Source–sink Dynamics for a Generalist Insect Predator in Habitats With Strong Higher-Order Predation

The functional importance of higher-order predators in terrestrial ecosystems is currently an area of active inquiry. In particular, an understanding of the relative influences of prey availability and higher-order predation on predator populations is of immediate relevance to the theory of biological control of herbivorous arthropods. Biological control workers have repeatedly speculated that one cause of failure to establish predators and parasitoids in novel environments is the strong mortality imposed on released agents by higher-order predators. Nevertheless, the ability of higher-order predators to create a habitat where mortality exceeds natality (a “sink” habitat) has never been tested experimentally with a biological control agent in nature. Although in isolation the predatory lacewing Chrysoperla carnea can consistently produce strong suppression of populations of the aphid Aphis gossypii, the full community of predators when tested together exerts minimal aphid control. The age structure of Chrysoperla spp. populations in cotton fields harboring low to intermediate densities of aphid prey is characterized by a sharp drop in densities from the egg to the first larval instar; this observation is consistent with heavy mortality during either the egg or first larval stage. Egg cohorts followed under unmanipulated field conditions showed relatively high rates of successful hatch, suggesting that the vulnerable developmental stage is the young larva. Larval survival is relatively high in the absence of hemipteran predators, suggesting that prey availability is not the primary limiting factor. Depressed survival is observed in the presence of Geocoris spp., Nabis spp., and Zelus renardii, all common hemipteran predators in cotton. Predation on lacewing larvae appears to disrupt the strong top-down control of aphid populations in cotton. Chrysoperla spp. densities declined in fields harboring intermediate aphid densities when lacewing subpopulations were experimentally caged to block immigration and emigration. In one year (1993) Chrysoperla spp. densities fell to very low levels, suggesting that the field was either a true sink habitat or a pseudosink with a very low equilibrium density. In a second year (1994), densities declined to what appeared to be a lower but stable density, suggesting that the habitat was a pseudosink. Thus, in both years, declines in Chrysoperla spp. densities were observed following caging, suggesting that Chrysoperla spp. populations are spatially subsidized. Aphid prey availability and higher-order predation interacted strongly in their influence on C. carnea survival: larval survival in the presence of higher-order predators was 5.6% when prey availability was intermediate and 40.5% when prey were superabundant. Spatial heterogeneity in aphid prey densities modulates the intensity of higher-order predation and thereby appears to produce source–sink dynamics of Chrysoperla spp. in cotton fields.