Michael J. Pitcairn

DNA phenotyping to find a natural enemy in Uzbekistan for California biotypes of Salsola tragus L.

Abstract Salsola tragus L. (Russian thistle, Chenopodiaceae), a weed of Central Asian origin, has two biotypes in California, type A and type B. The gall midge Desertovellum stackelbergi Mamaev (Diptera: Cecidomyiidae), which attacks S. tragus in Uzbekistan, is a candidate biological control agent for this weed in the United States. In a field test conducted in Uzbekistan with plants of the two biotypes of S. tragus from California, both biotypes were attacked by the insect, although type A was the preferred host. Accessions of S. tragus from Uzbekistan, Greece, and Ukraine were similar to the California type A when compared using RAPD and ISSR analyses, while California type B was distinct. Since both California biotypes were hosts to the gall midge, further studies on the biology and host specificity of the insect are justified. Genetic characterization of target weeds can provide information useful for the selection of natural enemies.

Post-release dispersal of the introduced lygus bug parasitoid Peristenus relictus in California

Abstract Lygus spp. (Heteroptera: Miridae) are serious pests of numerous field and fruit crops in North America. In an effort to reduce crop damage, parasitoids known to attack these species in Europe were introduced into the USA beginning in the 1970s. Permanent populations of the nymphal endoparasitoid Peristenus relictus (Hymenoptera: Braconidae) were established at two locations in California during the 2000s. Both populations are associated with significant reductions in lygus bugs attacking alfalfa, commercially produced strawberries grown organically and in wild vegetation. Beginning in 2009, in an effort to determine the extent to which P. relictus has spread from the Sacramento Valley and Monterey Bay region, populations of lygus bug were sampled at increasing distances from their original release sites. P. relictus has dispersed at least 213 km in the central region of California and 150 km along the coastal mountains. These respective populations have averaged 16.6 km/year and 17.7 km/year since they were released. While not directly examined, the spread of P. relictus south into the central and major growing region of California, the San Joaquin Valley, where previous releases have failed, suggests this population may be evolving greater heat tolerance, relative to the populations originally introduced into California.

Biological control of water hyacinth in California’s Sacramento–San Joaquin River Delta: observations on establishment and spread

ABSTRACT Water hyacinth (Eichhornia crassipes (Martius) Solms-Laubach) is a serious invasive weed in the Sacramento–San Joaquin River Delta of California. Three insects: Neochetina eichhorniae Warner and Neochetina bruchi Hustache (Coleoptera: Curculionidae) and Niphograpta (=Sameodes) albiguttalis (Warren) (Lepidoptera: Crambidae) were released during 1982–1987 at four locations for the biological control of water hyacinth. Observations in 1985 suggested that all three species had established. By 2002, water hyacinth populations in the Delta still required an aggressive chemical control campaign and the status of the biological control agents was in question. In late 2002, a field survey to determine the distribution and abundance of the released insects was performed. Water hyacinth plants were collected by boat in the main water channels and from land at smaller sloughs and examined for insects. In total, 27 sites with water hyacinth distributed across the Delta were examined of which 21 had weevils. Weevil abundance ranged from 0 to 10.9 weevils per plant, with an average of 0.93 (±0.47 SEM) adult weevils per plant. All weevils (n = 518) were identified as N. bruchi. No N. eichhorniae were recovered and no larvae or evidence of larval feeding by N. albiguttalis were observed. A total of 322 weevils were examined for microsporidia and none was found infected, indicating an infection rate of less than 1%. These results suggest that N. bruchi may be the only established biological control agent of water hyacinth in the Delta and that infection by microsporidia does not appear to be limiting its population abundance.

Assessing the biological control of yellow starthistle (Centaurea solstitialis L): prospective analysis of the impact of the rosette weevil (Ceratapion basicorne (Illiger))

Yellow starthistle (Centaurea solstitialis L) (YST) is an invasive weed native to the Mediterranean region with a geographical centre of diversity in Turkey. It is widely established in Chile, Australia, and western North America. It arrived in California as a contaminant in alfalfa seed in 1859 and, by 2002, had infested >7.7 million hectares in the U.S.A. Biological control of YST using capitula feeding weevils, picture wing flies and a foliar rust pathogen has been ongoing in the western U.S.A. for more than three decades with limited success. Modelling and field research suggest natural enemies that kill whole plants and/or reduce seed production of survivors are good candidates for successful biological control. A candidate species with some of these attributes is the rosette weevil Ceratapion basicorne (Illiger). In the present study, a model of the rosette weevil is added to an extant system model of YST and its capitula feeding natural enemies and, in a GIS context, is used to assess YST control in the Palearctic region and the weevils potential impact on YST in western U.S.A. The results obtained suggest densities of mature YST plants in western U.S.A. would be reduced by 70–80% in many areas.