Brandon Lingbeek, 2007

Calvin College

Why does the population of the specialist herbivore, Lema daturaphila, decline as its host plant, Physalis longifolia, becomes more abundant?

The three-lined potato beetle, Lema daturaphila, is a specialist herbivore of the plant Physalis longifolia, and the population of this beetle declines drastically by the middle of the summer. The availability of food is known to limit the population size of many species, but the population of Lema daturaphila decreases as the amount of its host plant increases. A total of nine experiments were performed in order to determine why the population of Lema daturaphila declines. Two experiments testing the genotypic preference of adult Lema daturaphila were performed to determine if some genotypes were unsuitable host plants. A larval survival test was performed to see if some plant genotypes better supported the development of Lema daturaphila. Larval development on plants that had previously been defoliated was studied to determine if induced defensive compounds reduce secondary colonization of Physalis plants later in the summer. Two experiments were performed to determine if Lema daturaphila prefers Datura stramonium to Physalis longifolia and shifts away from Physalis later in the summer, when Datura becomes abundant. Potential predators were released into enclosures along with Lema daturaphila to determine who the most effective predators are, and two experiments testing the effect that all possible predators have on the Lema daturaphila population were performed. The results showed that Lema daturaphila does not prefer particular host plant genotypes, but some genotypes appear to better support larval development. Secondary metabolites do not reach fatal concentrations after defoliation, and Lema daturaphila strongly prefers to use Physalis longifolia over Datura stramonium as a host plant. Predators appear to have a small affect in reducing the population of Lema daturaphila in June, but very strong effects by late July: In July, no unprotected larvae survived to adulthood, while most larvae protected from large invertebrate predators were parasitized by tachinid flies. Thus, mid summer predation and parasitism seem to play stronger roles in limiting Lema daturaphila populations on Physalis longifolia than plant chemistry, plant genetics, or insect host plant preference.

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