Chapter 11. Host Preference Testing for Parasitoids of a Eucalyptus Borer in California

ASSESSING HOST RANGES OF PARASITOIDS AND PREDATORS ______

CHAPTER 11. HOST PREFERENCE TESTING FOR PARASITOIDS OF A EUCALYPTUS BORER IN CALIFORNIA

T. D. Paine,1 J. G. Millar,1 and L. M. Hanks2 1Department of Entomology, University of California, Riverside, California [email protected] 2Department of Entomology, University of Illinois, Urbana, Illinois

DESCRIPTION OF PEST INVASION AND PROBLEM Of the more than 700 species in the genus Eucalyptus L’Heritier native to Australia and New Guinea, approximately 90 species have been introduced into North America over the last 150 years (Doughty, 2000). Eucalyptus trees were first propagated in California from seed brought from Australia. Insect pests and diseases associated with living trees were not introduced with the seeds. As a result, the trees growing in California were relatively free of pests until the last two decades of the twentieth century (Paine and Millar, 2002). Phoracantha semipunctata (Fabricius) (Coleoptera: Cerambycidae) is native to Australia but has been accidentally introduced into virtually all of the Eucalyptus-growing regions of the world, including California, and is causing significant tree mortality in many of those areas (Paine et al., 1993, 1995, 1997). The beetles are attracted to volatile chemical cues produced by downed Eucalyptus and Angophora Cav. trees, broken branches, or standing stressed trees that are suitable larval host material (Chararas, 1969; Drinkwater, 1975; Ivory, 1977; Gonzalez- Tirado, 1987; Hanks et al., 1991). After mating on the bark surface, females oviposit under loose, exfoliated bark. The neonate larvae mine through the outer bark and feed in the nutri- tious inner bark, cambium, and outer layers of xylem (Hanks et al., 1993). Fresh host material that has a moisture content below a critical threshold (Hanks et al. 1991, 1999) is most suitable for larval development. Freshly cut logs attract more oviposition than aged logs (Paine et al., 2001), which is consistent with observation that the adults locate available sites for oviposition by olfactory cues (Hanks et al., 1991). As logs dry, the emission of volatile cues declines (Hanks et al., 1998). In addition to the reduction in volatile emissions, the moisture content of the wood decreases with age and this reduces the quality of the host material for larval development (Hanks et al., 1993). Larvae feeding on poor quality host mate-

138 Chapter 11. Host Preference Testing for Parasitoids of a Eucalyptus Borer ______ASSESSING HOST RANGES OF PARASITOIDS AND PREDATORS

rial have prolonged development and smaller adult size (Hanks et al., 1995). The prolonged development time exposes the larvae to increased risks of parasitism (Paine et al., 2001).

TESTING RATIONALE FOR AGENTS INTRODUCED Although P. semipunctata has caused significant mortality of eucalypt trees in areas of the world where it has become established, the insect has not been a significant problem in Australia (Duffy, 1963). Mortality caused by a guild of natural enemies in Australia (Austin et al., 1994; Austin and Dangerfield, 1997) may be critical in regulating the population and limiting its pest status. Of the parasitoids present in Australia, the encyrtid egg parasitoid Avetianella longoi Siscaro and the braconid solitary larval parasitoid Syngaster lepidus Brullè have been introduced into California.

THE EGG PARASITOID AVETIANELLA LONGOI SISCARO This egg parasitoid arrives on trees that are attractive to ovipositing P. semipunctata adults (Hanks et al., 1996) and lays its eggs in beetle eggs that are less than 24 hours old (Luhring et al., 2000). Since the arrival of the host beetle and parasitoid are virtually simultaneous, we expect that female wasps use cues similar to those that help adult beetles locate trees suitable for oviposition and larval development. The beetle’s eggs are found in a specific microhabitat under exfoliated bark or tight crevices. Once on the host tree, female parasitoids conduct localized searches to find beetle egg masses, perhaps in response to both mechanical and odor cues. While it would have been possible to conduct formal laboratory tests with various nontarget host species found in California, in this system it is clearly the attraction of the parasitoid to the eucalypt habitat that shapes the host contacts of the parasitoid. Eucalyptus species are all introduced species in North America and are one of only a few groups of myrtaceous plants in California. Munz (1968) lists no members of this family as native to the state. There are very few, if any, species of herbivorous arthropods North American arthropods that have shifted onto Eucalyptus species as hosts. This fact strongly suggests that the risk of this parasitoid encountering alternative hosts is extremely low because of the parasitoid’s strong attraction to Eucalyptus before searching for host eggs. Consequently, no formal host range testing was done prior to this species’ release in California.

