Using Trichogramma and Bt for European Corn Borer Control

Trichogramma are tiny parasitic wasps that lay their eggs in and feed on eggs of moths and butterflies, many of which are agricultural pests in their immature larval, or "worm" stage. Pest eggs parasitized by Trichogramma turn black, which allows us to easily determine whether eggs found in the field are parasitized. Several species of Trichogramma occur naturally and a few species are commercially available from insectaries. Trichogramma ostriniae, which was imported into the U.S. from China, shows great promise for European corn borer (ECB) management in commercial sweet corn. In New York, on-farm research and demonstrations have been conducted using two different approaches; inundative releases, and inoculative releases.

The research and demonstrations were initiated and supported by Mike Hoffmann's lab in the Department of Entomology at Cornell. We started out using an inundative approach similar to what has been used for Trichogramma releases in Europe. An inundative approach involves releasing large numbers of beneficial insects throughout a field almost like you would use an insecticide. Releases are repeated as long as new pests keep reinfesting a field. The goal of the inundative releases was to provide commercially acceptable levels of ECB control using only Trichogramma or Trichogramma in combination with Bt. This approach would make the most sense for organic growers or other growers with a market for insecticide-free sweet corn.

Because T. ostriniae are very mobile and very good at finding hosts and because they successfully reproduce in the field after they are released, Mike Hoffmann started looking into an inoculative approach, in which smaller numbers of wasps are released early in the season, reproduce, and move into new plantings as the season progresses. The goal of the inoculative releases was to reduce the number of insecticide applications needed to achieve commercially acceptable ECB control. Growers who use chemical insecticides may be able to reduce the number of applications needed with a small early-season investment in Trichogramma wasps.

Releasing wasps in the field. Trichogramma wasps are reared on the eggs of a grain moth that is less expensive to rear than the agricultural pests they are used against. The Trichogramma arrive as pupae inside grain moth eggs, which look like large grains of black pepper. Commercial insectaries often glue the eggs to cards. Trichogramma are usually placed in the field by hand, as the technology for mechanical application is still under development. The adult wasps then emerge from the parasitized grain moth egg and begin to search for a place to lay their eggs. In sweet corn, the parasitized grain moth eggs needed to be protected from being eaten by predators like lady beetles and lacewings, which are very common in sweet corn fields. For these trials, the eggs were put out in the field in small ice cream cartons with screened tops or in paper cups with release holes poked through with a pin.

Inundative releases.
Release timing: starting at late whorl
Release rate: 120,000/A
Frequency of releases: 2-3 times/planting at weekly intervals

We have conducted on-farm demonstrations of inundative releases during three seasons with varying levels of ECB infestation. We targeted the demonstrations to fields harvested before the end of August, before the fall armyworm (FAW) and corn earworm (CEW) flights start in western NY. Wasps were placed in the field in a 100' x l00' grid, starting at late whorl stage when the corn becomes most attractive to egg-laying female ECBs. The fields were scouted weekly, and percent parasitism was assessed by taking any egg masses we found back to the lab to see if they turned black. Bt applications were recommended if the fields were over the 15% infestation threshold at tassel emergence or the 5% infestation threshold during the silk stage. If unparasitized egg masses were found when scouting, they were held for 2-4 days to see if they turned black due to Trichogranima parasitization, and parasitized egg masses were excluded from the calculation when deciding if a field was over threshold. Bt products were applied at the maximum label rate. At harvest, 200 ears were evaluated in each field for ECB damage or infestation. The results are summarized below in Table 1. It was not possible to have untreated controls for comparison in these demonstrations because the wasps disperse from field to field so readily.

 Table 1.
 Year Farm/
 Range of % Infestation  N Tricho. Releases  Range of % Parasitism  N Bt Applic.  % Clean Ears
1997  K  2-28%  4  100%  2  100%
   R  17-44%  4  60-90%  2  78%
   L1  0-24% 4  90%  1  90%
   L2  7-10%  4  60-88%  1  91%
1998  T1  7-26%  3  100%  0  95%
   T2  12-30%  2  No EM found  0  96%
   Al  4-32%  3  100%  0  93%
   A2  13-36%  3  100%  0  93%
   A3  1-26%  3  No EM found  0  100%
   P  0-25%  3  100%  1  98%
 1999  H  2-20%  3 100%   1  97%
   S1  0-2%*  3  100%  0  99%
   S2  0-5%*  3  80-100%  0  89%**
   G  1-16%  3  100%  0  97%
   M  0-3%*  3  100%  0  94%
 * ECB egg masses only
 ** 1/17 ears infested with ECB, others were CEW and FAW

Based on the results of the inoculative releases discussed below, it appears that it may be possible to decrease the number of wasps released and frequency of releases when using an inundative strategy. Further research is needed to determine the optimal numbers.

Inoculative releases.
Release timing: beginning of the first generation moth flight
Release rate 30,000/A
Frequency of releases: once per field at "knee high" stage

Research on the inoculative release approach started in 1997. Initial research was conducted in individual fields and established that, after the initial release, the wasps were able to move up to 340 feet and persist in a field for up to 30 days. In 1999, a single release of 30,000 wasps was made per field, and egg masses were sampled in fields throughout the season. The average percent parasitism of egg collected masses was 65% (range 0-100%). At harvest, the infestation level in ears was similar in release and non-release fields regardless of whether insecticides were applied or not. The wasps were able to persist throughout the season even in fields receiving insecticide applications. They disappeared from only one release site, which had very low numbers of ECB eggs.

Things to keep in mind when using Trichogramma.
Trichogramma are living organisms and must properly handled so they are alive when put out in the field. Do not expose to temperatures below 50o F or above 80o F.

Trichogramma cannot be stored on the shelf. They must be ordered when they are needed in the field and released as soon as possible after arrival.

Activity and movement can be influenced by weather conditions, especially cool temperatures or long periods of rain. Parasitism may be reduced under less than ideal conditions.

Wasps live longer when they have access to flower nectar. Flowering plants in the field may enhance parasitism levels.

T. ostriniae does not control fall armyworm or corn earworm.

For the portion of the growing season when ECB is the only sweet corn pest to contend with, using Trichogramma ostriniae for ECB management can benefit all sweet corn growers, whether they use insecticides or not. Organic growers and growers who do not have sprayers suitable for sweet corn have an effective ECB management option. With new insecticides on the market that have less impact on natural enemies, growers who use insecticides can maximize the impact of the Trichogramrna when making inoculative releases, and reduce the number of applications needed to produce good quality corn early in the season. T. ostriniae should be available from commercial insectaries next season and the cost for the wasps is quite reasonable, probably in the range of $11- 12. per acre. For small acreages, shipping cost can be discouraging, equal to or more than the cost of the wasps themselves. Smaller growers who live near each other may want to pool their orders to manage the shipping costs.

Abby Seaman, Area Extension Educator, Cornell Cooperative Extension, IPM Program Office, NYSAES, Geneva, NY 14456, (315) 787-2422

Originally published: Proceedings. 1999. New England Vegetable and Berry Growers Conference and Trade Show, Sturbridge, MA. p.194-196.

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