Food Chains

Although cockroaches generally feed on dead plant and animal material, they are also well known as primary consumers. Many blattids in tropical forests are cryptic herbivores and some are overtly herbivorous, particular on young vegetation (Chapter 4). Roth and Willis (1960) were surprised that the role of cockroaches as plant pests is rarely discussed, and detailed the abundant records of the phenomenon in the literature. Most of the evidence comes from commercially grown crops, particularly in the tropics and in greenhouses. One field study, however, found that the frequency of herbivore damage on new leaves in rainforest canopy (Puerto Rico) was significantly correlated with the abundance of Blattaria (Dial and Roughgarden, 1995). It is therefore possible that cockroaches may have an undocumented but significant ecological and evolutionary impact on vascular tropical flora, as well as on nonvascular plants in the phylloplane.

At the next level of the food chain, cockroaches are prey for numerous taxa, including pitcher plants (Sarracenia and Nepenthesspp.) (Roth and Willis, 1960) and a variety of invertebrate and vertebrate predators (Fig. 10.4). The principal food of the grylloblattid Galloisiana kurentzovi in East Asia is juveniles of Cryptocercus relictus (Storo-zhenko, 1979), and small blattellid cockroaches climbing on low vertical twigs and grass blades constitute 92% of the prey of the Australian net-casting spider Menneus unifasciatus (Austin and Blest, 1979). In desert sand dunes of California, Arenivaga investigata makes up 23% of the prey biomass taken by the scorpion Paruroctonus mesaensis (Polis, 1979). Examination of the excrement of the South American frog Phyllomedusa iheringii indicates that cockroaches are a major part of its diet (Lagone, 1996). Blattellid cockroaches of the genus Parcoblatta are a high proportion of the menu of endangered red-cock-aded woodpeckers (Picoides borealis) in the Coastal Plain of South Carolina (Horn and Hanula, 2002). Cockroaches were consistently taken by all observed birds, made up 50% of the overall diet, and were 69.4% of the prey fed to nestlings (Hanula and Franzreb, 1995; Hanula et al., 2000). Pycnoscelus indicus on Cousine Island in the Seychelles is the favored prey of the endangered magpie robin (Copsychussechellarum) (S. Le Maitre,pers. comm. to LMR); the birds feed on American cockroaches as well. Attempts to control urban infestations of Periplaneta americana with toxic insecticides may have contributed to the decline of this species on Fr├ęgate Island. The birds feed close to human habitations and take advantage of dead and dying insecticide-treated cockroaches. Lethal doses accumulated in the birds, with subacute effects on their behavior. The current use of juvenile hormone analogs for cockroach control appears to result in good control of the pests while posing a negligible hazard to the birds (Edwards, 2004). These few examples (see Roth and Willis, 1960 for more) suffice to emphasize that in their role as prey, cockroaches may significantly influence the population structure of insectivores in terrestrial ecosystems. They may also be a link between terrestrial and aquatic food chains at river and stream edges, and in delicately balanced cave ecosystems. Cave-dwelling cockroaches accidentally introduced into water are one of the

Fig. 10.4 Scorpion feeding on the ground-dwelling cockroach Homalopteryx laminata, Trinidad. Photo courtesy of Betty Faber.

principal foods of some cavernicolous fishes; they are 26% of the diet of Milyeringa veritas (Humphreys and Feinberg, 1995). Cockroaches are considered the base of the food web in South African bat caves and support a large community of predators and parasites. Their feces are also an important food source for smaller invertebrates (Poulson and Lavoie, 2000). Hill (1981) noted that for most of the guano community in Tamana cave, Trinidad, the incoming supply of energy was in the form of cockroach, not bat, feces.

At the top of the food chain, there are numerous reports of cockroaches preying on other insects (detailed by Roth and Willis, 1960). Most of these accounts are observations of opportunistic predation on a broad range of vulnerable taxa and life stages, particularly eggs and larvae. Instances of cockroaches controlling prey populations of crickets and bedbugs in urban settings are frequent in the historic literature but largely anecdotal and unverified. One ecological setting in which cockroaches do have potential for influencing population densities of prey is in caves (Chapter 4).

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