Tolerance and compensatory response of rice to sugarcane borer (Lepidoptera: Crambidae) injury

A 3-yr field experiment was conducted to evaluate the tolerance and compensatory response of rice (Oryza sativa L.) to injury caused by sugarcane borer, Diatraea saccharalis (F.), as affected by cultivar (Cocodrie, Francis, and Jefferson), stage of crop growth during which the injury occurred (third tiller stage, panicle differentiation stage, and heading stage), and sugarcane borer density. The proportion of rice tillers with sugarcane borer injury (leaf and leaf sheath injury and/or stem injury) was lower when injury occurred at the third tiller stage (0.05) than at panicle differentiation (0.19) and heading (0.18). When injury occurred at the two latter stages, both the proportion of tillers with injury and the proportion of tillers with stem injury were negatively correlated with rainfall. Rainfall resulted in dislodgement and mortality of sugarcane borer eggs and larvae before the larvae entered the stems. Rice plant density in this study (111.1 plants/m2) was higher than recorded for previous research on rice compensation using potted rice or conducted in low-density hill production systems (26.7-51.3 plants/m2). Two mechanisms of within-plant tolerance/compensation were observed. Stem injured plants produced approximately 0.69 more tillers than uninjured plants, whereas tillers with leaf and leaf sheath injury produced larger panicles, up to 39.5 and 21.0% heavier than uninjured tillers, when injury occurred at third tiller stage and at panicle differentiation, respectively. Rice yield was not reduced with up to 23% injured tiller and up to 10% injured stems at the third tiller stage, 42% injured tillers and 17% injured stems at panicle differentiation, and 28% injured tillers and 14% injured stems at heading. Significant between-plant compensation was not detected, suggesting competition between adjacent plants is not significantly reduced by injury. Our results suggest that rice can tolerate and/or compensate for a level of stem borer injury previously considered to be economically damaging.     

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.     

A hybrid Bacillus thuringiensis delta-endotoxin gives resistance against a coleopteran and a lepidopteran pest in transgenic potato

Expression of Bacillus thuringiensis delta-endotoxins has proven to be a successful strategy for obtaining insect resistance in transgenic plants. Drawbacks of expression of a single resistance gene are the limited target spectrum and the potential for rapid adaptation of the pest. Hybrid toxins with a wider target spectrum in combination with existing toxins may be used as tool to mitigate these problems. In this study, Desiree potato plants were genetically modified to resist attack by insect species belonging to the orders Coleoptera and Lepidoptera, through the insertion of such a hybrid gene, SN19. Transgenic plants were shown to be resistant against Colorado potato beetle larvae and adults, potato tuber moth larvae, and European corn borer larvae. These are the first transgenic plants resistant to pests belonging to two different insect orders. In addition, the target receptor recognition of this hybrid protein is expected to be different from Cry proteins currently in use for these pests. This makes it a useful tool for resistance management strategies.     

Validation of a nested error component model to estimate damage caused by corn rootworm larvae

The corn rootworm complex (Coleoptera: Chrysomelidae) constitutes a significant threat to maize production in the United States, and more recently, in Europe. We conducted an analysis of readily available field trial data to validate an existing damage function for corn rootworm larvae. We used a nested error component model with unbalanced panel data to describe the relationship between yield loss and root injury caused by these insects. These data were collected by personnel with the Insect Management and Insecticide Evaluation Programme (Department of Crop Sciences, University of Illinois) and represent 19 location‐years. To our knowledge, this is the largest data set used to estimate a damage function for corn rootworm larvae. Unlike many experiments examining the relationship between root injury and yield loss caused by corn rootworm larvae, the data set used for our analysis includes many Bt maize hybrids. Our model suggests that for each node of roots injured by corn rootworm larvae, a yield loss of approximately 15% can be expected. Statistically significant variance components included an effect of location and experimental error. We speculate that variation in weather across experimental sites was the principal factor contributing to the significant effect of location. The substantial experimental error observed for our model highlights the limitations of utilizing a multi‐year, geographically diverse damage function for predicting yield loss because of root injury on a small scale. We discuss major factors contributing to the variance components estimated by our model and suggest techniques for improving future analyses of the damage function for corn rootworm larvae.     

