Feasibility of Biological Control of European Corn Borer in Peppers

April Satanek, Brent Rowell, and Ric Bessin

Introduction

Integrated pest management practices have aided farmers in reducing the amount of pesticides used to control insects and in using them more effectively when needed. Biological control is a component of IPM that is growing in popularity, especially among organic growers. Biocontrol uses one living organism to control the population of an unwanted pest. Tiny parasitic wasps (less than 0.5 mm long) from the genus Trichogramma have been used as biocontrol agents in sweet corn against European corn borer (ECB, Ostrinia nubilalis). ECB is also the most serious insect pest in peppers in Kentucky. Pepper crops are often damaged by second and third generation ECB larvae in the middle to later part of the growing season. Once hatched, the larvae are difficult to control because they quickly tunnel into the caps of pepper fruits. Once inside the fruit, ECB larvae cannot be killed with insecticides. Subsequent injury to the fruit may go undetected until they decay from bacterial soft rot which occurs as a result of ECB damage.

Trichogrammae ostriniae is an egg parasite which was introduced to the US from China. This species has been tested and found effective in reducing the number of insecticide treatments for control for ECB in sweet corn in the northeast. Little is known about the wasps' effectiveness in controlling ECB in peppers, and to our knowledge this species has not been previously evaluated for its effectiveness in peppers. In this preliminary trial, we released T. ostriniae in a small, unreplicated trial in order to get an idea of its potential for ECB control in bell peppers and to learn scouting and other procedures in order to better plan replicated trials for 2002.

Materials and Methods

Two bacterial spot-resistant bell pepper cultivars, 'Early Sunsation' and 'Defiance', were planted in each of two 50 ft by 50 ft plots which were prepared at separate sites at the University of Kentucky Horticultural Research Farm in Lexington. One of these identical plots was designated the release plot and the other the control plot (no T. ostriniae released). The plots were approximately 300 yards apart, with fields of other crops, a gravel parking lot, and a building between them. The release plot was located downwind from the control plot.

Peppers were seeded on April 19 and transferred to 72-cell trays on May 16. The plants were set on May 30 into raised beds with black plastic mulch and drip irrigation. Standard commercial practices were used: plants were grown in double rows with 12 in. between plants within the row and 15 in. between the double rows. Each plot consisted of six double-row beds (384 plants) bordered by guard rows on each side (128 plants). The plants were irrigated as needed based on tensiometer readings.

One hundred and fifty pounds per acre of ammonium nitrate (50 lb N/A) was incorporated into each plot prior to planting. Plots were fertilized with P and K according to soil test results. An additional 10 lb N/A as ammonium nitrate was applied in the release plot and 7 lb N/A in the control plot in 3 fertigations. Total season N applications including preplant were 60 lb N/A for the release plot and 57 lb/A for the control plot. Maneb and copper (TennCop) were applied weekly to both plots to protect against bacterial spot. The insecticide Spintor was mistakenly applied once in the control plot; no other insecticide treatments were applied in the control or release plot. A pheromone trap was placed adjacent to the release plot to monitor ECB activity.

Thirty thousand T. ostriniae-parasitized Ephestia eggs (glued inside two paper cups with 15,000 each) were obtained from Cornell University and placed in the release plot on July 11. This was the date predicted as the first flight of second generation ECB moths for Lexington by the University of Kentucky ECB degree day model. That flight turned out to be very light and we decided to obtain and release a second batch of 30,000 a week later on 18 July in the same field. Paper cups containing the egg parasites had been folded and stapled shut in order to protect against predators and exposure; numerous pinholes had been made in the cups to allow the parasites to emerge. Releases were simply a matter of hanging the two paper cups under the leaf canopy of a plant in the center of the plot.

Plots were scouted twice weekly and the number of parasitized and unparasitized ECB egg masses recorded. Once eggs were located and their status recorded, leaves with eggs were flagged with a plastic marking ribbon and given a number. These egg masses were visited twice weekly and their condition recorded until hatching or their disappearance.

All green mature fruits were harvested on August 1 and 18. Marketable fruits were graded and weighed according to size class (U.S. No. 1 extra large, large, medium). Each fruit was carefully examined for signs of ECB feeding or injury; all fruits with noticeable signs of ECB activity were dissected to determine the presence of larvae. Only fruits with one or more larvae inside were recorded as having ECB damage.

