Effects of Water Mite Parasitism on the Demographics of an Adult Population of Ischnura posita (Hagen) (Odonata: Coenagrionidae)

Abstract

The demographics of an adult population of Ischnura posita (Hagen) (Odonata: Coenagrionidae) at a N Texas pond were studied using mark-recapture techniques. Twenty-three percent of marked females were recaptured vs. 20 percent of the males; the highest recapture rate was for mature females not parasitized by water mites. The average daily probability of survivorship ranged from 0.73 for mite-free mature females to 0.61 for mite-parasitized mature females. Water mites were present on one-half the population and were more frequent on males than females. The number of individuals displayed a marked peak during the study. This peak was coincident with the only observed cases of adult cannibalism. INTRODUCTION Ischnura posita is a small (body length: 18-29 mm) nearctic damselfly found throughout the eastern United States and Canada. It occurs in varied habitats including both lotic and lentic environments (Bick, 1957; Walker, 1953), and tends to be found at the margin of ponds and streams amidst dense emergent and peripheral vegetation. Unlike many adult damselflies, the aquatic environment does not serve it solely as a rendezvous site for copulation and oviposition, but as the ecological arena for virtually the entire life span of both sexes. Coenagrionid damselflies are often parasitized by water mites. These mites attach during the damselfly's nymphal stage and migrate from the exuvia to the adult during emergence. They attach primarily to the ventral surfaces of the insect's thorax and abdomen, and may often be found on an individual in extraordinary abundance. Utzeri (1980) reviewed reported cases of adult cannibalism in Zygoptera and suggested that it is the result of a situation where intraspecific communication fails. There is no published evidence indicating density-dependent adult cannibalism. STUDY SITE The study site is a 0.3-ha pond located in Veterans Park, Arlington, Texas. The pond is 127 m long by 36 m wide and has a mean depth of 1.6 m. It is surrounded by the following plant species: Eclipta alba, Juncus diffusissimus, Ambrosia trifida, Paspalum sp., Eleocharis sp. and scattered Salix sp. The dominant emergent plants are Typha sp. and Ludwigia peploides. The nearest aquatic site is approximately 0.4 km away and does not support a population of Ischnura posita. Ischnura posita was the most abundant zygopteran species present during the period of this study. Other zygopterans collected were Anomalagrion hastatum, Enallagma basidens, E. traviatum, Telobasis salva and Argia apicalis. METHODS Ischnura posita were collected daily from 21 May 1980 to 22 June 1980 (except 25 May). Individuals were uniquely marked on their wings with indelible ink immediately after capture and released near the point of their capture. Data regarding the individual's sex, its maturity and presence of mites were recorded. The presence of mites was determined visually without the aid of supplementary optics. The set of mite species was thus not completely determined. Samples of these mites indicate the presence of Hydrachna sp. Coloration was used to determine maturity of the damselflies.