Ostrinia nubilalis, larvae. A putative Cry1Ab receptor (OnBt-R(1)) molecule was cloned and sequenced from a cDNA library prepared from midgut tissue of O. nubilalis larvae. The 5.6 Kb gene is homologous with a number of cadherin genes identified as Cry1 binding proteins in other lepidopterans. Brush border membrane vesicles were prepared using dissected midguts from late instars. A 220-kDa protein was identified as a cadherin-like molecule, which bound to Cry1Ab toxin and cross-reacted with an anti-cadherin serum developed from recombinant expression of a partial O. nubilalis cadherin peptide. Two additional proteins of smaller size cross-reacted with the anti-cadherin serum indicating that Cry1Ab binds to multiple receptors or to different forms of the same protein. Spodoptera frugiperda (SF9) cells transfected with the OnBt-R(1) gene were shown to express the receptor molecule which caused functional susceptibility to Cry1Ab at concentrations as low as 0.1 microg/ml. These results in combination suggest strongly that a cadherin-like protein acts as receptor and is involved with Cry1Ab toxicity in O. nubilalis.     

Impact of plant resistance on southwestern corn borer (Lepidoptera: Crambidae) biology and plant damage

Southwestern corn borer, Diatraea grandiosella Dyar (Lepidoptera: Crambidae), is a major insect pest of corn, Zea mays L., in the southern United States. Germplasm lines with resistance to southwestern corn borer have been developed and released by the USDA-ARS. Two single-cross hybrids produced by crossing germplasm lines with resistance to southwestern corn borer and a susceptible single-cross hybrid were infested with southwestern corn borer larvae in a 2-yr field test conducted in Mississippi. The susceptible hybrid sustained significantly more leaf damage and stalk tunneling than either resistant hybrid. The number of tunnels and the length of tunneling were significantly lower on the resistant hybrids. In 2003, up to 15 times more tunneling was observed on the susceptible hybrid. Larvae feeding on the resistant hybrids were delayed in their movement from the whorl to the stalk and larval survival was 50% lower on the resistant hybrids than on the susceptible hybrid. Larvae recovered from the susceptible hybrid 7-14 d after infestation weighed twice as much as those recovered from the resistant hybrids. Similar differences in larval weight were observed in the laboratory when larvae were reared on diets prepared from lyophilized tissue from the three hybrids. These results provide a foundation for other investigations designed to identify and determine the roles of specific genes and gene families associated with southwestern corn borer resistance in corn.     

Management of feeding damage and survival of southwestern corn borer and sugarcane borer (Lepidoptera: Crambidae) with Bacillus thuringiensis transgenic field corn

The efficacy of transgenic corn hybrids expressing an insecticidal Bacillus thuringiensis (Bt) delta-endotoxin from different transformation events was evaluated in field corn, Zea mays L., against the southwestern corn borer, Diatraea grandiosella Dyar, and sugarcane borer, Diatraea saccharalis (F.). Susceptibilities of neonates and third instars were determined on Bt and non-Bt corn plants (V6 and R1 stages) in field plots and corn leaf tissue feeding exposure in laboratory bioassays. Bt corn hybrids associated with MON810 and CBH351 transformation events sustained significantly less injury by southwestern corn borer and sugarcane borer during mid-whorl stage infestations compared with their respective non-Bt hybrid equivalents. Southwestern corn borer and sugarcane borer feeding injury to ear leaf-sheath and husk tissues during the silking stage of corn was significantly reduced in MON810 and CBH351 Bt corn compared with their respective non-Bt hybrids. However, resistance levels to feeding injury in Bt hybrids associated with the MON810 event were significantly higher than that in the hybrid associated with the CBH351 event. Southwestern corn borer and sugarcane borer caused more feeding injury to husk tissue than to ear leaf-sheath tissue in both Bt and non-Bt hybrids infested during the silking stage. Laboratory performance of the MON810 event against southwestern corn borer and sugarcane borer varied among hybrids associated with the same event. Third instars of southwestern corn borer were highly susceptible to MON810 Bt corn hybrids in leaf tissue experiments. However, sugarcane borer larvae were susceptible to the MON810 event only in one of the Bt hybrids evaluated. Sugarcane borer mortality was significantly lower after 96 h of feeding exposure on CBH351 Bt corn leaf tissue than on MON810 Bt corn leaf tissue. Plant resistance to southwestern corn borer and sugarcane borer increased as plants matured, independent of the presence of a Bt construct. These results are essential to estimate the importance of Bt transgenic corn in areas of southern United States and other areas where mixed populations of southwestern corn borer and sugarcane borer are predominant and cause severe damage to corn production.     

