Spider mites cause significant damage in California vineyards leading grape growers to treat more than 100,000ha annually with pesticides. The Pacific spider mite Tetranychus pacificus McGregor and the Willamette spider mite Eotetranychus willamettei (McGregor) (Acari: Tetranychidae) are the two most common spider mites in vineyards. The western predatory mite, Galendromus occidentalis (Nesbitt) (Acari: Phytoseiidae), is an important natural enemy of both spider mites.
Temperature is a critical factor influencing pest outbreaks. We evaluated the effects of temperature on life histories of the three mites and found that higher temperatures are more favorable for T. pacificus than for E. willamettei and G. occidentalis. The lower development threshold lay around 10oC for all three mites, while the upper development threshold was estimated to be 31, 37 and 40oC for E. willamettei, G. occidentalis and T. pacificus, respectively. T. pacificus completed its immature development significantly more rapidly than E. willamettei above 23oC, whereas G. occidentalis developed significantly faster than either spider mite from 11 to 36oC. The intrinsic rate of increase of the three mites followed a pattern similar to development time. The intrinsic rate of increase of G. occidentalis feeding on T. pacificus at 28oC was not significantly different than when feeding on E. willamettei.
Water stress is another factor influencing spider mite outbreaks. A field study showed that increases in ambient temperature and plant water stress increased grape leaf surface temperature. Densities of T. pacificus increased significantly with increasing frequency of leaf temperatures above 31oC while those of E. willamettei showed no relationship and those of predatory mites (Acari: Phytoseiidae) showed a negative relationship with high leaf temperatures. These results help to explain why outbreaks of T. pacificus occur in hot or water stressed vineyards, while E. willamettei develops higher populations in cool or well irrigated vineyards.
Applications of pesticides against other vineyard pests may affect biological control by G. occidentalis. The insecticides imidacloprid and buprofezin negatively affected the population growth of G. occidentalis but had no effect on T. pacificus. The fungicide wettable sulfur significantly decreased T. pacificus population growth but it did not affect G. occidentalis. In addition, the insecticides methoxyfenozide and the fungicides tebuconazole and trifloxystrobin had no effect on the population growth of G. occidentalis or T. pacificus.
Populations of T. pacificus from vineyards that reported miticide failures in recent years developed statistically significant 11-fold resistance to pyridaben, seven-fold resistance to bifenazate and four-fold resistance to propargite compared to a susceptible laboratory population. These results underline the importance of alternation of products from different mode of action groups to delay resistance development.