European corn borer injury effects on lignin, carbon and nitrogen in corn tissues

Plant herbivores often stimulate lignin deposition in injured plant tissue, but it is not known whether corn (Zea mays L.) reacts to European corn borer (ECB, Ostrinia nubilalis Hubner) injury in this manner. Bt (Bacillus thuringiensis) genetic modification is also reported to affect lignin in corn. This study evaluated the effects of ECB injury and the Bt gene on the chemical composition and decomposition of corn tissues. Eight near isolines (Bt and NBt) were grown in pots and half were infested with ECB. The experiment was repeated in 2 years. ECB injury increased the lignin concentration in corn leaves in one of 2 years and lowered the C:N ratio in injured stems. Lignin concentration in leaves was greater in Bt than NBt corn in 1 year and Bt stems had greater N concentration than NBt stems in 1 year of the 2 year study. ECB injury affected the composition of lignin-derived phenols, however ECB infested and non-infested stems lost the same amount of mass after 5 months in buried field litterbags. In conclusion ECB injury and the Bt gene had subtle effects on the chemical composition of corn tissue, which did not alter the short-term decomposition of corn residues.     

Analysis of quantitative trait loci (QTLs) and quantitative trait alleles (QTAs) for potato tuber yield and starch content

 Using RFLP markers, QTLs for tuber starch-content and tuber yield were mapped in two F₁ populations derived from crossing non-inbred di-haploid potato breeding lines. QTLs were identified and mapped, based on both single-marker tests and interval analyses. A model specifically developed for interval QTL analysis in non-inbred plant species was successfully applied for the first time to experimental data. Results of both methods of QTL analysis were similar but not identical. QTLs for tuber starch-content and tuber yield were analysed in segregating populations K31 and LH in five and two environments, respectively. Population K31 was fully genotyped whereas population LH was selectively genotyped according to high and low tuber-starch content. Eighteen putative QTLs for tuber starch-content were identified on all 12 potato linkage groups and eight putative QTLs for tuber yield were identified on eight linkage groups. Twenty of twenty six putative QTLs were reproducibly detected in at least two environments and/or mapping populations. Few major QTLs for tuber starch-content were highly stable across environments but were detected in only one of the two mapping populations analysed. Most QTLs for tuber yield were linked with QTLs for tuber starch-content suggesting that the effects on both traits are controlled by the same genetic factors. The results are discussed with respect to marker-assisted selection in potato. Received: 9 March 1998 / Accepted: 29 April 1998     

Transgenic Bacillus thuringiensis corn hybrid performance against univoltine ecotype European corn borer (Lepidoptera: Crambidae) in South Dakota

The performances of Bt-corn hybrids against univoltine ecotype European corn borer larvae were evaluated in South Dakota from 1997 to 1999. The corn hybrids were exposed to natural seasonal fluctuations of a univoltine ecotype European corn borer population. Larval injury parameters, grain yields, and gross incomes were quantified during the 3-yr study. In general, the Bt-corn hybrids had significantly higher yields than the untreated non-Bt isolines in 1997 and 1998 when corn borer pressures were high. However, most of the Bt-corn hybrids did not produce significant yield advantages in 1999 when the European corn borer pressure was low. Some of the Bt-corn hybrids even produced significantly lower yields than their untreated non-Bt counterparts in 1999. The performances of non-Bt isolines that were treated with permethrin granules at whorl stage were similar to Bt-corn hybrids in 1998 when the corn borer pressure was high and similar to the untreated non-Bt isolines in 1999 when the corn borer pressure was low. Injury-free corn produced by Bt-corn hybrids did not necessarily translate into higher yields in some hybrid groups. Grain moisture at harvest, which can result in moisture penalty or dockage, was significantly higher in most Bt-corn hybrids. Gross incomes of the Bt-corn hybrids were generally higher than the untreated non-Bt isolines when the corn borer infestation was high, but were either similar to or lower than the untreated non-Bt isolines when the corn borer infestation was low.     

