Laboratory Rearing of Hydrometra australis (Hemiptera: Hydrometridae)

Abstract

The surface-dwelling aquatic insect Hydrometra australis Say (Hemiptera: Hydrometridae) is difficult to rear. Although most field-collected nymphs reared on a diet of Drosophila in the laboratory develop to the adult stage, mate, and reproduce successfully (Lanciani 1975), only a small percentage of individuals reared from the egg stage on this food reach adulthood. In previous laboratory rearing attempts using different container sizes, different substrata, and Drosophila as food, the maximum survival I have observed in individuals reared from egg to adult is 40%. The closely related species Hydrometra martini Kirkaldy has been cultured on Drosophila (Sprague 1956), but its survival rate was not given. Collembolans have been cited as a common food source of hydrometrids, e.g., H. martini (Sprague 1956) and Hydrometra stagnorum (Linnaeus) (Arrow 1895). In northern Florida, H. australis can be often seen in the field feeding on aquatic collembolans, which are abundant in many shallow aquatic habitats frequented by H. australis. These observations suggested a rearing method for H. australis using aquatic collembolans as food. Six female H. australis were collected from Lake Alice, Gainesville, Florida, U.S.A. on 17-IX-1989, placed individually in plastic petri plates lined with moistened absorbent paper, and fed 3 frozen Drosophila melanogaster Meigen per day. Eggs removed from these plates to establish a laboratory parental generation were placed individually in plastic vials (3 cm in diameter and 7 cm in height) that were half to three-fourths full of water. The water surface was completely covered with plants of 2 species of duckweed, Lemna minor Linnaeus and Spirodela polyrhiza (Linnaeus). A circular hole cut in each vial cap was covered with a piece of screen. The vials were placed in a constant-temperature chamber held at 28?C with a 12-h light-dark cycle. Approximately 20 or more live collembolans (Sminthurides sp.) of various sizes collected from a local temporary pond were added to each vial when the Hydrometra eggs hatched. (Collembolans were easily field-collected by slowly skimming the duckweed-covered pond surface with the edge of a plastic box.) As the number of collembolans in the vials dwindled, more were added so that prey were always abundant. Uneaten collembolans survived well, so contamination by death of excess prey did not occur. As individuals of the parental generation of H. australis reached the adult stage, females and males were paired. The first 27 eggs laid, which were from two females, were then placed individually in plastic vials to establish the subject generation. This generation was reared as was the parental generation, except that the vials were removed briefly from the constant-temperature chamber 4 times each day (within the periods 7-9a.m., lla.m.-lp.m., 4-6p.m., and 8-lOp.m.) to be checked for molts. Twenty-six of the 27 subject-generation eggs successfully developed to the adult stage on an exclusive diet of collembolans. The mean stadium of each of the 5 immature instars is listed in Table 1. These stadia are consistently shorter than those (Lanciani 1975) determined on Drosophila-fed individuals (Table 1; P < 0.001 for each instar, t-test). The comparisons of instars 2 to 5 are not ideal because in the Drosophila-fed group, data for instars 2 to 5 were obtained from field-collected instars 1 to 4, respectively, that were maintained in the laboratory through the next instar only. The Collembola-fed