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Project description

Genetic selection and behavior modification to circumvent differential susceptibility of eucalyptus longhorned borers to attack by the egg parasitoid, Avetianella longoi. (02XU027)
Program Exotic Pests and Diseases Research Program
Principal
investigators
J.G. Millar, Department of Entomology, University of California Riverside
T.D. Paine, Department of Entomology, University of California Riverside
Host/habitat Eucalyptus; Landscape Trees and Shrubs
Pest Eucalyptus Longhorned Borers Phoracantha semipunctata and P. recurva
Discipline Entomology
Review
panel
Urban Systems
Start year (duration)  2002 (Three Years)
Objectives Determine if the behavior of the egg parasitoid wasp, Avetianella longoi, can be altered to increase acceptance (parasitoid attack rate) of Phoracantha recurva longhorned borer eggs, via conditioning (associative learning) or genetic selection.

Determine the basis of variability in wasp virulence (survival), host preference and determine fitness costs (effects on size, fecundity and/or longevity) in maintaining virulence of the egg parasitoid A. longoi against P. recurva in the laboratory.

Determine the basis of variability in host resistance against the egg parasitoid A. longoi over time (seasonal, age-related), and fitness costs of maintaining resistance within a lab population of P. recurva beetles.

Determine the physiological/biochemical factors involved in the host/parasitoid resistance/virulence interaction by comparing parasitized and non-parasitized eggs of P. semipunctata to those of P. recurva.

Determine if parasitoids in southern California are naturally adapting to the use of P. recurva as hosts as these beetles replace P. semipunctata in the eucalyptus environment.

Project
Summary
The project goal is to develop more effective strains of the parasitoid, Avetianella longoi, for controlling the Eucalyptus longhorned borer Phoracantha recurva. This borer is relatively resistant to the parasitoid when compared with its congener, P. semipunctata. However, given no choice, wasps accept P. recurva eggs and the resulting progeny survive, with variable success. Sources of the variable host resistance and parasitoid virulence will be determined, using microscopy, histology, and biochemical analyses to identify morphological, physiological and developmental differences between developing wasps and eggs of the two beetle species. Wasp behavioral modifications and development of isofemale (genetic) lines of both the wasp and the beetle host will provide insight into the mechanisms of the host/parasitoid relationship, and provide the basis for development of more effective parasitoid strains. Field evaluations of wild pest and parasitoid populations will be conducted parallel with lab studies.
Final report Laboratory experiments were conducted to determine whether a biological control agent, Avetianella longoi, could be manipulated to improve its effectiveness against the Eucalyptus longhorned borer, Phoracantha recurva. This beetle host was found to produce an active cellular immune response which often killed the developing egg or larva of the wasp. Host eggs that were stung multiple times and were 24 to 48 hours old were less likely to survive parasitism than eggs that were stung once, or that were younger than 24 hours. Forced rearing of the wasp on P. recurva eggs over more than 20 generations did not significantly improve its parasitic success, or change its behavior toward the host. Even wasps that had been reared continuously for more than 20 generations, when given a choice, greatly preferred the susceptible host, P. semipunctata, remained highly variable in their stinging rates on P. recurva, and continued to have poor survival in P. recurva eggs. Furthermore, a large percentage of parasitized P. recurva eggs continued to survive and produce a beetle larva, further indicating that the wasp was not improving in its ability to commandeer the host egg.

Selective breeding of the beetle tended to yield larger and more fecund beetles, but there was no indication of increased or decreased resistance to the wasps. Biochemical studies of changes in the host egg after wasp attack indicated a number of protein differences, but it was unclear, in many cases, whether these proteins were parasitoid- or host-derived. Distinct differences became apparent only 72 to 96 hours post-parasitism. Also, there were distinct biochemical differences between the susceptible hosts, P. semipunctata, and resistant hosts, P. recurva, both pre- and post-parasitism.