Genetic basis of resistance to fall armyworm (Lepidoptera: Noctuidae) and southwestern corn borer (Lepidoptera: Crambidae) leaf-feeding damage in maize

Leaf-feeding damage by first generation larvae of fall armyworm, Spodopter frugiperda (J. E. Smith) (Lepidoptera: Noctuidae), and southwestern corn borer, Diatraea grandiosella Dyar (Lepidoptera: Crambidae), cause major economic losses each year in maize, Zea mays L. A previous study identified quantitative trait loci (QTL) contributing to reduced leaf-feeding damage by these insects in the maize line Mp704. This study was initiated to identify QTL and their interactions associated with first generation leaf-feeding damage by fall armyworm and southwestern corn borer. QTL associated with fall armyworm and southwestern corn borer resistance in resistant line Mp708 were identified and compared with Mp704. Multiple trait analysis (MTA) of both data sets was then used to identify the most important genetic regions affecting resistance to fall armyworm and southwestern corn borer leaf-feeding damage. Genetic models containing four and seven QTL explained southwestern corn borer and fall armyworm resistance, respectively, in Mp708. Key genomic regions on chromosomes 1, 5, 7, and 9 were identified by MTA in Mp704 and Mp708 that confer resistance to both fall armyworm and southwestern corn borer. QTL regions on chromosomes 1, 5, 7, and 9 contained resistance to both insects and were present in both resistant lines. These regions correspond with previously identified QTL related to resistance to other lepidopteran insects, suggesting that broad-spectrum resistance to leaf feeding is primarily controlled by only a few genetic regions in this germplasm.     

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.     

Volatiles induced by the larvae of the Asian corn borer (Ostrinia furnacalis) in maize plants affect behavior of conspecific larvae and female adults

Abstract  Effects of maize ( Zea mays L.) volatiles induced by larvae of the Asian corn borer, Ostrinia furnacalis (Guenée), on the orientation behaviors of Asian corn borer larvae and oviposition of the females were investigated. Nineteen volatile chemicals, with terpenes being the major components, were identified from maize plants attacked by third instar Asian corn borer larvae. Coupled gas chromatographic-electroantennographic detection (GC-EAD) analyses revealed some electroantennographic differences between female and male Asian corn borer antennae in response to larvae-induced maize volatiles; female responded to ( E )-2-hexenal, nonanal, ( Z )-3-hexen-1-ol and three unknown compounds while the male only responded to ( E )-2-hexenal, nonanal and one unknown compound. In laboratory orientation bioassays, Asian corn borer neonate larvae were attracted to extracts collected from Asian corn borer-damaged plants as well as to synthetic farnesene, but were repelled by ( Z )-3-hexen-1-ol. In laboratory oviposition bioassays, gravid females laid fewer eggs on plants damaged by larvae than on mechanically damaged plants or undamaged plants. Adult Asian corn borer females deposited fewer eggs on wax paper treated with ( E )-2-hexenal or ( Z )-3-hexen-1-ol than on wax paper treated with hexane (control). The results suggest that Asian corn borer can affect the behaviors of conspecific larvae and adults by changing host plant volatiles.     

Plant resistance and its effect on the peritrophic membrane of southwestern corn borer (Lepidoptera: Crambidae) larvae

The southwestern corn borer, Diatraea grandiosella Dyar (Lepidoptera: Crambidae), is a serious pest of corn, Zea mays L., in the southern United States. Corn germplasm lines with conventional genetic leaf-feeding resistance to this pest, the fall armyworm, Spodoptera frugiperda (J.E. Smith), and other lepidopterans have been released to the public by USDA-ARS scientists located in Mississippi. Recent studies suggest the insect resistant lines disrupt the integrity of the peritrophic membrane of the fall armyworm. The objectives of this study were to investigate any morphological differences in the structure of the peritrophic membrane of southwestern corn borer larvae feeding on resistant and susceptible corn hybrids and to quantify the damage. Larvae were reared under field and laboratory conditions on three corn hybrids (two resistant and one susceptible). Scanning electron microscopy was used to examine the peritrophic membrane for abnormalities such as holes or tears and to count the holes or tears in the membrane. Differences in the degree of damage to peritrophic membrane of larvae fed on resistant and susceptible plants were not detected. Up to five distinct layers of the membrane were observed in each larva. Variation in the amounts of damage to the peritrophic membrane observed from larvae feeding on all plant material was high. Plant resistance adversely affects growth and development of southwestern corn borer larvae, and further investigations are needed to explain the role of plant resistance and its relation to peritrophic membrane in southwestern corn borer larvae.     

