Apr 18, 2016

Pacific Spider Mite present in Washington vineyards

A survey of mites on wine grapes in Washington during 2013 to 2015 provided some interesting results. The most startling find was that California’s number one spider mite pest of grapes, the Pacific Spider Mite (Tetranychus pacificus), has been confirmed as present in some of WA’s vineyards. Confirmation of the presence of this species in Washington grapes has come from two spider mite taxonomists in Canada and Florida who have identified samples we sent to them last fall.

From the beginning of wine grape cultivation in eastern Washington, we have had two problem spider mites, Twospotted spider mite (Tetranychus urticae) and McDaniel’s spider mite (Tetranychus mcdanieli). McDaniel’s spider mite has been the dominant species. It now looks like many of the infestations thought to be McDaniel’s are in fact Pacific Spider Mite. The two species are very similar and can only be separated by examination of their genitalia (try doing that with a hand-lens!).

So what is the significance of this find of Pacific Spider Mite in Washington wine grape vineyards? We are not sure at this point. The economic impact of Pacific Spider Mite may be similar to McDaniel’s Mite and Twospotted Mite.

Spider mites generally occur in high, damaging densities on grapes under hot, dry conditions but there may be differences between the three species in this response. Spider mite populations are also exacerbated by the use of certain broad-spectrum insecticides as well as imidacloprid. But again, there may be differences between species.

We also need to know the identity of our pests for regulatory reasons and to optimize our integrated pest management programs. So the news of Pacific Spider Mite being a new addition to our grape pest fauna in Washington is not insignificant.

We currently have no idea of the extent of Pacific Spider Mite within our state and whether it’s a recent arrival or has been here undetected for some time. Interestingly, Pacific Spider Mite within California has become a bigger problem in recent years by establishing in more northern and coastal wine grape areas where it formerly was not a problem. Its impact within its traditional range has also increased. Clearly, it would be a good idea to establish the distribution of Pacific Spider Mites within our state. as well as its abundance in comparison to McDaniel’s and Twospotted Spider Mites.

The arrival of Pacific Spider Mite in Washington wine grape vineyards means that we now have four spider mite pest species. Aside from McDaniel’s and Twospotted, we also have Willamette Spider Mite which was also discovered for the first time during this same mite survey. Our survey also showed that few wine grape vineyards experienced damaging populations of spider mites but those that did were most often caused by Willamette Spider Mite. A relationship between frequent use of neonicotinoid insecticides and Willamette Spider Mite outbreaks was also suggested.

A more complex pest mite fauna in our vineyards is not necessarily a bad thing but it is certainly a situation that requires a better understanding of the roles each species plays in the potential for spider mite damage.

— David James, WSU-IAREC

Source: Washington State University Extension