Third-year
progress
The project goal is to gain a better understanding of the physiological and behavioral intricacies of the host/parasitoid relationship between the eucalyptus longhorned borer, Phoracantha recurva, and the egg parasitoid, Avetianella longo, in an effort to develop better biological control of this pest of eucalyptus trees. This borer is relatively resistant to the parasitoid, in contrast to a congener, P. semipunctata, the preferred host for A. longoi. Despite the development of several different genetic lines of the parasitoid, the plasticity in parasitoid behavior toward P. recurva remained. Superparasitism, the deposition of more than one parasitoid egg per host, was advantageous in the use of P. recurva as a host with the number of emerging wasps per P. recurva egg increasing with increased levels of superparasitism. Variability in oviposition behavior, particularly in very young or very old wasps, is still problematic. Therefore, we looked to the beetle as the source for host resistance.

We developed eight isofemale lines of the borer, and these genetic lines differed in their attractiveness to and suitability for parasitoids. The line also varied in vigor. Attempts to produce consistently resistant beetle lines have been unsuccessful. Limited testing of the resistant lines indicated that there was no overt resistance against the parasitoid because female wasps accepted and used these hosts to successfully produce progeny. The ability to produce resistant beetle eggs appears to be constant for the first three weeks of a beetle oviposition period. Older beetles tend to produce a greater proportion of infertile eggs that are more susceptible to parasitoid development than fertile eggs. In the time remaining on this project, we will concentrate on elucidating the biochemical differences between suitable (i.e., P. semipunctata) and more resistant (i.e., P. recurva) host eggs.

Second-year
progress
The project goal is to develop more effective strains of the parasitoid, Avetianella longoi, for controlling the Eucalyptus longhorned borer Phoracantha recurva. This borer is relatively resistant to the parasitoid in comparison to its congener, P. semipunctata. However, when given no choice, wasps will accept P. recurva eggs and the resulting progeny survive with variable success. The development of isofemale lines of this parasitoid and testing these genetic lines for host acceptance, host resistance, and parasitoid success has shown that attack rates (host acceptance) can increase with selection and breeding; however, there remains tremendous variability within the wasp populations. Without stringent genetic selection, it is doubtful that an effective strain of A. longoi parasitoids can be reared for sustained use against P. recurva. Individuals of the same line and generation differ greatly in their behavioral responses towards the resistant host. Many refuse to oviposit in P. recurva eggs, whereas others are much less discriminatory. Analyses of single female lines indicate that wasps can oviposit continually for up to three weeks but the success of parasitism dwindles with increasing age beyond 10 days. Typically, females will deliver only single eggs as they become older and these eggs are not successful. This is possibly due to depletion of some factor necessary to overcome the host egg's immune response against the parasitoid. Due to this wasp variability we will now focus on the beetle egg as the source for host resistance for the remainder of the funding period.

First-year
progress
This project has been operating since Jan. 23, 2003 when official notification of funding was received. Due to the current questionable status of the state budget, we thought it irresponsible to begin experiments any earlier. Very limited results have been obtained in this short period of operation and the data collected has not yet been analyzed. Individual female wasps of both strains have been exposed to successive oviposition bouts with P. recurva eggs. Isofemale lines of both strains have been established. Twenty lines (ten lines of each strain) were started. We currently have four healthy and strong lines of both strains (eight total lines) selectively reared on P. recurva. This result indicates that the use of P. recurva as a host remains highly detrimental to the majority of parasitoids and that genetic or behavioral variability still exists within the parasitoid colonies. The lines have been reared selectively for approximately five generations. Female wasps still require a three-day maturing period before they will attempt to oviposit into host eggs. This means that any mass release of parasitoids for biological control would be best served by allowing wasps to emerge, mate and feed for three days prior to release, particularly in an area where P. recurva exists. Data indicate that females can live up to three to four weeks under laboratory conditions and will oviposit continually during that time but at very low levels (one to two eggs/day), particularly late in life.

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