A field experiment was conducted to compare sweet corn pest management by conventional insecticides with the biological control strategies of predator conservation and predator enhancement. We are attempting to conserve predators by applying microbial insecticide (B.t.), and to enhance predators by a sprayable sugar-protein product (Pred-Feed). In an early planting of 'Seneca Horizon' at both Fremont and Columbus, a split-plot experimental design was used. Main plot treatments were Pred-Feed applied weekly vs. no Pred-Feed. Sub-plot treatments, replicated four times, were 1) B.t. granules in whorls, 2) permethrin granules in whorls, 3) B.t. sprays to silks, 4) thiodicarb sprays to silks, 5) B.t. granules in whorls plus B.t. sprays to silks, 6) permethrin granules in whorls plus thiodicarb sprays to silks, 7) untreated check. A similar trial was conducted in late plantings of 'Lancelot' at both locations but with only sub-plot treatments 3, 4, and 7. Plots were monitored in the whorl and tassel stages for pests and predators, and ear quality was evaluated at harvest. Results of harvest (Table 1) and scouting were subjected to analysis of variance. Results were similar to trials conducted in 1993 and 1994; there was no significant effect of the Pred-Feed attractant; conventional insecticides provided the best control, and B.t. provided control intermediate between conventional and untreated.
To determine the efficiency of O. insidiosus and C. maculata in searching and destroying egg masses of European corn borer, a field cage study was conducted at the OSU Horticulture Farm in Columbus on 3-4 September 1995. Cages were placed over individual corn plants in the pollen shedding stage. The day before the experiment was conducted, caged plants were sprayed with pyrethrin to remove any predator or prey insects. Adult O. insidiosus and C. maculata were collected in the field and starved for 24 hours before release. Egg masses of European corn borer were obtained from a laboratory colony. Three egg masses per plant were pinned on the ear husk, and on the bottom surface of the leaf immediately above and immediately below the ear. Twelve treatments in a three by four factorial design were: O. insidiosus at densities of 0, 4, 8, and 12 per plant, C. maculata at densities of 0, 1, 3, 5 per plant, and combined O. insidiosus and C. maculata at the same densities. There were four replicates per treatment. Egg masses were retrieved 24 hours after release, and examined under a microscope to determine egg mortality. Statistical analysis will determine effects of egg mass placement on the plant, egg mortality at different predator densities, and interactions between predators as related to egg mortality. Data have not yet been analyzed, but we observed that C. maculata was generally better than O. insidiosus at finding and consuming eggs, and eggs on the leaf above the ear were more frequently consumed than eggs at other locations.
The effect of prey density on predator feeding was studied in a growth chamber in September and again in December 1995. Adult O. insidiosus, adult C. maculata, and the first instar of the lacewing Chrysoperla carnea were kept in containers with cotton wicks saturated with water, and starved for 24 hours before testing. Individual adult O. insidiosus were placed in containers with 0, 10, 20, 30, and 40 corn borer eggs and 0, 10, 20, 30, and 40 corn borer first-instar larvae. Consumption was measured and prey replaced every 2 hours for 12 hours. The procedure was the same for C. carnea and for C. maculata except that for the C. maculata tests, prey density was 0, 50, 75, 100, and 125 per container. There were six replicates each for male O. insidiosus, female O. insidiosus, unsexed C. maculata, and unsexed C. carnea. Analysis of variance and regression analysis were done to determine the functional response curves. Preliminary results show that C. maculata exhibits a type II functional response to corn borer larvae, O. insidiosus females exhibit a type II functional response to larvae while males exhibit a type I response to larvae, and C. carnea exhibits a type II functional response to larvae. Maximum prey consumed in 24 hours was 80 eggs or 123 larvae by C. maculata, and 8 eggs or 40 larvae by O. insidiosus.
Table 1. Insect damage on harvested sweet corn, 1995.
| Mean percentage of ears unmarketable | ||
|---|---|---|
| Early planting | Late planting | |
| Location effect: | ||
| Columbus | 13.3 a | 60.5 a |
| Fremont | 1.3 b | 23.0 b |
| Main plot effect: | ||
| With attractant | 6.8 a | 43.2 a |
| No attractant | 4.8 a | 39.0 a |
| Sub-plot effects: | ||
| Untreated control | 15.3 a | 73.5 a |
| Microbial insecticides | ||
| B.t. granules | 5.8 ab | -- |
| B.t. sprays | 7.0 ab | 38.8 b |
| B.t. granules + sprays | 4.7 ab | -- |
| Conventional insecticides | ||
| Permethrin granules | 2.9 b | -- |
| Thiodicarb sprays | 7.7 ab | 14.0 c |
| Permethrin + thiodicarb | 1.3 b | -- |
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