William B. Showers

Reduced Fusarium Ear Rot and Symptomless Infection in Kernels of Maize Genetically Engineered for European Corn Borer Resistance

ABSTRACT Field experiments were conducted in 1994, 1995, and 1996 to evaluate the incidence and severity of Fusarium ear rot and the incidence of symp-tomless Fusarium infection in kernels of maize hybrids genetically engineered with Bacillus thuringiensis genes encoding for the delta-endotoxin CryIA(b). Treatments included manual infestation with European corn borer (ECB) larvae and insecticide applications to limit ECB activity to specific maize growth stages or mimic standard ECB control practices. Fusarium symptoms and infection were affected by the specific cryIA(b) transformation used in each hybrid that determines tissue-specific expression of CryIA(b). In hybrids expressing CryIA(b) in kernels, incidence and severity of Fusarium ear rot and incidence of symptomless kernel infection were reduced compared with near-isogenic hybrids lacking cryIA(b) genes. In plants that were manually infested with ECB, ear rot incidence was reduced by 87, 58, and 68%; severity was reduced by 96, 54, and 64%; and incidence of kernel infection by Fusarium species was reduced by 17, 38, and 38% in 1994, 1995, and 1996, respectively. Results were similar in treatments that were not manually infested, but differences between transgenic and nontransgenic hybrids were smaller. Most kernel infection was due to F. moniliforme, F. proliferatum, and F. subglutinans (section Liseola) collectively, and it was within this group that transgenic hybrids exhibited reduced infection. Expression of CryIA(b) in plant tissues other than kernels did not consistently affect Fusarium symptoms or infection. Disease incidence was positively correlated with ECB damage to kernels. Insecticide applications also reduced Fusarium symptoms and infection when applied to nontransgenic plants.

Aggregation and Dispersal Behavior of Marked and Released European Corn Borer (Lepidoptera: Crambidae) Adults

Abstract To observe the aggregation and dispersal behavior of adult European corn borer, Ostrinia nubilalis (Hübner), males in search of mates, two populations were marked, each with a different dye. One population was continuously reared in the laboratory (>5 yr) and the other was collected annually from the field. From 1986 to 1988, marked adults were released in two release sets per year, with three to five releases per release set, coinciding with the spring and summer flights of European corn borer in central Iowa. Traps for recapture contained lures baited with 40 μg of synthetic 97:3 Z:E-11-tetradecenyl acetate. Traplines extended from 200 m to 48 km. Each trap was assigned a compass direction. Males from the laboratory-reared population dispersed similarly to males just 1 generation from the wild. European corn borer males and females dispersed 23–49 km and some were recovered 14 km from the release site within 100 min after release. Sampling of aggregation sites demonstrated that on the nights of release, many adults aggregated in adjacent dense vegetation and did not disperse until the following night. Upon dispersal, these adults seemingly moved many meters or kilometers before settling again. Recapture of marked adults at 200 m might have been influenced by open landscapes (short, vegetative-growth corn). Recapture at 800 m or beyond, however, was unaffected by open landscapes, and in 1988 a greater proportion of marked males was recaptured while the landscape was closed. (tall, mature-growth corn). In 1987, during the first flight of European corn borer, displacement to 800 m was southeasterly, south, or west, but during first flights in 1986 and 1988, displacement to 800 m was predominately northeasterly. During the second flight in midsummer, displacement to 800 m for all 3 yr of the study also was northeasterly. Recapture results from 1986 suggest that male movement >800 m is common. During the first release set (early summer), 37% of the males recaptured flew 800 m or more and 8% flew 3.2 km or more. During the second release set (late summer), 51% of the males recaptured flew 800 m or more and 11% flew 3.2 km or more. The recommendation for placement of nontransgenic corn (Zea mays L.) as refuge in the Corn Belt is a half mile or closer to Bacillus thuringiensis Berliner (Bt)-corn if the refuge corn is not sprayed and a quarter mile or closer when the refuge corn is sprayed. Based on the dispersal results from this study, at least in Iowa, a half-mile proximity recommendation should be robust. Studies still should be conducted in other regions, especially where corn is commonly irrigated, to determine whether European corn borer adult movement patterns are similar.