A picture is worth a thousand words; just stare at entomologist Frank Zalom’s photo.
Zalom’s PowerPoint slide of a severely-damaged almond orchard caused by spider mites generated gasps from PCAs at a tree and vine seminar during the California Association of Pest Control Adviser’s 35th annual meeting in Sparks, Nev., in October.
The photo taken by Zalom in eastern Stanislaus County in August 2001, showed a partly-defoliated orchard with almonds still on the tree, but partially hidden by an eerie silver haze created by the web-spinning mites. The damage reduced vigorous bloom and foliage the following year.
Zalom is a mite guru. He serves as an entomology professor at the University of California, Davis. Zalom was director of the University’s IPM program for 16 years. He was selected as a Fellow of the Entomological Society of America in 2008, classifying him as one of the top insect scientists in the world.
“I consider spider mites to be a secondary pest in nut crops when compared to the damage caused by the Navel orange worm,” Zalom told the PCAs. He launched into a descriptive discussion on spider mite species which target grapes, nuts, pome, and stone fruit.
The Pacific spider mite, Tetranychus pacificus, and the twospotted mite, Tetranychus urticae, both web-spinning mites, are the primary spider mites that target California orchard crops, Zalom says. Other spider mites (non-web-spinning) include the European red mite, Panonychus ulmi, and the brown almond mite, Bryobia rubrioculus.
All mites suck out leaf cell contents resulting in leaf stippling (spotting) and yellowing, plus leaf drop. Studies indicate mite feeding over several years can reduce fruiting spurs and yields.
Zalom says spider mites typically appear in orchards after a “disruption” including hot and dry conditions, irrigation practices that can promote water stress (including plugged emitters), pesticide use which promotes secondary pests, and changes in insecticide and fungicide use patterns.
“A good thing to ask when spider mites are found is what am I doing differently in the field,” Zalom said. “It could be new insecticide or fungicide use. Some newer products can reduce natural predators.”
The Pacific spider mite with its broad host range occurs more frequently in warmer growing areas. Eggs are white and spherical. Adults vary in color from slightly amber to greenish or reddish with two larger spots located on the upper body and two on the rear. Spots are found on the insect’s side. The front legs have a more red-orange appearance than the twospotted mite.
The twospotted spider mite with its whitish front legs is found worldwide, but mostly in cooler growing areas. The twospotted mite has a broader host range than the Pacific mite. The eggs are spherical and are often laid in the webbing. Adults vary in color from whitish or greenish to amber and typically have two large spots that may merge together.
The European red mite, Panonychus ulmi, includes higher numbers in the spring to early summer in orchards. The insect overwinters in the egg stage. Eggs are red with a stipe while adults are dark red with long, curved hair protruding from the white spots. Five to 10 generations a year are typical.
The European red mite causes leaf stippling, but rarely leaf drop. Yellowed leaves from spring feeding can actually green up. Chronic infestations can cause yield loss.
“A well-watered orchard can usually tolerate a heavy European red mite infestation early in the season,” Zalom said. “For growers on the West Side with water-stressed trees a heavy mite buildup is possible with an increased potential for leaf drop.”
The brown almond mite also attacks nut and pome and stone fruit trees. The brown mite overwinters in the egg stage. Red eggs without a stipe are laid in masses on the tree bark. Juveniles are bright-red colored while adults are reddish-brown with a longer front pair of legs.
Brown almond mite feeding produces whitish gray spots on leaves. Adult mites are absent from the foliage during the day. Defoliation can occur at densities of 50 brown mites per leaf.
The difference between the eggs of the brown mite and the European mite is the brown mite egg lacks a small white stipe on the top. Adult brown mites crawl more than the European mite.
Successful control of the brown and European mites includes oil applied in a late dormant spray.
To gain accurate egg counts, Zalom suggests taking dormant spur samples from mid-November to late January. Randomly clip two-to-three spurs from the inside of each tree’s canopy (100 spurs total). Clip the spur at the base and include some old spur wood.
Examine the spurs in groups of 20 under magnification for mite eggs and record the number of spurs — present or not. Treat with oil if more than 20 percent of the spurs have mite eggs.
Natural enemies of spider mites in orchards include lacewings, spider mite destroyer, six-spotted thrips, and predatory mites.
“Six-spotted thrips are one of the most important predators in orchards, but their appearance is very unpredictable,” Zalom said.
“Pesticides provide good spider mite control including the more selective products,” Zalom says. He cautions that new pesticides can harm general predators and predacious beetles.
The Western orchard predator mite is probably the most common widespread predator in Central Valley orchards, Zalom says. In most predaceous mites in almonds, the adult females are narrowly oval in shape, shiny white to slightly yellow or reddish in color, and move faster than spider mites.
Eggs are elliptical and three to four times larger than the spherical eggs of spider mites. Predator mites often overwinter primarily as mated, adult females on the bark.
The two main spider mite problems in California vineyards include the Willamette spider mite, Eotetranychus willamettei, and the Pacific spider mite.
Cultivated and wild grapes are the main hosts for the Willamette mite. The early-season mite prefers the cooler, shadier parts of the plant. The Williamette mite is more common in the coastal valleys and the Sierra Nevada foothills. Heavy mite infestations continue into August.
The spherical eggs, slightly smaller than Pacific mite, have fine hair that tapers at the top. Adults are yellow with side spots. Feeding in the mid-to-late growing season causes foliage to turn a yellowish-bronze color and can even open canopies. Mite densities in the 30-50 per-leaf-range reduce brix levels.
The Pacific spider mite prefers the warmer upper canopy (sunny areas) of vines and thrives during hot and dry spells. The mite produces more webbing and tends to cluster. Damage begins as yellow leaf spots followed by necrotic (dead) areas. Large populations can burn grape vine leaves.
Pacific mites are present earlier in the grape season and are becoming an increasing problem into September and in the coastal valleys.