Geographic variation in susceptibility of Chilo suppressalis (Lepidoptera: Pyralidae) to Bacillus thuringiensis toxins in China

Geographic variation in the susceptibility of the striped stem borer, Chilo suppressalis (Walker) (Lepidoptera: Pyralidae), in China to Bacillus thuringiensis (Bt) insecticidal crystal proteins Cry1Ac and Cry1Ab was studied to establish baseline information for comparing the future response of populations with increased exposure to Bt products. Rice is the major host of C. suppressalis, and Bt rice ma) be released in China in the near future. Twelve populations of the pest were collected from the major rice-growing regions of China. LC50 estimates were determined for all populations for Cry1Ac and for eight populations for Cry1Ab. The bioassay results indicated that the range of LC50 in neonate larvae to Cry1Ac and Cry1Ab was from approximately 15 to approximately 157 mg (AI)/L and approximately 2 to approximately 34 mg (AI)/L, respectively. LC50 values were lower for Cry1Ab than for Cry1Ac, and there was a significant positive correlation between the two toxins tested.     

Screening for Bacillus thuringiensis crystal proteins active against the cabbage looper, Trichoplusia ni

Toxicity tests were performed to find among Cry1 and Cry2 Bacillus thuringiensis crystal proteins those with high activity against the cabbage looper. Tests were performed with neonate larvae on surface-contaminated artificial diet. The crystal proteins found to be toxic were, from higher to lower toxicity: Cry1Ac, Cry1Ab, Cry1C, Cry2Aa, Cry1J, and Cry1F (LC50 of 1.14.1, 3.4-4.4, 12, 34, 87, and 250 ng/cm2, respectively). Cry1B, Cry1D, and Cry1E can be considered nontoxic (LC50 higher than 2500 ng/cm2). Cry1Aa was moderately toxic to nontoxic, depending on the source (LC50 of 420 ng/cm2 from PGS and 8100 ng/cm2 from Ecogen). In vitro binding assays with trypsin-activated 125I-labeled Cry1Aa, Cry1Ab, and Cry1Ac crystal proteins and brush border membrane vesicles from midgut larvae showed a direct correlation between toxicity and binding affinity. Heterologous competition experiments indicated that Cry1Aa and Cry1F bind, though only at very high concentrations, to the Cry1Ab/Cry1Ac shared high-affinity binding site.     

Lipids and fatty acids of the southwestern corn borer,Diatraea grandiosella

The lipids of the adults and of several immature stages of the southwestern corn borer, Diatraea grandiosella, were studied after they were fed natural corn stalks or artificial diets. Linoleic acid (18:2) was the major fatty acid of the neutral lipids in both the natural and the artificial diets, but aleic acid (18:1) was the principal neutral lipid in all insect stages. Also, linoleic acid and oleic acid were the principal acids in the insect phospholipids of all stages. The content of linoleic acid in the natural diet was also high, but that in the artificial diet appeared to be much too low for insect requirements. Phosphatidyl choline (PC) and phosphatidyl ethanolamine (PE) were the major phospholipids in all growth stages. Thus, in larvae diapausing in the field, the unsaturated fatty acid content of PC was 59·3 per cent, primarily 16:1 and 18:1, and PE was 87·4 per cent, primarily 18:1, 18:2, and 18:3, and the fatty acids in the number 1- and 2-positions of PC were 53·6 and 97·2 per cent unsaturated, respectively. The haemolymph of diapausing southwestern corn borer larvae contained primarily glycerides but also had some PC and PE. Fat body from diapausing larvae contained primarily 16:0, 16:1, and 18:1 in a ratio of 1 : 1 : 2. Thus lipids of the southwestern corn borer do not reflect dietary lipids as closely as do other insects studied.     