Characterization of blackleg and soft rot from potato in northern Thailand

Blackleg and soft rot of potato cause economic loss through reduced yield and quality. The causal agents of bacterial blackleg and soft rot of potato were identified based on biological data and sequence analyses of the 16S rDNA gene. Between 2016 and 2018, diseased potato stems and tubers were collected in Chai Prakan District, Chiang Mai Province, and Chiang Khum District, Pa Yao Province. The symptoms included black stem lesions, soft rot on tubers, wilting, break down of the stem vascular ring and foliar yellowing. Of 13 bacterial isolates, five were identified as Pectobacterium carotovorum subsp. brasiliense, four‐Dickeya dadantii, two‐Pseudomonas putida and two‐Bacillus altitudinis. Pathogenicity tests of P. carotovorum subsp. brasiliense and D. dadantii resulted in lower leaves turning yellow and wilting followed by blackleg symptoms on lower stems and maceration of tuber tissue. Symptoms caused by P. putida were yellowing and wilting of leaves. B. altitudinis caused yellowing of the lower leaves and wilting followed by drying of leaf tissue. This is a first report of these bacterial pathogens causing blackleg and soft rot of potato in Thailand.     

The sweet potato sporamin promoter confers high-level phytase expression and improves organic phosphorus acquisition and tuber yield of transgenic potato

The sweet potato sporamin promoter was used to control the expression in transgenic potato of the E. coli appA gene, which encodes a bifunctional enzyme exhibiting both acid phosphatase and phytase activities. The sporamin promoter was highly active in leaves, stems and different size tubers of transgenic potato, with levels of phytase expression ranging from 3.8 to 7.4% of total soluble proteins. Phytase expression levels in transgenic potato tubers were stable over several cycles of propagation. Field tests showed that tuber size, number and yield increased in transgenic potato. Improved phosphorus (P) acquisition when phytate was provided as a sole P source and enhanced microtuber formation in cultured transgenic potato seedlings when phytate was provided as an additional P source were observed, which may account for the increase in leaf chloroplast accumulation (important for photosynthesis) and tuber yield of field-grown transgenic potato supplemented with organic fertilizers. Animal feeding tests indicated that the potato-produced phytase supplement was as effective as a commercially available microbial phytase in increasing the availability of phytate-P to weanling pigs. This study demonstrates that the sporamin promoter can effectively direct high-level recombinant protein expression in potato tubers. Moreover, overexpression of phytase in transgenic potato not only offers an ideal feed additive for improving phytate-P digestibility in monogastric animals but also improves tuber yield, enhances P acquisition from organic fertilizers, and has a potential for phytoremediation.     

Evaluation of transgenic Bacillus thuringiensis corn hybrids against Cry1Ab-susceptible and -resistant sugarcane borer (Lepidoptera: Crambidae)

A Louisiana strain of the sugarcane borer, Diatraea saccharalis (F.) (Lepidoptera: Crambidae), was selected for resistance to the CrylAb protein of Bacillus thuringiensis (Bt) by using an F2 screening procedure. Survival of Bt-resistant, -susceptible, and -heterozygous genotypes of sugarcane borer was evaluated on vegetative and reproductive stages of five non-Bt and seven Bt field corn, Zea mays L., hybrids in a greenhouse study. Larval survival was recorded 21 d after infestation of neonates on potted plants. Larval survival across the three sugarcane borer genotypes and five non-Bt corn hybrids after 21 d ranged from 23.6 +/- 5.2% (mean +/- SEM) to 57.5 +/- 5.2%. Mean survival of Cry1Ab-resistant larvae on vegetative and reproductive plant stages was 12 and 21%, respectively. During the vegetative stages, all seven Bt corn hybrids were highly efficacious against Cry1Ab-susceptible and -heterozygous genotypes of sugarcane borer, with a larval survival rate of <2% for the Bt-susceptible genotype and < or =5% for the heterozygotes. However, 8-18% of the heterozygous genotype survived on reproductive stage plants for four of the seven Bt corn hybrids tested. The variation in performance of Bt corn cultivars at vegetative and reproductive growth stages against Cry1Ab resistant sugarcane borer suggests differential seasonal expression that may hasten resistance in the field. Bt corn hybrids expressing a "high dose" for European corn borer, Ostrinia nubilalis (Hübner), may not produce a sufficient high dose for the sugarcane borer.     

