Vegetable Crops News

Exploring the Use of Gels to Apply Predator Eggs to Crop Foliage

In many cases, augmentative release of predator or parasite eggs could be a very useful approach in biological control. Eggs are small and easy to ship, and do not need to be fed in transit. Also, in contrast to mobile adult stages, eggs and new larvae will not fly away from your release site! One of the problems which limits the use of eggs in augmentative releases is how to apply them in the field. Current methods include scattering eggs in a carrier such as corn cob grits or vermiculite or the placement of egg cards/sheets on plants. Both of these methods are labor intensive, result in imprecise or clumped placement and in some cases require novel equipment. A practical and economical method that mimics the natural placement of eggs on crop foliage by female natural enemies could be very useful in biological control of crop pests.

Work at the University of Delaware has focused on the use of the predaceous stink bugs Podisus maculiventris and Perillus bioculatus to control the Colorado potato beetle in potatoes. Researchers explored the use of gels to apply and stick stink bug eggs to crop foliage. Gels of various compositions have been used for fluid-seed drilling, seed germination and storage of germinated seed since the 1970's. They also have uses as soil conditioners in arid and semi-arid climates. These gels are generally not toxic to plants, mix easily with water of different pH and mineral content, break down in soil and are relatively inexpensive. In addition, equipment for application of such gels already exists in the form of easily modified peristaltic fluid-drilling planters.

Of the seven different formulations tested, polyacrylamide gels allowed the highest percent hatch of stink bug eggs. After being applied to foliage, the polyacrylamide gels dried within 24 hours, effectively sticking eggs to the leaf but allowing most predators to successfully emerge. However, the polyacrylamide gels are not thought to break down readily in the environment. These promising results have spurred the search for the ideal gel that would stick readily to leaf surfaces, allow gaseous exchange so eggs can respire, dry fast with minimal residue and degrade rapidly in the environment.

- Daniel J. Jenkins, University of Delaware

Using Trichogramma ostriniae for Biological Control of the European Corn Borer

The European corn borer (ECB), Ostrinia nubilalis, is a major pest of corn throughout much of the US and Europe. On a worldwide basis, the use of Trichogramma egg parasitoids to control ECB and other borers is increasing. We carried out studies to evaluate the effectiveness of using an oriental species, T. ostriniae, for the control of the ECB.

In one experiment, we studied the effect of plant size and weather on the distribution of egg parasitism of the ECB by T. ostriniae in sweet corn. Our results indicated that with each increase of 100 cm2 of leaf area per corn plant, there was a 7-9% decrease in the odds of egg parasitism. Also, with each meter away from the point of wasp release, there was a 7-8% drop in the odds of egg parasitism. T. ostriniae preferred to parasitize eggs on the lower and middle portion of the corn plant and eggs exposed to T. ostriniae for two days were about two times more likely to be parasitized than eggs exposed for one day. Finally, temperature extremes reduced the level of ECB egg parasitism by T. ostriniae. The results suggested that inundative releases of T. ostriniae should be made every two to three days, with multiple release points per field. In addition, temperature, plant height and leaf area must be taken into consideration to maximize levels of parasitism.

In another experiment, we compared ECB egg parasitism by releasing T. ostriniae, and a native US species, T. nubilale, either alone or in combination. The overall level of egg parasitism by T. ostriniae was found to be 15% higher than that by T. nubilale, and 20% higher than that by the combination of the two species. These results suggest that T. ostriniae would be a better candidate for augmentative releases to control the ECB than T. nubilale. In addition, simultaneous release of the two species in a corn field should be avoided.

- Baode Wang and David N. Ferro, Department of Entomology, University of Massachusetts, Amherst

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