THE LARVAL PARASITOID SYNGASTER LEPIDUS BRULLÈ The larval parasitoid S. lepidus (Figure 1) also uses char- acteristics of the host tree to find P. semipunctata larvae in suitable stages of development. Female parasitoids are probably attracted to the plant host from a distance by volatiles emanating from the degrading/ drying plant tissue. In studies conducted in Australia, levels of parasitism were significantly greater in fresh logs than logs that had been aged (Paine et al., 2001). Fresh logs, even after the 10-day period for larval de- Figure 1. Syngaster lepidus Brullè. Photo: Don Hwan Choe. velopment, may have been a better source of those (UGA1295001)

Chapter 11. Host Preference Testing for Parasitoids of a Eucalyptus Borer 139 ASSESSING HOST RANGES OF PARASITOIDS AND PREDATORS ______

volatiles than old logs (Hanks et al., 1998). This response to volatiles produced relatively early in the process of colonization is particularly important for S. lepidus because females of this species prefer to use significantly smaller beetle larvae as hosts than do other species of the parasitoid guild (Paine et al., 2000). Short-range cues for location of individual larvae of subcortical beetles by parasitoid females may be a combination of sound and substrate vibration (Ryan and Rudinsky, 1962; Mills et al., 1991). Once on the larval host, it is probable that female S. lepidus use a combination of chemical and physical cues (sonic cues or surface vibrations) to detect and evaluate the size of beetle larvae beneath the bark. Phoracantha semipunctata larvae feed on both the hard outer xylem tissue and the softer inner bark, producing both perceptible surface vibrations and sounds audible to the human ear for several meters. Adult wasps may perceive these vibrations through either tarsal or antennal contact with the substrate (Hanks et al., 2001). Females respond to the larval stimuli and allocate the sex of their offspring directly in relation to the size of the host larvae; male eggs are oviposited on small host larvae, while female eggs are allocated to the largest larvae (Joyce et al., 2002). Host range testing of the larval parasitoid was undertaken to evaluate risk to a threatened insect, the valley elderberry longhorned beetle (Desmocerus californicus dimorphus Fisher). This cerambycid colonizes elderberry, Sambucus spp. (Caprifoliaceae), in riparian habitats in the California Central Valley. The larvae of the elderberry beetle mine the interior wood of the host plant during their 1-2 year larval life cycle. The plant host families and the larval habitats within the host trees of P. semipunctata and D. californicus are completely different, making a comparison for host testing very difficult. Also, because D. californicus has a protected status, it was virtually impossible to obtain speci- mens for laboratory testing. Consequently, we designed a study that tested the ability of S. lepidus to identify and use its correct host if it was found within the plant host of the threatened insect species. We hypothesized that the parasitoid relied on host plant volatiles to locate the correct larval host habitat, and if there was a volatile stimulus from the larvae that was associated with the correct host habitat, then it would be present in both the Sambucus and Eucalyp- tus treatments tested in the experiment. Fresh elderberry branches were collected from the field, split open enough to expose the central pith, P. semipuntata larvae of a size preferred for oviposition by S. lepidus were placed inside, and the branches were sealed. These infested branches were placed into a bioassay cage with Eucalyptus logs infested with similar sized larvae and, as a control, a section of polyvinylchloride pipe. All assay items were approximately the same size. Female wasps were introduced into the assay cage and their positions were re- corded at regular intervals. The results were unequivocal. They clearly demonstrated that there were no significant differences in landing and searching on either the wrong host (elderberry) or the plastic pipe. Landing on either elderberry or plastic pipe was confined to occasional periods spent resting. More importantly, the wasps spent significantly more time landing and searching in the correct host habitat than either of the other choices. There were no oviposition attempts on the P. semipunctata larvae in the elderberry branches, but test parasitoids did suc- cessfully oviposit on the P. semipunctata larvae in the Eucalyptus host logs.

