Quantifying predator dependence in the functional response of generalist predators

A longstanding debate concerns whether functional responses are best described by prey-dependent versus ratio-dependent models. Theory suggests that ratio dependence can explain many food web patterns left unexplained by simple prey-dependent models. However, for logistical reasons, ratio dependence and predator dependence more generally have seen infrequent empirical evaluation and then only so in specialist predators, which are rare in nature. Here we develop an approach to simultaneously estimate the prey-specific attack rates and predator-specific interference rates of predators interacting with arbitrary numbers of prey and predator species. We apply the approach to field surveys and two field experiments involving two intertidal whelks and their full suite of potential prey. Our study provides strong evidence for the presence of weak predator dependence that is closer to being prey dependent than ratio dependent over manipulated and natural ranges of species abundances. It also indicates how, for generalist predators, even the qualitative nature of predator dependence can be prey-specific.Author contributionsCW contributed to method development, KC and IS performed the caging experiment, and MN conceived of the study, carried out the fieldwork and analyses, and wrote the manuscript.

Biological-Control of Tetranychus-Urticae (Koch) (Acari, Tetranychidae) in Southern New-South-Wales Peach Orchards - the Role of Amblyseius-Victoriensis (Acarina, Phytoseiidae)

Populations of mite predators and Tetranychus urticae (Tetranychidae) occurring in peach orchards on two farms at Leeton, New South Wales, were monitored during 1987/88. In both orchards the key pest Cydia molesta (Busck) (Tortricidae) was controlled by a pheromonal mating disruption strategy which avoided the use of broad-spectrum insecticides. During both years substantial populations of a native predatory mite, Amblyseius victoriensis were associated with an absence or very low populations of T. urticae from mid-summer until leaf fall. During spring 1987 some control of T. urticae was provided by a suite of predatory insects. Late spring applications of fungicide delayed development of A. victoriensis populations and a mid-summer miticide spray delayed establishment of the predator. Typhlodromus occidentalis occurred only when A. victoriensis populations were low or eradicated by sprays. During autumn, 1987 A. victoriensis occurred on orchard weeds but populations declined to a low level during winter. Substantial overwintering populations of reproductive T. urticae occurred on weeds and invaded the trees at bud burst. The generalist feeding behaviour of A. victoriensis is considered to be the key to the success of this predator. Dependence on biological control for T. urticae management is considered to be a practical option for southern New South Wales peach orchardists.