Managing mites in walnuts takes a sharp eye, monitoring, and predator awareness in the orchard, says Kris Tollerup, University of California Cooperative Extension (UCCE) integrated pest management advisor at the Kearney Agricultural Research and Extension Center in Parlier.
Tollerup and David Havilland, UCCE farm advisor in Kern County, Calif., have talked in various settings about the damage the pests cause, what they look like, and how to keep pest populations down.
Both note that there are two primary mites in walnuts - the Pacific and Two-spotted mites. Both have six legs.
Two-spotted mites have a dark spot on both sides of their body when actively feeding. Pacific mites have a second pair of dark spots near the posterior ends. Often the spots are barely visible and may combine to form large darks areas, which makes it difficult to distinguish between the two species.
Feeding on the leaves by the mites causes stippling and browning of the leaves which results in leaf drop. Defoliation early in the year can greatly reduce nut yield and quality. Defoliation late in the year will impact the harvest, Haviland said.
Tollerup said damage the mites create reduces photosynthesis and the effect they have on stomata results in increased water stress as “they let water out.” The mites insert a stylophore into epidermal cells and remove their contents. They cause some direct, cosmetic damage to the fruit.
The mites have natural enemies that include the Lacewing, six-spotted thrips, and the spider mite destroyer. But Tollerup cautioned that he has been in orchards where he found virtually no predators, and he speculated that may have been tied to chemical usage to treat the orchard.
Tollerup said growers and others should take into account the economic level of injury caused by the pest. UC has a protocol for sampling for spider mites once a week, starting in June or early July through August.
Tollerup said it’s a “presence-absence” approach; finding on an egg or an adult indicates infestation.
Havilland said it works like this: Randomly select 10 trees in the orchard and pick five leaflets from lower branches and five leaflets from high branches in each tree.
Look for web spinning spider mites, predator mites, and six-spotted thrips in the low and high branches and record the finding. If predaceous mites are found on at least half the leaflets, there is a good probability that natural enemies will control mites, unless a broad spectrum spray is applied for codling moth.
If predaceous mites are not present on at least half of the leaflets, monitor again in three or four days to determine if the web-spinning mite population is increasing or decreasing. Monitor in three days if the weather is hot.
If mite populations don’t build up by mid-August, treatment may not be necessary.
In orchards where no organophosphate or pyrethroid applications are made, follow these guidelines.
For predators found on fewer than 10 percent of mite-infested leaves, spray if 30 or 40 percent of the leaves are mite infested. For predators present on 20 to 50 percent infested leaves, spray if 40 or 50 percent of leaflets are mite infested.
Don’t treat if predators are present on more than 50 percent of mite-infested leaves.
In orchards using organophosphate or pyrethroid applications with predators present on fewer than 10 percent of mite-infested leaves, spray when 10 percent of the leaves have spider mites.
If predators are present on more than 10 percent of mite-infested leaves, spray when 20 percent of the leaves have spider mites.
Havilland recommends several orchard management practices to reduce mite populations. These include:
- Minimize dust by oiling orchard roads and maintaining ground cover;
- Well irrigated and vigorous trees have reduced mite problems;
- When possible, avoid using pyrethroids, organophosphates, and carbamates until later in the season.
Mites overwinter as reddish-orange mature females in protected areas on the tree, in the soil, and in trash on the ground. In the spring, the females will begin to feed on walnut leaves and ground cover in the orchard.
Colonies of mites will develop on the underside of the leaves. If heavy populations build up, they will also be found the upper side of the leaves. The mites produce several generations a year.
Damage caused by mites is highly variable by regions of the state, Tollerup said.
Miticides include a mite growth regulator that requires contact and results in females laying sterile eggs. It is fast acting and has good knock-down and residual activity, Tollerup said.
A lipid biosynthesis inhibitor is most effective against immature stages, he said. It’s slower acting but has good residual activity.