Colonization history of the western corn rootworm (Diabrotica virgifera virgifera) in North America: insights from random forest ABC using microsatellite data

First described from western Kansas, USA, the western corn rootworm, Diabrotica virgifera virgifera, is one of the worst pests of maize. The species is generally thought to be of Mexican origin and to have incidentally followed the expansion of maize cultivation into North America thousands of years ago. However, this hypothesis has never been investigated formally. In this study, the genetic variability of samples collected throughout North America was analysed at 13 microsatellite marker loci to explore precisely the population genetic structure and colonization history of D. v. virgifera. In particular, we used up-to-date approximate Bayesian computation methods based on random forest algorithms to test a Mexican versus a central-USA origin of the species, and to compare various possible timings of colonization. This analysis provided strong evidence that the origin of D. v. virgifera was southern (Mexico, or even further south). Surprisingly, we also found that the expansion of the species north of its origin was recent—probably not before 1100 years ago—thus indicating it was not directly associated with the early history of maize expansion out of Mexico, a far more ancient event

Influence of temperature on the reproductive and demographic parameters of two spider mite pests of vineyards and their natural predator

This research was supported by scholarships to MC Stavrinides from the Robert and Peggy van den Bosch Memorial Scholarship in Biological Control and the Cyprus-America Scholarship program administered by the Fulbright Commission in Cyprus. Funding was also provided by the American Vineyard Foundation, the Viticulture Consortium West and the California Raisin Marketing Board. We thank A. Goldman, J. King, R. Lara and the undergraduate research apprentices who helped with laboratory observations. Steve Welter and Wayne Sousa read earlier versions of this manuscript and provided comments for its improvement. We also thank Duarte nurseries for supplying grape plants for this study.We evaluated the influence of temperature on demographic parameters of two common vineyard pests, the Pacific spider mite, Tetranychus pacificus McGregor, and the Willamette spider mite, Eotetranychus willamettei (McGregor) (Acari: Tetranychidae). Additionally, we investigated the effects of temperature on their shared predator, the western predatory mite, Galendromus occidentalis (Nesbitt) (Acari: Phytoseiidae). The intrinsic rate of increase (r (m) ) was higher for T. pacificus than E. willamettei at 15 and 28A degrees C, but similar at 22A degrees C. G. occidentalis achieved a higher r (m) than T. pacificus from 15 to 28A degrees C, but the difference was significant only at 22A degrees C. At 34A degrees C, the r (m) for both T. pacificus and G. occidentalis was negative, while E. willamettei did not develop at this temperature. Prey species did not affect demographic parameters of G. occidentalis. These results suggest that higher temperatures favor T. pacificus over the less damaging E. willamettei, and may also reduce the effectiveness of G. occidentalis.Robert and Peggy van den Bosch Memorial Scholarship in Biological Control, Cyprus-America Scholarship, American Vineyard Foundation, Viticulture Consortium West, California Raisin Marketing Boar