Evaluation of Trichogramma ostriniae (Hymenoptera: Trichogrammatidae) as a biological control agent against the European corn borer, Ostrinia nubilalis (Lepidoptera: Pyralidae): Biological, behavioral and ecological perspectives
The biology and behavioral ecology of Trichogramma ostriniae, an introduced egg parasitoid of Ostrinia nubilalis and other lepidopteran species were studied in the laboratory and under field conditions. The inter- and intra-specific discrimination, competition, and the effectiveness of T. ostrinia and T. nubilale were also investigated.
For T. ostriniae to complete its life cycle, it took 6.7 days at 33$\sp\circ$C and 20 days at 17$\sp\circ$C. Comparison of several theoretic models indicated that a transformed day-degree model and the Hilbert-Logan model were better for predicting the development of this parasitiod. The wasps reared on O. furnacalis by the seventh generation and at 27$\sp\circ$C had a lower level of parasitism than wasps from other generations and at other temperatures. The differences among generations for the time female wasps spent drumming host eggs did not show any host and generation related trend.
European corn borer egg parasitism was positively correlated with egg density. Female T. ostriniae might exhibit the Type II or Type III functional response depending on the temperature. The field data suggested a Type II functional response when data were pooled for all replicates.
The effects of weather, plant size, and distribution of egg masses on egg parasitism by the wasp were studied in a sweet corn field. Logistic regression analyses indicated that percentage of eggs parasitized was negatively related to an increase in leaf area and distance of eggs from release point. Eggs on leaves in the upper third of a corn plant received much less parasitism than those on the middle and lower third. Temperature as well as the length of exposure of egg to wasps also affected the level of egg parasitism.
Laboratory test on the cross breeding between T. ostriniae and T. nubilale indicated no successful hybridization. The results of the inter- and intra-specific discrimination and competition between the two species revealed that only female T. ostriniae discriminated eggs parasitized by females of the same species. When both species oviposited the same egg, T. ostriniae was most likely to be able to complete development.
The level of egg parasitism by releasing T. ostriniae alone was found to be 15% higher than that by releasing T. nubilale alone, and 20% higher than by releasing a combination of the two species. Further analyses using the logistic regression model for independent and correlated data indicated T. ostriniae to be more efficient at discovering host egg masses than T. nubilale. Mutual interference between the two species was the main factor for the lower level of egg parasitism when the two species were released together.
The results suggest that T. ostriniae is the better candidate for augmentative releases for control of the European corn borer, and the two species should not be released into a corn field at the same time.