Results and Discussion

ECB egg masses were first discovered in the trial on July 13. The second generation ECB moth flight was not as concentrated as in past years, and only small numbers of moths were caught until the end of the season. Our plan had been to release T. ostriniae to coincide with the predicted flight of second generation ECB around July 10; however, a total of only 7 ECB moths had been trapped by July 25. The detected initiation of the moth flight occurred about a week later than predicted. This may have been due to low ECB numbers. A total of 19 egg masses in the release plot were located and monitored until hatching or disappearance from July 13 until August 10.

Only 3 ECB egg masses were found in the control plot; however, more than double the number and weight of ECB-damaged fruits occurred in this plot than in the release plot (see table below). The release plot yields were slightly higher than control plot yields. It is not known why ECB masses in the control plot were not as easily detected as those in the release plot. Many T. ostriniae-parasitized ECB egg masses were also found in an adjacent sweet corn trial after ECB egg numbers in the pepper plot had declined. Only a few very small clusters of 1-3 ECB eggs were found in the pepper plots towards the end of the growing season.

Many of the flagged egg masses, whether parasitized or not, seemed to disappear during the course of the trial. These disappearances might have been caused by the feeding of predatory insects such as lady beetles, by egg casings being eaten after ECB or wasp emergence, or simply by becoming detached and lost.

Notes on scouting. Scouting techniques included brushing pepper leaves up with one's arm to expose undersides of the leaves. ECB eggs were most often found half way up from the bottom of the plant on the undersides of leaves and fruit. ECB moths laid eggs indiscriminately on both hail-damaged and whole leaves alike. Viable ECB eggs are scale-like, circular, milky-white with an iridescent casing; they are deposited in clusters (masses). When T. ostriniae parasitizes a ECB egg, the inside of the egg turns solid black. This parasitized condition should not be confused with the "black head" stage of ECB eggs. The "black head" stage occurs in non-parasitized eggs when heads of the ECB larvae become visible about 24 hours prior to hatching. The black head stage is not as completely black as in parasitized eggs. T. ostriniae are not able to parasitize eggs in the black head stage. Adult female T. ostriniae lay their eggs in ECB eggs, sometimes depositing more than one egg in each ECB egg. The wasp larva hatches inside the ECB egg, feeds on the contents, and pupates. The adult wasp chews a circular escape hole in the ECB egg casing and vacates the egg. These escape holes were visible with a hand lens. It takes about 10 days from egg deposition to wasp emergence. Because of their extremely small size, there appears to be no other practical way of monitoring T. ostriniae activity other than locating and recording parasitized eggs.

Many environmental factors are known to affect the success of T. ostriniae. The wasps prefer temperatures between 62 and 89F, and relative humidities between 45% and 95%. The adults probably feed on nectar of the pepper plant flowers. Strong winds, dust and rain will affect the wasps' performance. Studies conducted by Cornell University in New York have shown that T. ostriniae can persist even when pesticides are used because the developing wasps are protected inside the ECB egg.

Further trials. It was not our intention to determine if T. ostriniae could successfully control ECB in peppers in this preliminary trial. We hoped that these observations might indicate whether the technique looked promising. The results do seem to suggest that one or two inoculative releases of T. ostriniae have potential to help control ECB in peppers. The test also provided insights on scouting procedures and data collection for use in future trials.

Acknowledgements

Our thanks to Larry Blindfold, Spencer Helsabeck, John Holden, Dave Lowry, Bonnie McCaffrey and Kirk Ranta for help with harvest and data collection. We would also like to gratefully acknowledge the assistance of Mike Hoffmann, Mark Wright, and Sylvie Chenus of the Department of Entomology at Cornell University for generously providing the parasites and for their helpful suggestions and advice.

Note: see www.uky.edu/Ag/Entomology/entfacts/fldcrops/ef106.htm for more information regarding the degree day model for predicting flights of European Corn Borer in Kentucky.

European corn borer damage and pepper yields in T. ostriniae release and control plots at Lexington, KY, 2001. Data are season totals from two harvests.