Effects of Nonhost Plant on an Insect Herbivore in Diverse Habitats

Abstract

Vegetation texture (i.e., plant density, species diversity, and structural complexity) may influence the abundance of a herbivore by affecting its movement, altering the suitability of host plants, and by changing the herbivore's vulnerability to attack by natural enemies. We investigated these effects on the Mexican bean beetle (Epilachna varivestis) on snap bean (Phaseolus vulgaris) growing in high—and low—density monocultures and intercropped with short (0.5 m) or tall (1.2 m) corn plants (Zea mays). Beetle populations consistently reached higher densities on beans in monocultures than on beans intercropped with tall corn. Neither bean plant density nor plant species diversity alone significantly affected beetle abundance. Instead, the presence of tall corn plants contributed most to lower bean beetle density in the tall corn—bean intercrop, primarily by reducing adult colonization. However, the tall corn also increased the suitability of intercropped bean plants to the beetles because adults preferred to feed on and larvae developed faster on beans in the tall corn—bean habitat. Host plant effects were independent of the influence of microclimate and risk of predation. Abundance of natural enemies of the bean beetle was low and overall, predator density and bean beetle larval survival did not differ significantly among habitats. However, it is difficult to predict the effect or mortality factors, such as natural enemies, on bean beetle populations because larval mortality over time was affected by habitat type (i.e., there was a significant interaction between habitat type and time). Our results were partially consistent with the resource concentration hypothesis. Whereas Mexican bean beetles colonized bean monocultures more readily than they did tall corn—bean diculture, the presence of more suitable host plants in the diculture acted to increase, rather than decrease, bean beetle density in the tall corn—bean habitat. However, higher adult feeding preference and faster larval development on beans in the tall corn—bean habitat did not offset lower adult colonization in this habitat.

Relation between plant density and arthropod density in cabbage fields

“…Although this hypothesis predicts that the density of herbivores per host plant increases with increasing plant density, most of the experimental results in the literature contradict this prediction; the number of herbivores per plant is smaller in densely planted fields in most cases (Luginbill and McNeal 1958;Pimentel 1961;Way and Heathcote 1966;A'Brook 1968;Farrell 1976;Solomon 1981;Latheef and Ortiz 1983;Root and Kareiva 1984;Power 1987;Segarra-Carmona and Barbosa 1990; Thompson and Quisenberry 1995). Only a few experiments supported the resource concentration hypothesis (Ralph 1977;Turchin 1988) and several experiments did not detect consistent results (Mayse 1978;Bach 1980;Boiteau 1984;Power 1989;Coll and Bottrell 1994). Therefore, it is valid to adopt the opposite hypothesis, which may be called the 'resource diffusion hypothesis': herbivores use more efficiently hosts that are diffused (i.e., sparsely distributed).…”

Can cruciferous agroecosystems grown under variable conditions influence biological control ofPlutella xylostella(Lepidoptera: Plutellidae)?

“…On the other hand, insect pest damage tends to be lower in diverse agroecosystems (Andow 1986;Coll and Bottrell 1994). Root (1973) has proposed two hypotheses to explain lower pest problems in diverse agroecosystems.…”

Testing the enemies hypothesis in forest stands: the important role of tree species composition

“…what densities these organisms can reach in monocultures and polycultures. Studies in agro-ecosystems have indicated that populations of herbivorous insects generally reach higher levels in simple systems than in diverse ones (Pimentel 1961;Coll and Bottrell 1994). Root (1973) proposed two explanations for this pattern, which are not mutually exclusive: (1) the enemies hypothesis, which states that predators and parasites are more effective in diverse systems than in simple ones; and (2) the resource concentration hypothesis, according to which specialist herbivores more easily find, stay in, and reproduce in simple systems of their host plants.…”