Selitrichodes globulus

Salle, John La, Arakelian, Gevork, Garrison, Rosser W. & Gates, Michael W., 2009, A new species of invasive gall wasp (Hymenoptera: Eulophidae: Tetrastichinae) on blue gum (Eucalyptus globulus) in California, Zootaxa 2121, pp. 35-43 : 36-37

The Selitrichodes globulus outbreak in California

On November 5, 2008, the Entomology Lab of the Los Angeles County Department of Agricultural Commissioner/ Weights & Measures received a single blue gum eucalyptus branch from a homeowner (Monrovia, Los Angeles County) with a note stating "infested tree in the backyard, cancerous growth on branches/twigs". Examination revealed multiple galls on branches containing live larvae of Eulophidae wasps. Since this was an unusual damage caused by an apparently new pest on eucalyptus for the US, an 'official' sample (per protocol) was collected at the property on November 6. The infested blue gum eucalyptus tree was in poor shape with heavy, hanging branches and twigs covered with galls ( Figs 1–2 ). Galls consisted of multiple chambers containing larvae and pupae, and could occur almost continually along branches; there were up to as many as 20 specimens per 5 mm of branch. At the same time some leaves had isolated galls on them. Several smaller branches and twigs were entirely dry with 10–20 mm cracks along galls that could possibly promote fungal infection and decay spreading to larger branches.

Cut eucalyptus branches were stored in plastic bags punctured with #4 insect pins (to prevent adult escape and to keep material from fast drying). From November 8–10, these bags were placed in the lower (fruit/ vegetable) section of a refrigerator to stop fungal growth and to possibly stimulate the emergence of adults. Adults started emerging on November 12 and were observed emerging until December 2. About 120– 130 adults were reared along with two Erythmelus gracilis (Howard) (Mymaridae) , which are egg parasitoids and were probably present as contaminants brought in on the twigs. There were high larval and pupal densities present in galls, with adults emerging at the same time.

A second infestation was subsequently found at the Los Angeles County Arboretum and Botanical Gardens located in Arcadia (by J.Turney, Los Angeles Co. Dept. of Agricultural Commissioner/Weights & Measures). As yet no adults have emerged from these samples; the larvae were submitted to CDFA to confirm the new location.

Subsequent rearings recovered additional species of wasps associated with these galls. The most interesting was the rearing of five males of an Ophelimus species ( Eulophidae : Ophelimini). Ophelimus is an unusual Australian genus which contains over 50 described species and many more undescribed. Species have been assumed to always be gall inducers ( Bouček 1988), and this is probably the largest group of gall inducing eulophids on Eucalyptus. However actual biology is known for relatively few species, and there is some indication that at least some species are not true gall inducers but are associated with galls in other ways, perhaps as inquilines or even parasitoids (La Salle 2005).

The presence of Ophelimus sp. questions the status of the two wasp species associated with blue gum galls in California. Selitrichodes species appear to be mainly parasitoids of gall inducing wasps (Kim et al. 2008), and Ophelimus are predominantly gall inducers. In this case, it appears that these roles are reversed. Further studies to clarify the exact roles of all the species associated with these galls will be of interest. However, it should be pointed out that when this paper was submitted, samples of gall infested E. globulus had produced almost 600 S. globulus , and less than 20 male Ophelimus . Under these circumstances, it is difficult to imagine S. globulus as being anything except the primary gall inducer.

Figures

FIGURES 1 – 6. Selitrichodes globulus. 1 – 2, Gall damage on Eucalyptus globulus. 3, Galls showing adult emergence holes. 4, Dissected section of stem showing galls containing larvae and pupae. 5 – 6, Larvae within dissected galls.