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Tree nuts face tough spider mite pressure

Management of spider mites on San Joaquin Valley tree nut crops demands more than selection and timing of the best miticide, especially in a year like this one.

That's the word from David Haviland, Kern County farm advisor, who shared some tips recently at a field day for walnut and almond growers and PCAs outside Hanford.

Species most economically important in tree nut crops in the region are Pacific and two-spotted mites, although others also occur.

“Often, managing mites is not one factor, but a combination of several that can really nail you if you don't keep them in mind,” he said.

One important element is the condition of the leaves. “If the orchard is well-watered and there's little dust, you might get by with some things that otherwise wouldn't work.”

However, a dry, moisture-stressed tree is a magnet for mites. In addition, dry, hardened-off foliage does not take up miticides as well as tender, soft leaves.

Heavy dust on foliage, like that in rain-short Kern County this year, he said, further interferes with spraying. “A lot of growers were trying to go with 100, maybe 150, gallons of water with Agri-Mek at two to three miles an hour, but this year with the dust they needed more volume and reduced speed.”

Conditions with mites in Kern County almonds are so severe this year that some growers had made their second miticide application by mid-June and were considering a third, even before temperatures got out of the 90s.

Haviland said Kern growers and PCAs recalled only normal conditions by mid-June of 2005, which later turned out to be the worst mite year in the last decade for almonds, walnuts, and grapes.

Compounding the worries are water shortages. But he said a string of 100-degree days and 80-degree nights could put almonds under more stress than irrigations could correct, even if water were available.

This year in some orchards, early thrips infestations, not usually a problem, were so heavy that they hardened off leaves. Drying wind storms in Kern County also had the same effect.

Another issue is simply coverage. “In miticides, particularly systemics, coverage of the tree is absolutely essential,” Haviland said. “Even at a volume of 400 gallons, if the rig is driven too fast, you'll get lots of water on the outside of the tree without reaching inside the canopy where it's needed.”

Too often, he added, growers complain about a miticide failure when they don't know the water volume and the speed of the rig. “We are seeing the scenario out there of growers trying to apply a miticide that works on contact, but never reaches the mites.”

In a reminder to PCAs, he said even though they specify the volume and speed in their recommendation, it's a good idea to follow up and monitor just what the applicator is actually putting on.

Mite density is another factor. “It's one thing to deal with two mites per leaf, but another trying to get through after leaves have cupped and become covered with webbing and the mites safely inside. It doesn't much matter what volume you put on, you still won't get them all.”

Turning to material selection, Haviland said the later generation pyrethroids, Brigade and Warrior, for example, gaining favor lately for navel orangeworm and codling moth control, have an impact on beneficials and may be an issue in mite management later on.

Miticide timings for almonds and walnuts differ and miticide data on walnuts is lacking, but some generalizations can be made by extrapolating data for almonds.

Haviland suggested walnut growers use the typical June application of Agri-Mek, but also try other products, such as Monitor, Zeal or Envidor, in some blocks to develop a rotational program. “Some growers have said that going away from Agri-Mek to something else for a year meant that it worked better when they came back to it.”

Agri-Mek, he noted, is also an excellent material in several crops for six-spotted thrips, a highly effective, natural enemy of spider mites.

In almonds, common practice in recent years has been to apply Agri-Mek in May as a preventive program, even if mites are not present. But Haviland said he has data that shows newer growth regulator miticides will perform well from June, or whenever a treatable mite threshold appears, until hull-split.

When heavy mite pressure occurs at hull-split, slower acting growth regulators don't work and a contact knock-down is needed, with some caution, Haviland said.

Fujimite has shown knock-down performance but is also hard on predatory mites. Haviland said it might also work well in June, but he does not have supporting data, pending trials currently underway.

Acramite can provide good knock-down, but tends to be shorter-lived in the southern SJV. In “a rescue situation,” he said, Kanamite has good knock-down for an even shorter period, but is easier on beneficials and has a three-day pre-harvest interval.

“All the new miticide products are very effective, if you get good coverage and treat before the mites get too much out of control,” Haviland said.

He added, however, that many of the new products can only be applied once per year and growers with heavy mite pressure this year are working through the list.

How predatory mites, mainly Galendromus occidentalis or western predatory mite, are applied in a biological control practice was demonstrated by Don Thomas, owner of Advanced Agricultural Services, Hanford, Calif.

He said the Galendromus, reared by commercial insectaries, is well-adapted to high temperatures and has a life cycle of seven or eight days. It generally appears naturally in mid-season.

Although predatory mite practices, Thomas said, were found to have great potential decades ago, new miticides took out both beneficial and target species and set back adoption of releasing beneficials. With softer miticides released since, there's been a revival of interest.

His company uses ATVs fitted with dispensers and blowers to distribute the predatory mites clinging to particles of vermiculite well into the foliage. Kept in cold storage until application, 2,500 predatory mites are applied per acre.

Thomas said releases are not an in-season “fix” but a long-term preventive program used in hopes of backing away from pesticides.

A precedent for the release concept exists around Hanford where grower Doug Verboon once tossed cornstalks covered with predatory mites into the crotches of walnut trees as his sole spider mite control program. He used the practice for 15 years before the advent of harsher materials.

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