Abstract
We examined oviposition decisions by Hippodamia convergens Guerin in semi-natural arenas in the laboratory. Gravid females were presented individually with an array of four young sorghum plants, Sorghum bicolor, bearing (1) no additional stimu- lus, (2) an established colony of greenbug, Schizaphis graminum Rondani, (3) residues of conspecific larvae, and (4) greenbugs plus residues of conspecific larvae. Females laid no egg masses on type 3 plants, significantly fewer than expected by chance on type 4 plants, and significantly more on type 1 plants, with type 2 plants receiving expected numbers. Females laid 50% of egg masses on elements of the arena other than the plants, especially the cage screen, suggesting that females sought to distance their eggs farther from larval residues than the spacing of plants in the arena permitted (15 cm). When the experiment was repeated with plants exposed to larvae of Coleomegilla maculata DeGeer, the repellency was weaker. Once again, clean plants were the most preferred and aphids did not increase the acceptability of plants with larval residues. Nevertheless, only 18% of egg masses occurred off the plants and larval residues did not reduce the acceptability of aphid-bearing plants as did conspecific larval residues. Simultaneous choice tests conducted with individual third instars of both species revealed that C. maculata larvae consumed H. convergens eggs as readily as conspecific eggs, but H. convergens larvae preferred conspecific eggs to those of C. maculata. We conclude that H. con- vergens oviposition decisions are shaped by the risks of both egg cannibalism and predation.
Highlights
Most aphid species typically exhibit an aggregated distribution on their host plants (Wratten, 1974; Pennachio & Tremblay, 1986; Michaud, 1999) and this is true for cereal aphids such as the greenbug (Kring & Gilstrap, 1983)
Gravid female H. convergens did not lay any egg masses on sorghum plants that were contaminated with residues of conspecific larvae and lacked aphids (Fig. 1)
Plants bearing aphids and residues of conspecific larvae received only three egg masses, and two of these were laid by the same female
Summary
Most aphid species typically exhibit an aggregated distribution on their host plants (Wratten, 1974; Pennachio & Tremblay, 1986; Michaud, 1999) and this is true for cereal aphids such as the greenbug (Kring & Gilstrap, 1983). The clustered egg masses of aphidophagous coccinellids are prime targets for both cannibalism and intraguild predation (Mills, 1982; Schellhorn & Andow, 1999a; Gagne et al, 2002; Cottrell, 2005), so adult females must select oviposition sites that afford their offspring sufficient resources to complete development, and minimize their exposure to predictable sources of mortality. Some of the most intensive studies on coccinellid oviposition responses to larval residues (RĤžiþka, 1997, 2001, 2002, 2003) all employed ten females at a time in experimental arenas This is a possible cause for concern because studies examining both aphid parasitoids (Michaud & Mackauer, 1995) and coccinellid predators (Hemptinne et al, 1993) have demonstrated that oviposition behavior can be strongly influenced by contact with conspecific females. We assessed the deterrency of both conspecific (H. convergens) and heterospecific (C. maculata) larval residues on coccinellid oviposition behavior using solitary H. convergens females foraging on plants in semi-natural arenas, both in the presence and absence of aphid prey
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