A Herbivorous Mite Down-Regulates Plant Defence and Produces Web to Exclude Competitors

Herbivores may interact with each other through resource competition, but also through their impact on plant defence. We recently found that the spider mite Tetranychus evansi down-regulates plant defences in tomato plants, resulting in higher rates of oviposition and population growth on previously attacked than on unattacked leaves. The danger of such down-regulation is that attacked plants could become a more profitable resource for heterospecific competitors, such as the two-spotted spider mite Tetranychus urticae. Indeed, T. urticae had an almost 2-fold higher rate of oviposition on leaf discs on which T. evansi had fed previously. In contrast, induction of direct plant defences by T. urticae resulted in decreased oviposition by T. evansi. Hence, both herbivores affect each other through induced plant responses. However, when populations of T. evansi and T. urticae competed on the same plants, populations of the latter invariably went extinct, whereas T. evansi was not significantly affected by the presence of its competitor. This suggests that T. evansi can somehow prevent its competitor from benefiting from the down-regulated plant defence, perhaps by covering it with a profuse web. Indeed, we found that T. urticae had difficulties reaching the leaf surface to feed when the leaf was covered with web produced by T. evansi. Furthermore, T. evansi produced more web when exposed to damage or other cues associated with T. urticae. We suggest that the silken web produced by T. evansi serves to prevent competitors from profiting from down-regulated plant defences

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