Since moving to the southern end of San Joaquin Valley in California, I’ve seen more fruit and nut trees than I thought possible. Some of the common monoculture plantings include almonds, cherries, peaches, nectarines, walnuts, pistachios, plums and prunes. Each orchard has individual challenges depending on soil, cultural practices, climate and other environmental variables, but they often share similarities when it comes to insect and mite pests.

Spider mite

A two-spotted spider mite adult is on the left. An immature spider mite is on the right.

Photo Credit: ©2005 Buglady Consulting

Predatory mite

Neoseiulus californicus is an excellent predatory mite for the two-spotted spider mite.

Photo Credit: ©2003 Buglady Consulting

For example, stone fruits, including cherries, peaches and prunes (which are a fruit – not just dried plums as I used to think), all battle spider mites as a common pest. These detrimental eight-legged critters have the ability to explode into huge, uncontrollable populations that can defoliate a tree if left unchecked.

How do they damage a tree? A spider mite’s head is similar to a large straw. The pests insert it into plant cells and then draw the cell contents into their bodies to digest. This causes the leaves on the trees to get a stippled look because the green chlorophyll is no longer present, and a white spot or empty plant cell is left behind.

The tree doesn’t have the ability to repair that cell; all it can do is push out new growth to compensate for the damage on the older growth, which takes nutrients away from the following growing season. New growth often occurs late in the season, after the tree’s already fruited and been harvested. (FYI, spider mites don’t damage the fruit – other pests do that.) The tree wastes energy by producing new growth for leaves that’ll simply fall off another month or two later.

Spider mites can be seen with the naked eye, but it’s much easier to see them with a 10X magnifying loop. Looking for discolored leaves is a good way to spot the pests when they’re first getting started.

There are a number of management strategies for controlling spider mites. I tend to focus on using natural enemies or beneficials that are naturally occurring or can be purchased from a commercial insectary, then released into the orchard as augmentative biological control. (In other words, “releasing good bugs to eat the bad bugs.”)

Depending on the location of your orchard, different predatory mites, like Galendromus occidentalis or Neoseiulus californicus, can be used against spider mites. There are many generalists (or beneficials that go after many different pests), like lacewings and ladybird beetles, that can assist in getting the spider mite population under control, too. To attract beneficials to a specific area, cover crops and insectary plantings with pollen and nectar. Or, if you avoid using insecticides or miticides, naturally occurring beneficials will be drawn to your orchard. Contact your local cooperative Extension Service, check books, surf the Internet or contact your beneficial supplier to find the correct helper insect for your situation.

Spotting spider mite infestations early on and fighting back with beneficials is the best way to keep your stone fruit trees looking good and growing healthy for years to come.