Increased Offspring Viability

McKittrick (1964) was of the opinion that the burial and concealment of oothecae by oviparous females is a response to pressure from parasitoids and cannibals. Although few studies directly address this question, some evidence suggests that concealing oothecae may attract rather than deter hymenopterous parasitoids. The mu-copolysaccharides in the saliva used to attach egg cases to the substrate may act as kairomones, making oothecae more vulnerable to attack. Parasitic wasps may even expose buried oothecae by digging them out from their protective cover (Narasimham, 1984; Vinson and Piper, 1986; Benson and Huber, 1989). On the other hand, oothecae of P. fuliginosa that were glued to a substrate had a higher eclosion rate than those that were not glued, suggesting that salivary secretions may enhance egg viability in some unknown way (Gordon et al., 1994). Oothecae of

Fig 7.8 Parasitism of cockroach eggs. (A) Anastatus floridanus ovipositing into an ootheca carried by Eurycotis floridana. (B) Detail of oviposition by the parasitoid. Photos by L.M. Roth and E.R. Willis.

oviparous cockroaches are also prone to parasitism prior to deposition, while females are forming and carrying them. The window of vulnerability can be a wide one. Females of Nyc. acaciana, for example, can take 72 hr to form an ootheca (Deans and Roth, 2003). The parasitoid Anastatus floridanus (Eupelmidae) oviposits in egg cases attached to female Eur. floridana (Fig. 7.8) (Roth and Willis, 1954a). The cockroach can detect the presence of the wasp on the surface of the ootheca and tries to dislodge it with her hind legs (LMR,pers. obs.). Blattella spp. that carry egg cases externally until hatch are also vulnerable to egg parasitoids, and continue to carry the parasitized ootheca (Roth, 1985). External retention of egg cases, then, may be little better than concealment in conferring protection from parasitism.

The value of egg case burial lies primarily in protecting them from predation and cannibalism; concealment is almost 100% effective in saving oothecae from being devoured by other cockroaches (Rau, 1940). McKittrick et al. (1961) found that in Eur. floridana, burial of oothecae prevented cannibalism by conspecifics and predation by ants, carabids, rodents, and other predators. Conversely, exposed egg cases and those still attached to a female are subject to biting and cannibalism (Roth and Willis, 1954b; Willis et al., 1958; Gorton, 1979). These improprieties are countered with aggression on the part of the mother. Female P. brunnea, P. americana, and Paratem-nopteryx couloniana drive other females away from exposed oothecae (Haber, 1920; Edmunds, 1957; Gorton, 1979). Two behavioral classes of female can be distinguished in B. germanica; females carrying oothecae are more aggressive than females that had not yet formed them (Breed et al., 1975). Aggressive behavior is favored despite its attendant risks, given that one nip taken from an ootheca can result in the death of the entire clutch from desiccation (Roth and Willis, 1955b).

Ovoviviparity is viewed as a solution to this constant battle against predators and parasites, and is thought to have appeared in the Mesozoic as an evolutionary response to cockroach enemies that first appeared during that time (Vishniakova, 1968). Parasitoids have not been detected in the oothecae of ovoviviparous blaberids (LMR, pers. obs.). The eggs are exposed to the environment for only the brief period of time between formation of the ootheca and its subsequent retraction into the body, allowing only a narrow time frame for parasitoid oviposition. Once in this enemy free space, the eggs are subject only to "the vicissitudes that beset the mother" (Roth and Willis, 1954b). Nonetheless, nymphs of ovovi-viparous cockroaches are at risk from cannibalism at the time of hatch. Attempts by conspecifics to eat the hatch-lings as the female ejects the ootheca have been noted and may include pulling the still attached egg case away from the mother (Willis et al., 1958). We note, however, that laboratory observations of cannibalism in cockroaches of any reproductive mode may be of little consequence in natural populations, with the exception of highly gregarious species like cave dwellers. Females of at least one species of the latter are known to be choosy about where they expel their neonates. Darlington (1970) reported that pregnant females of Eub. posticus preferred one chamber of the Tamana cave for giving birth, and migrated into that chamber from other parts of the cave. Defense against pathogens as agents of egg mortality is unstudied, despite the disease-conducive environments typical of cockroaches.

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