140 Chapter 11. Host Preference Testing for Parasitoids of a Eucalyptus Borer ______ASSESSING HOST RANGES OF PARASITOIDS AND PREDATORS

CONCLUSIONS Our test results indicate the important role of the host’s habitat and the sequence of parasitoid behaviors that are used to locate individual hosts. The parasitoids orient initially to the stimuli associated with the habitat in which the host life stages are most likely to be found. Once within the habitat, the searching females may use different cues to locate the microenvironment of the host. In the Eucalyptus host plant system in North America, there are very few native plants that are closely related to eucalypts. Consequently, there are few host plants that produce similar stimuli that would attract searching parasitoids. Also, native herbivores are rarely found feeding on Eucalyptus plants. Since there have been no host plant shifts in more than 150 years, there are few opportunities for specialist introduced parasitoids searching the correct host to attack a nontarget native insect. While it is impossible to prove that an event will not happen, the lack of related plants, the specificity of the herbivores, and the searching behavior of the parasitoids suggest that the risk to native species is extremely low.

REFERENCES Austin, A. D., D. L. J. Quicke, and P. M. Marsh. 1994. The hymenopterous parasitoids of eucalypt longicorn beetles, Phoracantha spp. (Coleoptera: Cerambycidae) in Australia. Bulletin of Entomological Research 84: 145-174. Austin, A. D. and P. C. Dangerfield. 1997. A new species of Jarra Marsh and Austin (Hy- menoptera: Braconidae) with comments on other parasitoids associated with the eucalypt longicorn Phoracantha semipunctata (F.) (Coleoptera: Cerambycidae). Australian Journal of Entomology 36: 327-331. Chararas, C. 1969. Étude biologique de Phoracantha semipunctata F. (Coléoptère Cerambycidae) xylophage specifique des Eucalyptus en Tunisie et recherches sur la vitalité et l’adaptation de ces essences. Académie D’Agriculture de France, C.R. 55: 47-57. Drinkwater, T. W. 1975. The present pest status of eucalyptus borers, Phoracantha spp., in South Africa, pp. 119-125. In: Anon. Proceedings of the 1st Congress of the Entomological Society of Southern Africa, 1975. Entomological Society of Southern Africa, Pretoria, South Africa. Doughty, R.W. 2000. The Eucalyptus: a Natural and Commercial History of the Gum Tree. The Johns Hopkins University Press. Baltimore, Maryland, USA. Duffy, E. A. J. 1963. A Monograph of the Immature Stages of Australasian Timber Beetles (Cerambycidae). British Museum (Natural History), London. Gonzalez-Tirado, L. 1987. Tabla de vida para Phoracantha semipunctata Fab. (Col. Cerambycidae). Perforador de los eucaliptos en el sudoeste Español. Boletin de Sanidad Vegetal, Plagas 13: 283-301. Hanks, L. M., T. D. Paine, and J. G. Millar. 1991. Mechanisms of resistance in Eucalyptus against the larvae of the eucalyptus longhorned borer (Coleoptera: Cerambycidae) in California. Environmental Entomology 20: 1583-1588. Hanks, L. M., T. D. Paine, and J. G. Millar. 1993. Host species preference and larval perfor- mance in the wood-boring beetle Phoracantha semipunctata F. Oecologia 95: 22-29. Hanks, L. M., J. G. Millar, and T. D. Paine. 1995. Biological constraints on host -range expansion by the wood-boring beetle Phoracantha semipunctata (Coleoptera: Cerambycidae). Annals of the Entomological Society of America 88: 183-188. Hanks, L. M., T. D. Paine, and J. G. Millar. 1996. A tiny wasp comes to the aid of California’s eucalyptus trees. California Agriculture 50: 14-16.