转Bt基因抗虫玉米对亚洲玉米螟幼虫取食行为的影响

室内研究了转cry1Ab杀虫蛋白基因的Bt抗虫玉米MON81 0和Bt1 1对亚洲玉米螟Ostriniafurnacalis初孵幼虫和3龄幼虫的取食行为、取食选择性和存活率的影响。在48h的非选择性试验中玉米螟初孵幼虫在MON81 0和Bt1 1玉米心叶组织上的幼虫取食率随时间的增加而下降,在对照玉米上的幼虫取食率随时间的增加而上升,两者间差异极显著。初孵幼虫接虫到MON81 0和Bt1 1玉米叶片48h的累计死亡率分别为67 .5 %和47 .5 % ,而在对照玉米上死亡率均为0. 3龄幼虫在Bt和非Bt玉米穗轴组织上的幼虫取食率随时间的增加呈上升趋势,第48h时在Bt和非Bt玉米上的幼虫取食率分别达到77 5 %和1 0 0 % ,差异极显著。选择性试验中,第4～48h内,初孵幼虫在Bt玉米上的幼虫取食率呈下降趋势,第48h时MON81 0和Bt1 1与各自非Bt对照的组合中初孵幼虫的累计死亡率分别为2 5 .0 %和1 7. 5 % ,二者差异不显著。3龄幼虫在Bt玉米和非Bt玉米上的幼虫取食率均随时间的延长而增加,但在非Bt玉米的幼虫取食率增加速度快,与Bt玉米差异极显著。Bt玉米对玉米螟幼虫取食有抑制和忌避作用。     

An immunochemical study of the diapause-associated protein of the southwestern corn borer,Diatraea grandiosella

The distribution and titre of the diapause-associated protein (DAP), which accumulates in the fat body of diapausing larvae of the southwestern corn borer, Diatraea grandiosella, were determined using an antibody raised to the purified protein. Immunoblotting and immunodiffusion showed that the highest concentration of DAP was present in the fat body. Small amounts of the protein were present in the haemolymph and even lower levels were detected in other tissues. Rocket immunoelectrophoresis showed that DAP accumulated in the fat body at the beginning of diapause, reached a plateau, and gradually declined towards the end of diapause. Similarly, the titre of DAP in last instar non-diapausing larvae treated with a JH analogue increased, reached a plateau and then declined. Double immunodiffusion and immunoblotting using the DAP-antibody and extracts of the fat body of D. grandiosella and the southern cornstalk borer, Diatraea crambidoides, and the sugarcane borer, Diatraea saccharalis, revealed the partial immunochemical identity of the DAP of D. grandiosella and a protein present in these two species. Denaturing electrophoresis with immunoblotting showed that the DAP-related proteins of D. crambidoides and D. saccharalis have apparent molecular weights of 33,000 and 36,000, respectively, as compared to a molecular weight of 35,000 for DAP of D. grandiosella. A dot blot analysis showed that no cross reaction occurred between the “diapause proteins” present in the haemolymph of adults of the Colorado potato beetle, Leptinotarsa decemlineata, and the DAP of D. grandiosella.     

