Biological control of ECB with Trichogramma ostriniae
in Sweet Corn: Be ready for early releases!
A tiny wasp – smaller than the dot at the end of the sentence – that will search out and kill the egg masses of one of our major sweet corn pest – can this really work? A number of sweet corn growers in the Northeast have been testing Trichogramma ostriniae parasitic wasps over the past five years and have found that they do help to control European corn borer (ECB) in both corn and peppers. The use of these wasps in commercial sweet corn fields in Massachusetts has resulted in the reduction or elimination of foliar insecticide sprays, saving time, labor, pesticides, and fuel, reducing soil compaction, and maintaining and improving ear quality.
This method is an ideal IPM practice because it prevents the emergence and feeding of caterpillars in the first place, as opposed to rescuing the corn with sprays after the caterpillars have become a problem. Using Trichogramma to control ECB in early corn (corn to be harvested in July) is especially useful because timing sprays in the early corn can be tricky. Also, most of the caterpillar damage in early sweet corn is from ECB - thus, wasp release control measures are not complicated by the need to control other major caterpillar pests. Trichogramma can also be used for second generation ECB, which attacks both peppers and corn.
Biology
Trichogramma species are tiny parasitic wasps, smaller than the period at the end of this sentence. Female wasps lay their eggs in the egg masses of host insects. Trichogramma larvae feed and pupate inside the egg, killing the egg and preventing hatch. Trichogramma ostriniae lays its eggs in ECB egg masses. As they mature, unparasitized ECB egg masses turn from a cream color to white, to white with a black head mass in the center of each egg. When parasitized by Trichogramma, the entire egg turns black. T. ostriniae have excellent dispersal and ability to search for egg masses in the field. They do not overwinter but they will reproduce and contribute to the control of ECB throughout the season.
Release timing
While some native species of Trichogramma persist in the wild, T. ostriniae need to be reared at an insectary, shipped to the farm and released each season. Since Trichogramma control ECB by parasitizing egg masses, knowing when to release the wasps requires knowing when the ECB moths are laying eggs. Thus, knowing when ECB flight begins, reaches a peak, and ends in a given field is key to the proper timing of Trichogramma releases. You can use regional information about flight activity; however, to get the best coordination of timing on your farm, we recommend that you monitor ECB flight in your own fields.
ECB moths have two generations per growing season in Massachusetts; the first one emerges in late May or early June, while the second generation begins to emerge in late July and early August. Time the first release of T. ostriniae to the beginning of ECB egg laying, which will begin within a week after the first ECB moths are caught in traps. If the corn is less than 6 inches high, you may want to wait a few days. For corn maturing in the middle of moth flight, target releases to corn that is in the 4-6 leaf stage (12-16 inches tall).
To help align the concentrated presence of T. ostriniae with ECB host egg laying we recommend two to three releases, each approximately 7 days apart. Our current recommended release rates in early corn are 60,000 wasps per acre per release.
Degree days (DD) can help with timing. Using a base temperature of 50 degrees F, the first spring moths will emerge at 375 DD50, and the first eggs are laid at 450 DD50. Eggs require 100 degree days to hatch. Releases should be made when eggs are in the field, but before eggs hatch.
Handling Trichogramma
Trichogramma are shipped from the insectary as pupae inside protective cards. They are ready to emerge upon arrival, although there will be a range of pupal age so they will emerge gradually, over 1-7 days, depending on temperature. It’s best to put the cards out in the field the same day as they arrive. If you cannot release them upon their arrival, keep the cards in their shipping box in a cool location at about 50°F – not in the refrigerator! The insects are alive: avoid exposing them to extreme temperatures (below 40°F or above 90°F) so they will still be alive and in good shape when you put them in the field.
Releasing Trichogramma
Place the proper number of cards to provide the desired release rate in the center of the field, or at regular intervals through the field, away from the field edges. Trichogramma wasps will disperse well throughout the field- one to four release sites per acre is adequate. Tie cards securely to corn leaves or on a stake. Do not put them on the ground. Leave the packet stapled shut so that other insect predators do not consume them.
Scouting release fields
Where Trichogramma has been released, you can scout as usual. Eggs that were parasitized and did not hatch will never reach the larval stage, resulting in a lower rate of infestation with caterpillars. Use the standard ECB threshold (15% infestation in caterpillars or fresh damage) to decide whether to spray.
Spraying release fields
T. ostriniae will suppress ECB, but will not always provide complete control. In addition, an early corn earworm flight may arrive during silking. Thus, insecticide applications may still be needed to achieve high levels of clean corn. Use selective insecticides with low impact on natural enemies (aka beneficials). Trichogramma that are inside host eggs are somewhat protected from the spray and many will survive, but adult wasps may be killed by insecticides that are harsh on beneficial organisms.
Ordering Trichogramma
Trichogramma ostriniae may be ordered from IPM Laboratories in Locke, New York.
315-497-2063, www.ipmlabs.com. Order well in advance; it is a good idea to call IPM Labs with a general order in the early spring. Tell them the expected acreage, release rate and dates of release. A more specific date and amount can be ordered closer to the release time, but expect a 2-week turn around.
by Ruth Hazzard, Amanda Brown and Pamela Westgate, Extension Vegetable Program, University of Massachusetts-Amherst
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