How We Grow

2021 Jan/Feb How We Grow

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PEST MANAGEMENT Return of the Thrips Signals Less Need for Spring Miticides There's a new sheriff in town, but instead of a six-shooter, he carries six spots. The six-spotted thrip (Scolothrips sexmaculatus) is a friend to almond growers and a powerful enemy to leaf-damaging spider mites. Adult thrips can eat 60 mite eggs a day, while immature thrips can consume 10 mites a day. Even a low level of thrips can maintain adequate mite control in an almond orchard. That's good news for growers because left uncontrolled, mites can completely defoliate a tree and threaten future yields. In the past, that threat has led many growers to adopt a policy of spray first and ask questions later. But experts in mite control are urging growers to rethink that strategy, especially in the springtime – most of the time, it's okay to hold off on the spray in May and let the thrips do their work. "Historically, there have been reasons to spray preventatively in May, but those reasons typically no longer exist," said David Haviland, entomologist and farm advisor with the University of California Cooperative Extension (UCCE) in Kern County. "Nowadays, increased sustainability on the part of almond growers, specifically the reduction in use of dormant sprays and in-season application of organophosphates and pyrethroids, has caused increased prevalence of six-spotted thrips," Haviland said. "Relatively unknown in the past, six-spotted thrips have become a dominant predator capable of complete suppression of mites in the spring." In fact, holding off on a May miticide spray can help ensure that plenty of thrips will be around later in the season when they are needed to help growers control mites. The key to planning for thrips' help is monitoring the orchard to make sure there are some thrips and not too many mites, Haviland said. That means checking for both thrips and mites. To check for thrips, Haviland recommends using Trécé Predator Traps. Research funded by the Almond Board of California (ABC) shows these traps are most effective. The traps are yellow three-by-five-inch cards, sticky on both sides, that cost about 35 cents each. Haviland recommends placing two-to-four cards per orchard, using a small binder clip and short wire (unwound paper clip) to hang the traps in the outer periphery of the tree at eye level. For ease of checking, Haviland recommends placing a predator trap at each location where a navel orangeworm trap is hung. Predator traps should be checked weekly and replaced at least once a month or when they become dirty. Data has shown that mite populations are controlled by thrips when there is an average of 0.4 thrips per card in a week for every one mite per leaf. If using a UC treatment threshold of approximately 40% of leaves infested (around two mites per leaf), this means that no May sprays are needed if at least one thrip is found per card weekly. Haviland said that in the past five years he has never found an almond orchard with less than one thrip per card during weeks in late April and May. Checking for mites involves a weekly check from March through harvest of 15 leaves randomly selected from at least four places in the orchard. If mites are observed on 40 percent or more of the leaves, it's time to spray – but if observations show less than that, with thrips also present, the grower is better off not spraying, Haviland said. Tracy Miller, a Pest Control Adviser (PCA) in Stanislaus, Merced, and San Joaquin counties and a member of ABC's Pest Management Workgroup, said the emergence of thrips as a predator has led him to change his recommendation to growers. "I have been observing a lot more six- spotted thrips in my fields than what I ever used to see before," Miller said. "The last two years I have completely abandoned prophylactic sprays in May – I don't recommend them anymore." "The last two years I have completely abandoned prophylactic sprays in May – I don't recommend them anymore." — Tracy Miller S i x - s p o tt ed t h r i p s o n s t i c ky t r a p . P h o t o c r e d it D a vi d H a v il a n d Cultural practices, such as ensuring tree health and minimizing dust, can also help curb spider mite population growth. 9

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