Chapter 11. Host Preference Testing for Parasitoids of a Eucalyptus Borer 141 ASSESSING HOST RANGES OF PARASITOIDS AND PREDATORS ______

Hanks, L. M., T. D. Paine, and J. G. Millar. 1998. Dispersal of the eucalyptus longhorned borer (Coleoptera: Cerambycidae) in urban landscapes. Environmental Entomology 27: 1418-1424. Hanks, L. M., T. D. Paine, J. G. Millar, C. D. Campbell, and U. K. Schuch. 1999. Water relations of host trees and resistance to the phloem-boring beetle Phoracantha semipunctata F. (Co- leoptera: Cerambycidae). Oecologia 119: 400-407. Hanks, L. M., J. G. Millar, T. D. Paine, Q. Wang, and E. O. Paine. 2001. Patterns of host utiliza- tion by two parasitoids (Hymenoptera: Braconidae) of the eucalyptus longhorned borer (Co- leoptera: Cerambycidae). Biological Control 21: 152-159. Ivory, M. H. 1977. Preliminary investigations of the pests of exotic forest trees in Zambia. Commonwealth Forest Review 56: 47-56. Joyce, A. L., J. G. Millar, and T. D. Paine. 2002. The effect of host size on the sex ratio of Syngaster lepidus, a parasitoid of eucalyptus longhorned borers (Phoracantha spp.). Biological Control 24: 207-213. Luhring, K. A., T. D. Paine, J. G. Millar, and L. M. Hanks. 2000. Suitability of the eggs of two species of eucalyptus longhorned borers (Phoracantha recurva and P. semipunctata) as hosts for the parasitoid Avetianella longoi Siscaro. Biological Control 19: 95-104. Mills, N. J., K. Kruger, and J. Schlup. 1991. Short-range host location mechanisms of bark beetle parasitoids. Journal of Applied Entomology 111: 33-43. Munz, P. A. 1968. A California Flora. University of California Press, Berkeley, California, USA. Paine, T. D. and J. G. Millar. 2002. Insect pests of eucalypts in California: implications of manag- ing invasive species. Bulletin of Entomological Research 92: 147-151. Paine, T. D., J. G. Millar, T. S. Bellows, L. M. Hanks, and J. R. Gould. 1993. Integrating classical biological control with plant health in the urban forest. Journal of Arboriculture 19: 125-130. Paine, T. D., J. G. Millar, and L. M. Hanks. 1995. Biology of the eucalyptus longhorned borer in California and development of an integrated management program for the urban forest. California Agriculture 49: 34-37. Paine, T. D., J. G. Millar, T. S. Bellows, and L. M. Hanks. 1997. Enlisting an under-appreciated clientele: public participation in distribution and evaluation of natural enemies in urban landscapes. American Entomologist 43: 163-172 Paine, T. D., E. O. Paine, L. M. Hanks, and J. G. Millar. 2000. Resource partitioning among parasitoids (Hymenoptera: Braconidae) of Phoracantha semipunctata in their native range. Biological Control 19: 223-231. Paine, T. D., J. G. Millar, E. O. Paine, and L. M. Hanks. 2001. Influence of host log age and refuge from natural enemies on colonization and survival of Phoracantha semipunctata. Entomologia Experimentalis et Applicata 98: 157-163. Ryan, R. B. and J. A. Rudinsky. 1962. Biology and habits of the Douglas-fir beetle parasite, Coeloides brunneri (Hymenoptera: Braconidae) in Western Oregon. The Canadian Entomolo- gist 94: 748-763.

142 Chapter 11. Host Preference Testing for Parasitoids of a Eucalyptus Borer