INTRODUCTION: Several Mycogen NatureGard™ corn hybrids, produce an insecticidal protein derived from the bacteria Bacillus thuringiensis (Bt). In 7059, 7559 and 7959, the plant-produced protein (CRY1Ab) effectively controls European corn borer larvae without the need for foliar insecticides or special pesticide application equipment. These products were tested in 1995 under an Experimental Use Permit and have been approved for commercial sale by the Environmental Protection Agency. Information about the hybrids and how they control European corn borers are included below. The last section contains information on European corn borer resistance management.
HYBRID CHARACTERISTICS: 7059, 7559 and 7959 contain synthetic Bt genes which make plants produce the insecticidal CRY1Ab protein in green tissue and pollen. The level of protein production is approximately 20 times greater than that necessary for 100% kill of European corn borer larvae. The plants do not produce CRY1Ab protein in roots, pith, and kernels. As corn plants dry down, the CRY1Ab protein is no longer produced. Characteristics of hybrids commercially available in 1996 are shown below.
CHARACTERISTIC 7059 7559 7959
Relative maturity 111 Day 114 Day 116 Day
European corn
borer resistance Bt + native Bt Bt
resistance
Plant ht/ear ht Tall/medium high Tall/high Tall/high
Yield potential Excellent Excellent Excellent
Rec. Population High Low Low-Med
Area of I-80 east to South of I-70 South of I-70
Adaptation Indiana
CONTROL OF EUROPEAN CORN BORERS: European corn borers lay eggs on NatureGard hybrids just as they would on conventional corn. The CRY1Ab protein produced in the leaves and pollen of 7059, 7559 and 7959 is eaten by European corn borer larvae which feed on these tissues during their first two growth stages. Once ingested, the protein causes the larvae's gut to malfunction and feeding stops immediately. European corn borer larvae starve to death before they grow large enough to bore inside stalks and cause corn yield reductions. In research trials, the level of first brood European corn borer control has been nearly complete (see Table 1. below). Control of second brood European corn borer is very good (74% average, Table 1). However, because CRY1Ab protein is not present in kernels, some European corn borer larvae can survive be feeding on kernels. When these larvae are large enough (about 1 inch long), they can bore inside stalks and shanks.
Table 1. Reduction of first and second brood European corn borer with 7059 in 4 trials, 1995.
TEST COOPERATOR % 1ST BROOD CONTROL % 2ND BROOD CONTROL University of Illinois 100 86 Iowa State University 99 57 University of Nebraska 100 82 Missouri Valley 100 71 Agri-Service AVERAGE 100 74
Mycogen Seeds 1340 Corporate Center Curve, P.O. Box 21428, St. Paul, MN 55121-1428 1-800-MYCOGEN
OTHER INSECTS: The CRY1Ab protein is very selective and will control only European corn borers and perhaps a few closely related species. In 1996, Mycogen will conduct additional tests to determine control of southwestern corn borer, fall armyworm, and corn earworm. The CRY1Ab protein will not control the following insect pests of corn: black cutworm, common stalk borer, corn earworm (ear infestations), and corn rootworm. It is not toxic to important beneficial arthropod species such as parasitic wasps, spiders, lacewings, and ladybeetle larvae.
EUROPEAN CORN BORER RESISTANCE MANAGEMENT: Government, university, and industry entomologists are concerned that overuse of Bt hybrids might cause Bt resistance to develop in European corn borer populations. While the short term risk of this is very low, Mycogen is supporting university research on how to maintain the effectiveness of Bt hybrids for the long term. Until this research is completed, Mycogen recommends that Bt hybrids be used as part, but not all, of a European corn borer management program. Factors to consider in developing a European corn borer management program that incorporates several different approaches are below.
HYBRID SELECTION: Plant some fields to Bt resistance, some to native resistance, and some to conventional corn hybrids. This will ensure that European corn borers always face different control methods and makes it unlikely that they will develop resistance to any one method.
PLANTING DATES: Planting date sometimes affects European corn borer infestations. Early planting increases the risk of first brood European corn borer. Late planting increases the risk of second brood. If practical, consider the hybrid resistance level and local trends in European corn borer problems when planning which hybrids to plant earliest and which to plant latest.
PLANT POPULATIONS AND FERTILITY: High plant populations and fertility levels can increase European corn borer infestations. This information may be useful in choosing whether to plant a field to a conventional or Bt hybrid.
INSECTICIDES: Scout fields for European corn borer and treat with labeled insecticides if severe infestations develop. See your university extension or local crop consultants for guidelines on scouting, making a treatment decision, and treating for European corn borers. If severe European corn borer infestations develop on 7059, 7559, or 7959, please contact your Mycogen sales representative immediately.