Pint-sized pests

Tadpole shrimp problems seem to be on the upswing in California rice fields.

By Luis Espino

Tadpole shrimp (TPS) is a problem that seems to be on the upswing. During our winter meetings, I conducted a clicker survey and asked attendees about the importance of TPS as a rice pest.

About half of respondents thought TPS problems were about the same as 10-15 years ago, but 40 percent thought TPS problems were worse or much worse. About a third of respondents also had noticed a reduction in the efficacy of the pyrethroid insecticides used to control TPS.

Last year, I collected TPS from two fields where the pest had survived a pyrethroid application and conducted a laboratory test to determine their susceptibility to several rates of the insecticide.

TPS from the problem fields survived 24 hours in water treated with five times the label rate of the pyrethroid; TPS from a field with no issues were killed after 24 hours in water treated with the label rate. Copper sulfate killed all TPS from all fields after 24 hours.

Tadpole shrimp may be developing tolerance to pyrethroid insecticides. We have seen tolerance to other pesticides before; in the 1980s, TPS became tolerant to parathion after about 20 years of use. We are approaching the same timespan with the pyrethroids, which were introduced in the early 2000s.

Growers and pest control advisers need to take this into consideration when planning their TPS applications. Copper sulfate  is still effective and could be used as a rotational product.

Susceptible rice stages

The most susceptible stage of rice seedlings is while the coleoptile and radicle are emerging. TPS seem to prefer to feed on the white structures coming out of the seed.

If the coleoptile or the radicle is consumed, the rice seedling is killed and will not get established. By the time the coleoptile turns into a green spike, TPS will prefer to feed on the growing roots.

As TPS grow, they can uproot seedlings. But once a seedling’s root is well established, the likelihood of this happening is reduced.

Scouting for TPS presence during the period of rice germination is important to avoid significant stand reductions. One question I have been asked several times is at what size can TPS start injuring rice.

Laboratory experiments I have conducted have shown TPS can injure rice when their shell is about half the  size of a rice seed. In my experience, this is about the time when TPS can be detected in the flood water.

TPS eggs hatch two to three days after they have been hydrated, but the young TPS are so small they cannot be seen with the naked eye in the flood water.

Tadpole shrimp develop quickly, and after a few days they are large enough to be detected in the flood water after careful observation.

I have been able to see TPS of damaging size after five days of starting the flood in late May. If planting during warm weather or  in late May, make sure to scout for TPS very soon after seeding.

Dr. Luis Espino is a University of California Cooperative Extension rice systems adviser and county director in Colusa County. He may be reached at laespino@ucanr.edu.