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Project description

Prune Aphids in California: Introduction of Parasitoids and Quantification of Predation. (99BC001)
Program UC IPM competitive research grants program
N.J. Mills, ESPM Insect Biology, UC Berkeley
Host/habitat Prunes
Pest Mealy Plum Aphid Hyalopterus pruni; Leaf Curl Plum Aphid Brachycaudus helichrysi
Discipline Entomology
parasitoids; Aphidius spp.; Ephedrus spp.
Biological Controls
Start year (duration)  1999 (Three Years)
Objectives Acquire and produce insectary cultures of new exotic parasitoid species for the biological control of pest aphids on prune.

Field release and monitor the establishment of the exotic parasitoids on prune aphid populations on both prunes and summer host plants.

Quantify the impact of indigenous predators on mealy plum aphid populations in prunes, forming the basis of a forecasting system for inclusion of predation into management decisions.

Final report Traditionally, dormant sprays of organophosphates have been used to control invasive aphids as key pests in prune orchards. The goal of this project has been to reduce health risks to humans and wildlife from spray suspension in winter fog and contamination of major watersheds in the Central Valley from surface runoff during winter rains. This has been achieved through introduction and monitoring of exotic parasitoids in California, and elucidation of the impact and role of indigenous aphid predators.

Of two exotic parasitoids, Ephedrus persicaeand Aphidius colemani, introduced against the leaf curl plum aphid in the late 1990s, the latter species has become particularly widespread and abundant in orchard monitoring from 1999-2002. Leaf curl aphid damage in prune orchards has not been greatly reduced since the establishment of these parasitoids. Screening of numerous strains of the parasitoid Aphidius transcaspicus, collected from mealy aphids in Europe has shown tremendous variation in compatibility with mealy plum aphids in California. Strains collected from almonds in Israel and from plums and apricots in Spain proved incompatible, whereas a strain collected from almond in Spain in 2001 showed good compatibility both in glasshouse rearing and in sleeve cages in prune orchards. While extensive field releases of this latter strain were made during the 2002 field season, it is not yet known whether it has established in the Central Valley and what impact it will have on mealy plum aphid populations.

Mealy plum aphid infestations attract numerous indigenous predators the most important of which are Chrysoperla nigricornisand Harmonia axyridis. However, by combining monitoring of aphid and predator abundance in prune orchards with field estimates of aphid population growth and rates of predation for key predator species, we were able to demonstrate indigenous predators have little if any impact on the suppression of aphid infestations. As a result predator abundance should not be used for forecasting within season trends in mealy plum aphid populations in prune orchards.

Traditionally prune orchards have received dormant sprays of organophosphates for control of potential pests such as peach twig borer, mites, scales, and aphids. However, this practice has been questioned for two reasons; the health risks to humans and wildlife from spray suspension in winter fog, and contamination of major watersheds in the Central Valley from surface runoff during winter rains. The goal of this project is to establish exotic parasitoids in California from the region of origin of these aphids, and to monitor the impact of indigenous aphid predators.

During the 2001 field season leaf curl plum aphid colonies in the Sacramento valley were found to be infrequent and small, but consistently parasitized by Aphidius colemani and Ephedrus persicae, two parasitoids introduced during an earlier phase of this project. Although levels of parasitism were lower in 2001 than in 2000 the impact of these parasitoids appears to be reducing damage by this aphid in prune orchards. Four biotypes of the mealy plum aphid parasitoid, Aphidius transcaspicus, were collected from Spain in May 2001 from almond, apricot, peach, and plum. Field releases of an apricot biotype collected from Spain in 2000 proved unsuccessful, but initial field releases of a new almond biotype collected from Spain in 2001 led to substantial parasitism of mealy plum aphids in the release orchards. More substantial releases of this new biotype are planned for the 2001 field season.

During 2001 the population growth rate of mealy plum aphids was estimated in two prune orchards in the Sacramento Valley. These data were used together with earlier observations of the predation potential of key predators to estimate the impact of predation on mealy plum aphids in prune orchards. When aphid abundance is extreme early in the season, predation is not sufficient to be able to reduce aphid populations before they migrate to their summer host plants, but in more typical situations the impact of predation is important in reducing aphid abundance.

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