Late Season Leaf Defoliation due to Web Spinning Spider Mites

A few farm visits and phone calls have been coming in over the concern of late season spider mites. In these orchards, mite populations have flared up, causing defoliation. These growers have been asking is they should spray for mite control. As always, the answer depends, but here are some thoughts to consider when making this decision:

1. Since harvest has been delayed, and many trees have experienced longer periods of water stress than normal. Tree stress attracts mites, which creates hot spots within weaker areas of the orchard.

2. Tree defoliation caused by mites rarely causes significant effects on next year's crop. If defoliation occurs, some leafing out may be triggered by an irrigation. It is still advised to apply a post harvest irrigation even if the tree is defoliated. Damage is more significant on younger trees as loss of leaves in the fall reduces the amount of overwintering carbohydrates.

3. An application of a miticide should only be made in attempts to curb the population until cooler weather. In many cases it is not possible to control a flare up, but rather the application "buys time" until cooler temperatures/ less stressful conditions arrive. An application should be considered if the "hotspots" are starting to defoliate and the mites are moving outward into the orchard.

4. If mites are present, but very little defoliation is occurring, the best strategy is to "wait it out." Some later season defoliation may occur, but the "damage done" would be minimal, thus not worth the time and expense of the spray.

5. Miticides to apply should target the adults stage. These include the products Acramite (bifenazate), Vendex (Fenbutatin-Oxide), and Desperado (Pyridaben/Sulfur).

6. It is very difficult to get effective coverage with a miticide spray due to the hydro-phobic nature of the webbing. Drive slow and use the proper rates of water to ensure the best coverage possible.

7. If there is a large population of mites within the orchard, most likely there will be a large population of overwintering mites as well. This will require treatment next year even if a spray is made this fall. These mites will begin to emerge in early March through April. At that time, they will begin laying eggs. Making an application of a miticide that targets mite eggs at that time will help reduce mite pressure later season. If targeting adults, the sprays should be made later in the season, typically around mid-May.