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Project description

Hyperparasitoids and predators, two potential biotic factors disrupting the biological control of the walnut aphid in California. (04XA008)
Program Exotic Pests and Diseases Research Program
Principal
investigator
N.J. Mills, Insect Biology, UC Berkeley
Host/habitat Walnuts
Pest Walnut Aphid Chromaphis juglandicola
Discipline Entomology
Beneficial
organism
Trioxys pallidus; Syrphophagus aphidivorus
Review
panel
Agricultural Systems
Start year (duration)  2004 (Three Years)
Objectives Field sampling of the seasonal activity of Chromaphis juglandicola and its parasitoid Trioxys pallidus in relation to hyperparasitism and mummy predation in orchards where control has been sustained and in orchards in which control has recently been lost.

Laboratory determination of the host instar preference of T. pallidus and its host patch exploitation behavior in response to increasing host density and the presence/absence of Syrphophagus aphidivorus, a key hyperparasitoid.

Laboratory determination of the host age preference of the hyperparasitoid S. aphidivorus and its host patch exploitation behavior in response to increasing host density.

Project
Summary
The walnut aphid Chromaphis juglandicola is an invasive species originating from the Middle East, and known in California for more than a century. The successful introduction of the parasitoid Trioxys pallidus from Iran in 1969 has provided sustained control of this devastating pest for more than 30 years. However, within the last five years, localized outbreaks of walnut aphid have occurred in the Central Valley. We propose to investigate the potential role of hyperparasitoids and predators in the recent failure of this biological control program, through seasonal monitoring of hyperparasitoid and predator activity, and laboratory investigations on the indirect effects of hyperparasitism.

Final report The walnut aphid is an invasive species originating from the Middle East. The successful introduction of a parasitoid from Iran has provided sustained control of this devastating pest for more than 30 years. However, within the last eight years, localized outbreaks of walnut aphid have occurred in the Central Valley.

This project investigated the potential role of hyperparasitoids (parasites that attack the original parasitoid from Iran) and predators in the recent failure of this biological control program through seasonal monitoring of their activity in walnut orchards, and laboratory investigation of the presence of hyperparasitoids on the search behavior of the walnut aphid parasitoid.

Walnut aphids remained below the economic threshold in all the orchards sampled during 2004 to 2006, except one. As sampled orchards were selected based on their recent history of aphid outbreaks, this showed that walnut aphid outbreaks are not consistent from one year to another. Levels of hyperparasitism and predation of aphid mummies were on occasion very high, potentially compromising the ability of the parasitoid to provide effective control. However, further analysis suggested that predation on mummies is unlikely to compromise the parasitoid in controlling walnut aphid populations.

Investigations in the laboratory revealed an indirect influence of hyperparasitism in addition to parasitism of the walnut aphid parasitoid. Aphid parasitoids spent significantly more time on walnut leaflets in the presence of hyperparasitoids without increasing the number of walnut aphids parasitized. This behavioral change is unusual because the aphid parasitoids wasted time with no increase in their lifetime reproduction. Aphid parasitoids did not show the same response in the presence of another competing female, and, therefore, the hypothesis that they may be confused by the presence of a hyperparasitoid and respond inadequately by treating them as competitors was not supported.

Third-year
progress
The successful introduction of the parasitoid Trioxys pallidus in California from Iran in 1969 has provided sustained control of walnut aphid, Chromaphis juglandicola, for more than 30 years. However, within the last seven years, localized outbreaks of walnut aphid have occurred in the Central Valley. This project investigates the potential role of hyperparasitoids and predators in the recent failure of this biological control program. No walnut aphid outbreaks were observed in the orchards selected for sampling in 2006, confirming that walnut aphid outbreaks are not consistent from one year to another.

Overall, hyperparasitism rates were moderate in 2006 (26%-55%) although occasionally reaching 100%. Overall mummy predation was high (45-79%), seasonally reaching 100%. Investigations in the laboratory showed that T. pallidus females preferred to attack 4th instar aphids even though they were not the most suitable in supporting successful parasitism. T. pallidus females spent more time in the presence of hyperparasitoids—the two species tested were A. suspensus and S. aphidivorus—than when foraging alone. This unexpected behavioral response of T. pallidusto the presence of hyperparasitoids seems to be maladaptive because T. pallidus females waste time and eggs with no significant increase in lifetime reproduction. We are currently investigating a possible explanation for this behavior.

Second-year
progress
The walnut aphid, Chromaphis juglandicola, is an invasive species originating from the Middle East, and known in California for more than a century. The successful introduction of the parasitoid Trioxys pallidus from Iran in 1969 has provided sustained control of this devastating pest for more than 30 years. However, within the last five years, localized outbreaks of walnut aphid have occurred in the Central Valley. This project investigates the potential role of hyperparasitoids and predators in the recent failure of this biological control program. No walnut aphid outbreaks were observed in the orchards selected for sampling in 2005 showing that walnut aphid outbreaks are not consistent from one year to another. Investigations in the laboratory showed that T. pallidus females preferred to attack 3rd and 4th instar aphids. T. pallidus females spent more time in the presence of hyperparasitoids than when foraging alone. The two species tested were A. suspensus and S. aphidivorus. This unexpected behavioral response of T. pallidus to the presence of hyperparasitoids seems to be maladaptive because T. pallidus females waste time and eggs with no significant increase in lifetime reproduction.

First-year
progress
The walnut aphid, Chromaphis juglandicola, is an invasive species originating from the Middle East and known in California for more than a century. The successful introduction of the parasitoid Trioxys pallidus from Iran in 1969 has provided sustained control of this devastating pest for more than 30 years. However, within the last five years, localized outbreaks of walnut aphid have occurred in the Central Valley. This project investigates the potential role of hyperparasitoids and predators in the recent failure of this biological control program. Field monitoring of hyperparasitoid and predator activity will start in May 2005. Laboratory investigations on the host instar preference of T. pallidus are under investigation as a first step in the study of the indirect effect of hyperparasitism on the primary parasitoid T. pallidus. Preliminary results show that T. pallidus females prefer to attack larger rather than smaller instars.

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