Population Dynamics of a Relatively Rare Species of Milkweed Beetle (Labidomera)

Abstract

Labidomera clivicollis (Coleoptera: Chrysomelidae, Doryphorini), the milkweed leaf beetle, is relatively rare in New York State, although one of its host plants (Asclepias syriaca) is abundant. A related species, Leptinotarsa decemlineata, is an economic pest of potatoes. A comparison of the population dynamics of Labiodomera with previously published information on L. decemlineata suggests that the major differing factor controlling the two species' relative abundance is the intensity of predation to which the larvae are exposed. Predation on the young Labidomera larvae on the foliage of the host plants was the most important mortality factor in field experiments. Newly hatched beetle larvae were placed on 16 A. syriaca plants under natural conditions, in one of four treatment: (1) cage over the plant, with an open bottom; (2) sticky—trap barrier and groundcloth around the plant's base; (3) both barrier around base and cage over plant; and (4) control plants. The mortality could then be separated into four components: abiotic mortality, ground—dispersed predation, air—dispersed, and spider predation—plus—cannibalism. The rate of larval mortality on the control plants was high, averaging 35% per day; all of the 112 control larvae (on four plants) died by day 12 of the experiment. Ground—dispersed predation alone accounted for 54% of the total mortality. Important predators included Nabidae, Pentatomidae and Syrphidae (larvae); they were generalized, not specialized, arthropod predators. Another field experiment investigated the effect of different clutch sizes on larval mortality rates, with and without sticky barriers. The protective effect of the barriers, and hence the important of ground—dispersed predators, was confirmed in the second experiment. These results suggest that the beetle's preference for swamp milkweed for oviposition may be adaptive to avoid ground—dispersed predation. Swamp milkweeds frequently grow in marsh habitats with water around the base of the plants. Larvae on these plants would be protected by a "moat effect." A review of the literature on field experiments with Leptinotarsa suggests that an analogous "bare ground effect" protects the larvae on cultivated potatoes from effective predation.