Establishment of Cry9C susceptibility baselines for European corn borer and southwestern corn borer (Lepidoptera: Crambidae)

In 1997 and 1998, Cry9C susceptibility baselines were established for field-collected populations of European corn borer, Osrinia nubilalis (Hubner), and southwestern corn borer, Diatraea grandiosella Dyar. Bioassay of neonate European corn borer larvae of 16 colonies collected from the midwestern United States indicated LC50 values ranging from 13.2 to 65.1 ng of Cry9C protein per square centimeter. Neonate European corn borer LC50 values ranged from 46.5 to 214 ng/cm2. Neonate larvae of three colonies of southwestern corn borer collected from the southern and southwestern United States exhibited LC50 values from 16.9 to 39.9 ng of Cry9C protein per square centimeter. Southwestern corn borer neonate LC90 confidence limit values ranged from 40.3 to 157 ng of Cry9C protein per centimeter. The most sensitive southwestern corn borer colony was collected from the Mississippi delta exhibiting an LC50 value of 22.6 ng of Cry9C per cm2 and also displayed the widest LC0 confidence limits of 40.3-94.8 ng of Cry9C per cm2. Geographic baseline susceptibility data establishes the natural genetic variation and provides the foundation for future testing of insect populations exposed to increased use of Bacillus thuringiensis-based crops. Insect resistance management and stewardship of Cry9C will rely upon baseline data for the validation of discriminating dose assays for European corn borer and southwestern corn borer.     

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.     

Host plant constituents as oviposition stimulants for a generalist herbivore: European corn borer

Oviposition response of the polyphagous European corn borer,Ostrinia nubilalis (Hübner) (Lepidoptera: Pyralidae), to chemical constituents in host plants was investigated in the laboratory using two-choice bioassays. Foliar extracts of corn (Zea mays L.), pepper (Capsicum annuum L.) and potato (Solanum tuberosum L.) were prepared using the solvents pentane, acetone and methanol. In all three host plants, chemicals soluble in pentane stimulated oviposition. In potato, chemicals extractable in acetone also elicited a positive oviposition response. When presented with a choice between pentane extracts of corn and pepper, females preferred corn. No preferences were exhibited between pentane extracts of corn and potato or pepper and potato. Pentane extracts of corn husks, tassels, silk, and corn leaves from plants at early whorl and tassel (pre-pollen shed) stages of development also stimulated oviposition. Similar extracts from plants at 2-leaf and blister (when kernels resemble blisters) stages were not stimulatory. This indicates that plant phenology affects chemically mediated oviposition response in European corn borer. The potential use of plant chemicals for management ofO. nubilalis in the field is suggested.     

Regulation Effects of Water and Nitrogen on the Source-Sink Relationship in Potato during the Tuber Bulking Stage

The source-sink relationship determines crop yield, and it is largely regulated by water and nutrients in agricultural production. This has been widely investigated in cereals, but fewer studies have been conducted in root and tuber crops such as potato (Solanum tuberosum L.). The objective of this study was to investigate the source-sink relationship in potato and the regulation of water and nitrogen on the source-sink relationship during the tuber bulking stage. A pot experiment using virus-free plantlets of the Atlantic potato cultivar was conducted, using three water levels (50%, 70% and 90% of field capacity) and three nitrogen levels (0, 0.2, 0.4 g N∙kg⁻¹ soil). The results showed that, under all water and nitrogen levels, plant source capacity were small at the end of the experiment, since photosynthetic activity in leaves were low and non-structural reserves in underground stems were completely remobilized. While at this time, there were very big differences in maximum and minimum tuber number and tuber weight, indicating that the sink tuber still had a large potential capacity to take in assimilates. These results suggest that the source-supplied assimilates were not sufficient enough to meet the demands of sink growth. Thus, we concluded that, unlike cereals, potato yield is more likely to be source-limited than sink-limited during the tuber bulking stage. Water and nitrogen are two key factors in potato production management. Our results showed that water level, nitrogen level and the interaction between water and nitrogen influence potato yield mainly through affecting source capacity via the net photosynthetic rate, total leaf area and leaf life span. Well-watered, sufficient nitrogen and well-watered combined with sufficient nitrogen increased yield mainly by enhancing the source capacity. Therefore, this suggests that increasing source capacity is more crucial to improve potato yield.