Biological control of the mealy plum aphid, Hyalopterus pruni Geoffrey

The mealy plum aphid, Hyalopterus pruni (Hemiptera: Aphididae) is a pest of prune in California. The impact of aphids as pests is governed by their population growth rate, a parameter summarizing the age-specific development, survivorship, and fecundity. We estimated these life history characteristics at five temperatures in the laboratory. A non-linear model gave a developmental threshold of 9.39°C and a lethal maximum temperature of 35.49°C. Population growth rates for H. pruni from these data and from additional field cage observations gave estimates in the range rm = 0.14-0.22.Aphidius transcaspicus is a parasitoid selected for use as a biological control agent against H. pruni in California. We examined the life history characteristics, host instar preference and functional response of this parasitoid. The intrinsic rate of natural increase at 22.3°C was estimated to be 0.21. Host instar preference for parasitoids attacking aphids on field collected prune leaves was for the intermediate instars. The 24 hour functional response to host density using sleeve cages in prune orchards was linear (Type I).One laboratory and two field based methods were compared as approaches to estimate the daily consumption rates by larvae of Harmonia axyridis and Chrysopa nigricornis the two most abundant predators of H. pruni in prune orchards. There was little difference between consumption rates estimated from field enclosure and laboratory arena methods, while field-based observations gave higher rates of consumption for both predator species. The merits of the different methods are discussed in relation to the distribution (colonial versus dispersed) of the prey and the mobility of the predator.We determined the degree to which naturally occurring densities of aphidophagous predators in prune orchards are responsible for the observed seasonal changes in mealy plum aphid densities. We used a simple model of aphid population growth and predation, field sampling data on aphid and predator densities, independent estimates of H. pruni population growth rates, and stage- and species-specific per capita predation rates estimated from field observations. We conclude that predator densities in prune orchards are seldom sufficient to account for the observed changes in mealy plum aphid densities.