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Egg-Mass Variability and Differential Parasitism of Choristoneura parallela (Lepidoptera: Tortricidae) by Endemic Trichogramma minutum (Hymenoptera: Trichogrammatidae)

Abstract The fireworm Choristoneura parallela (Robinson) is a major pest in New Jersey cranberry bogs where it produces two generations each year. Eggs are laid in compact, single-layered masses of a few to >300 eggs on the leaves of various plant species, and are parasitized by the endemic generalist egg parasite, Trichogramma minutum Riley. In preparation for an augmentative biological control program using T. minutum, we assessed egg-mass variability and parasitism rates in a series of unmanipulated field populations. We examined 193,950 eggs from 1,789 egg masses on 17 plant species, which represented 12 samples taken from three bogs (one sample per generation per bog each year for 2 yr). Egg-mass size, shape, and density (number of eggs per unit area within masses) differed across samples and plant species. Egg masses on narrow-leafed species (e.g., redroot, swamp candles, and grasses) tended to be longer and narrower than those on broad-leafed species (e.g., brier, red maple, leatherleaf, marsh Saint John’s wort, and marsh fern). Relatively few eggs per mass on certain plant species (e.g., marsh fern) indicate possible host preferences, whereas unusually high (e.g., grasses, sweet pepperbush) or low (e.g., marsh fern) densities indicate additional host plant influences on egg mass structure. Overall, T. minutum parasitized 4,309 eggs (2.2%) from 271 egg masses (15.1%) on 12 plant species, which included the 11 most common fireworm host plants. Parasitism rates for egg masses varied from 1.6 to 44.9% across samples, and only 11 parasitized masses were completely parasitized (4.1%). Parasitized masses had fewer eggs and lower density than nonparasitized masses, but did not differ in length, width, area, or circumference. T. minutum exhibited an apparent preference for masses on sweet pepperbush and marsh Saint John’s wort over those on leatherleaf. This preference was not fully explained by egg number or density, and might be associated with other differences among egg masses or host plants. These results indicate potentially complex tritrophic interactions that could have important implications for the development, monitoring, and success of biological control programs.