Effects of transgenic bt corn on growth and development of the stalk borer Papaipema nebris (Lepidoptera: Noctuidae)

This study assessed the efficacy of two different genetic events, event Bt 11 (CrylAb) and event CBH351 (Cry 9C), in Bt corn against two instar classes of the stalk borer Papaipema nebris across three different plant stages (V1, V3, and V5) of corn, Zea mays. Class A includes instars 1 and 2, and class B includes instars 3 and 4. Stalk borer response and development over time were measured, and the data from 1999 and 2000 show that the Bt corn does have some effect on the feeding and development of P. nebris. Injury to the corn plant was reduced, although not eliminated. Stalk borer larvae caused significantly (P = 0.0001) more injury to the non-Bt plants than to the Bt plants over time. Growth and development of the larvae were slowed and mortality was higher for Bt corn than for non-Bt corn. These data suggest that planting Bt corn may benefit growers by reducing, but not eliminating, stalk borer infestations and subsequent plant injury.     

Pathogens associated with southwestern corn borers and southern corn stalk borers (Lepidoptera: Crambidae)

A study was undertaken to isolate entomopathogens of southwestern corn borer, Diatraea grandiosella Dyer, and southern corn stalk borer, Diatraea crambidoides (Grote). Field-collected diapausing larvae of southwestern corn borer (three sites in Mississippi) and southern corn stalk borer (one site in North Carolina), and a laboratory strain of D. grandiosella in the diapause state were maintained in a simulated winter followed by a simulated spring environnent. Few larvae (< or = 6%) collected from any of the field sites died in the winter environment, and most insect mortality (11-25%) occurred after transfer of the larvae to the simulated spring environment. Mortality during the simulated spring period differed among the collection sites, and the highest mortality was recorded for southwestern corn borers from Washington County (25%), followed by Marshall (16%) and Oktibbeha (11%) Counties. A high level of mortality was also observed in southern corn stalk borers during the simulated spring period (27%). No viruses were observed, but a number of bacteria, microsporidia, and fungi were isolated from both southwestern corn borer and southern corn stalk borer larvae and pupae. In most instances, numerous bacterial taxa were isolated from cadavers, but on some occasions a single taxon predominated. The most prevalent bacterial taxon from larval and pupal cadavers was Enterococcus faecalis (Andrewes & Horder) Schleifer & Kilppel-Balz, but Bacillus spp., Pseudomonas aeruginosa (Schroeter) Migula, and Serratia marcescens Bizio were frequently isolated as well. Few fungi (1-7%) were recovered from southwestern corn borer and southern corn stalk borer larvae and pupae. The most common entomopathogenic taxon isolated was Beauveria bassiana (Balsamo) Vuillemin from southern corn stalk borer larvae. Microsporidia were not isolated from southern corn stalk borers. However, Nosema spp. were isolated from southwestern corn borer cadavers from Washington (15%), Marshall (1%), and Oktibbeha (3%) Counties in Mississippi. In addition, we observed parasitism of southern corn stalk borer larvae by Macrocentrus cingulum Reinhard (Hymenoptera: Braconidae). No parasitism of southwestern corn borers was observed. Isolates of Bacillus, Beauveria, Entercoccus, Nosema, Pseudomonas and Serratia were all pathogenic to southwestern corn borer larvae under controlled environmental conditions, and with the exception of B. bassiana, these are novel pathogens of Diatraea corn borers.