Ovigeny in Selected Generalist Predators

Abstract

Ovigeny refers to egg production and develop ment in adult female insects. Extremes in ovigeny yield a dichotomy: species where adults emerge with a fixed complement of mature eggs are pro ovigenic; whereas, species that display continued egg development during adulthood are (Heimpel & Rosenheim 1998). Over 98% of parasitoid species surveyed emerged with at least some immature eggs (Jervis et al. 2001). A synovi genic insect may experience short-term egg deple tion, though lifetime fecundity limitation should be rare (Carbone & Rivera 2003). Egg limitation has been demonstrated in the field, affecting host parasitoid population dynamics, possibly result ing in inverse density-dependent parasitism (Heimpel & Rosenheim 1998). Jervis et al. (2001) suggested that synovigenic species tend to be longer-lived, and that the early reproductive cost in pro-ovigenic species incurred a biological cost. Most research in ovigeny has been performed on parasitoids (mostly Hymenopterans), possibly because of the direct relationship between egg load and parasitism rates, and hence reproduc tion. However, ovigeny is a potentially unifying concept in entomology and should be applicable to all insect species (Jervis et al. 2001). Previous studies of ovigeny in insects other than parasi toids include those of Boggs (1997) in Lepidop tera. Further, Cyzenis albicans Fallen (Diptera: Tachinidae) and hoverflies in general (Diptera: Syrphidae) were determined to be synovigenic by Hassell (1968) and Gilbert (pers. comm., 1990), respectively. Legaspi & Legaspi (2004) found evi dence of strong ovigeny in Podisus maculiventris (Say) (Heteroptera: Pentatomidae) by comparing egg loads at different times during female adult hood, against lifetime fecundity in the laboratory. Limited evidence of ovigeny in P. maculiventris was also found in the field (Legaspi & Legaspi 2005). In this study, ovigeny was examined in 4 com mon generalist predators: P. maculiventris, Orius insidiosus (Say) (Hemiptera: Anthocoridae), Geo coris punctipes (Say) (Heteroptera: Geocoridae), and Delphastus catalinae (Horn) (Cole?ptera: Coccinellidae). Dissection and feeding methods for P. maculiventris are described in Legaspi & Legaspi (2004). For the O. insidiosus, G. punctipes and D. catalinae, 40 newly-emerged females of each species were selected for study and placed individually in 10-cm Petri dishes with a damp cotton ball and lined with filter paper. The Petri dishes were secured with a rubber band. A male was placed with each female throughout the ex periment except on day zero. Each Petri dish com prised 1 replicate. The 40 females were assigned to one of 4 dissection times (10 replicates each). Prey was provided ad libitum as: Bemisia tabaci (Gennadius) (Homoptera: Aleyrodidae) for D. cat alinae and G. punctipes; and Ephestia kuehniella Zeller eggs and pollen. Oviposition substrates for the predators were stringbean (Phaseolus vul garis L.) for O. insidiosus and tomato (Lycopersi con esculentum Mill. syn. Solanum esculentum L.) leaf cuttings for D. catalinae and G. punctipes. Four dissection times were determined based on the expected lifespan of each species. At the be ginning of the experiment (dissection time = 0), 10 females were dissected, and the numbers of ma ture and immature eggs were recorded. At each of the remaining 3 dissection times, 10 females were dissected and examined. At each designated dis section date, total numbers of eggs laid were re corded. Numbers of mature and immature eggs were counted upon dissection. Results of dissections in P. maculiventris (Fig. 1A) showed that the numbers of immature eggs increased with the predator age, whereas the numbers of mature eggs declined (Legaspi & Le gaspi 2005) providing a clear indication of contin uous egg production during the adult stage. In the other predators that we studied, egg loads at time zero were always zero and tended to increase with time, except in D. catalinae (Fig. IB). However, the numbers of eggs laid all increased with time. Total numbers of eggs laid were higher than total egg count recorded in dissections. In O. insidio sus, 71 eggs (7.7%) were counted in dissections, versus 857 laid (92.3%). Results were similar for D. catalinae: 7 eggs (5.8%) found in dissections and 113 laid (94.2%); and G. punctipes: 189 (13.8%) versus 1183 (86.2%). These findings sug gested that egg production occurred during the adult stage in all these predators and that all predators studied were synovigenic to various de grees. In the case of D. catalinae, low egg load was likely a consequence of oviposition. Despite the fact that most studies on ovigeny have been done on parasitoids, it is not surprising that synovig eny is also common in other groups, including predators. Egg load dissections underestimate re productive potential of various species and may not be good criteria in evaluating predators as po tential biological control agents.