California Rice: Dimilin for Tadpole Shrimp Control

A FIFRA Section 2(ee) Recommendation has been issued for control of tadpole shrimp (TPS) using Dimilin 2L. The recommendation states to use Dimilin two to 10 days after the field is flooded; it also states that Dimilin does not appear to control adult TPS.

Dimilin is an insect growth regulator. In many insects, it interferes with the molting process of immatures. In rice water weevil, it sterilizes adult females and also kills eggs. TPS are not insects, but crustaceans; however, they also molt. TPS molt 10 to 15 times before becoming adults; after that, they continue to molt as they grow larger.

I evaluated a field application today in a field with a known TPS problem. Four days after the application, there doesn’t seem to be any surviving TPS and injury to the seedlings is minimal. The application went out soon after the field was flooded.

Last year, I conducted a trial to test several products for TPS control. Dimilin was one of them. As you can see below, the kill of TPS with Dimilin was slow, and survival was still 10% three days after the treatment.

Data 1

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The TPS used in this trial had a shell length size that ranged from 1/3 to 1/2 inch. It is difficult to determine when a TPS is an adult, since the immatures and adults look the same.

I have noticed that eggs appear when the TPS shell reaches about 1/4 inch in length, so the TPS used in the trial were most likely adults. This might explain why the results of the trial were not very striking.

I will continue evaluating Dimilin this season in commercial fields and I will repeat the trial conducted last year. The 2016 trial is described in the attached document.

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