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State recently approved for use in tree crops: Dimilin OK'd for PTB control

One of the first insect growth regulators introduced into agricultural pest control continues to take on new life.

Dimilin, a IGR developed in the 1970s and used for years in cotton, rice and soybeans as well as mosquito abatement, has recently been labeled in California for use against peach twig borer (PTB) in almonds, apricots, nectarines, peaches and plums/prunes.

“The registration of Dimilin is one more good alternative to the problems associated with dormant sprays and organophosphates,” said Walt Bentley, University of California regional integrated pest management advisor at the Kearney Ag Center, Parlier, Calif.

“We now have four good materials that can be used during bloom in almonds and stone fruit and all have different modes of action. I am really excited about Dimilin coming into this market because it offers an alternative in a resistance management program,” added Bentley, who has proven in his test Dimilin is effective in controlling PTB.

PTB can be a major problem early in both almonds and stone fruit. Bentley said they are particularly damaging in the latter where no damage can be tolerated. Almonds can tolerate a bit more PTB damage than fresh fruit.

Terminal ‘strikes’

PTB larvae emerge in the early spring, usually just before or during bloom. They migrate up twigs and branches where they attack newly emerged leaves and shoots. As shoots elongate, larvae mine the inside, causing the terminals to die back. These are known as “strikes.”

A second flight can emerge in the summer to attack either twigs or ripening fruit.

Growers have long relied or dormant oil sprays alone or mixed with organophosphate or pyrethroids for early-season PTB control. However, organophosphates are coming under increasing scrutiny of regulators who want them phased out. These sprays also have caused problems by disrupting beneficials and washing into waterways.

“I have seen a drop in organophosphate use for a couple of reasons,” said Bentley. One is the workers safety and training requirements, and the other is farmers are wanting to get away from some of the older materials that can cause problems.

“Most farmers realize they are under increasing scrutiny from the state's urban population and want to show people they can use a more reasonable approach when one is available that is effective,” said Bentley.

“That is what we are seeing with many new chemical registrations. I have been amazed at the products we have seen come into the so-called specialty markets…fungicides and pesticides that offer very good control and are environmentally friendly,” said Bentley.

Grower's analysis

Selma, Calif., peach, nectarine and raisin and wine grape grower Arnold Arnst agrees. He has been working with Crompton/Uniroyal product development specialist Charles Jackson in comparing a Dimilin/dormant and late-dormant treatments against Arnst's current standard of oil and a pyrethroid pesticide.

“As a farmer I am concerned about what I use. I look for products that are not only effective, but safe on beneficials and do not harm bees,” said Arnst. “I work with Charles on research projects because I am interested in not only what is new, but what is safer. Doing these comparison things is the only way you can learn.”

Arnst is pleased that he has seen no strikes in either the pyrethroid or Dimilin treatments. “I am happy with what I have seen in the Dimilin treatments,” he said.

“If I were a peach grower, I would definitely go with a delayed dormant oil treatment with Dimilin vs. a fully dormant spray,” he said.

“The closer you can get to bloom to spray, the better control you get with these new compounds like Dimilin,” Bentley added.

Another reason Arnst wants an alternative to his pyrethroid insecticide is to preclude mite flare-ups, a common side affect with pyrethroids. However, Jackson acknowledged that the pyrethroid Arnst is using, Asana, is less likely to flare mites than earlier generation pyrethroids.

Other pests controlled

Growers use dormant oil sprays to control overwintering PTB, but it also controls other pests, notable San Jose Scale. Dormant oils cover bark for control of pests overwintering in the trees.

Dimilin 2L was granted an accelerated review by the Environmental Protection Agency as an organophosphate replacement. California Department of Pesticide Regulation labeled it for use against PTB in April.

Optimum results require good coverage of the protective overwintering structure (hibernaculum) that covers the larvae in the bark crack and crevices. When the PTB larvae eats it way out of the hibernaculum, it ingests the Dimilin and becomes incapable of molting. Dimilin's mode of action interferes with the development of the insect's exoskeleton, which becomes weak and may rupture or be malformed during molting.

The standard rate for Dimilin is 12 to 16 ounces. The cost is about $24 per acre, according to Kelly Washburn, Crompton Uniroyal technical sales representative. The higher rate is recommended for orchards with a history of heavy PTB infestations.

Dimilin is applied in at least 50 gallons of water per acre for smaller trees and at least 100 gallons for larger trees.

Dimilin is a restricted use pesticide due to toxicity to micro crustaceans. It has low toxicity to mammals, birds, fish, honeybees and most aquatic invertebrates. It carries a caution label, but does not require posting or a notice of intent to apply.

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