When it comes to early season seedling injury, the damages wrought by rice seed midge and tadpole shrimp can be difficult to differentiate. Both are invertebrate pests that feed on young rice seeds and inhibit or terminate rice growth, and both can leave a field devoid of plants. However, taking a closer look at the affected seeds shows which pest has been there.
Tadpole shrimp eggs and rice share one thing in common; they are both “activated” by water. These eggs hatch very soon after the field is flooded, and the young shrimp grow quickly. Seeding into a field with shrimp present, even small shrimp, is asking for trouble. Small shrimp (about ½ an inch or less) can injure rice, just not as much as larger shrimp.
Tadpole shrimp tend to feed on the emerged radicle and coleoptile of the rice seedling, as seen in the picture below. You can see that whereas the two seedlings at the bottom of the picture showcase both shoots and roots, the remaining seeds have been pruned back. These injured seedlings are unlikely to continue growing, and if the tadpole shrimp are not controlled, the rice stand can take a huge hit.
Rice seed midge, on the other hand, tend to do things a little differently. Adult midge will lay eggs on the surface of the water as spring flooding begins. After the eggs hatch, midge larvae swim to the bottom to feed and build silken tubes for protection. These tubes quickly become visible as they become covered in mud and algae (picture below). As the larvae grow, so does their appetite; while they initially eat diatoms and algae, they’ll begin feeding on rice seeds and seedlings after about 5 days.
This feeding looks like tiny bore holes in the seeds. The midge will connect their tubes to the seeds so that they can easily feed from the protection of their tunnels. Sometimes, green algae will develop around the affected seeds. Similarly to the damage caused by tadpole shrimp, rice seeds are unlikely to continue growing, and untreated fields can be decimated by these pests.
Treatments
Bruce Linquist has conducted trials that show that the optimum rice stand is approximately 25 plants/sqft. Rice is resilient and can compensate for a reduced stand to an extent. For example, when the stand is reduced to 12.5 plants/ft2 (half the optimum), yield is reduced only 10%. If there is enough injury to reduce stand significantly, insecticides can be an option.
For tadpole shrimp, there are more insecticide options, such as clothianidin (Belay), carbaryl (Sevin), or copper sulfate. Always follow the label and use full label rates to avoid the development of resistance.
Pyrethroids are used to treat for both tadpole shrimp and rice seed midge. Lambda cyhalothrin, a common pyrethroid insecticide used for seedling pests in rice, has been used to manage both pests. Evidence of pyrethroid resistance in tadpole shrimp has been growing over the last few years, and in recent years, Ian Grettenberger’s work on midge has shown that some pyrethroid-treated fields are still damaged by midge. There is some evidence that indicates that midge larvae may not be as susceptible to this insecticide as we thought, possibly due to repeated applications of these insecticides through time. UCCE Rice is conducting field studies to confirm these observations and determine which insecticides could be used to help manage midge and tadpole shrimp.
In terms of cultural options for control, a brief drain will also work; however, take into consideration any effects the drain may have in weed control and fertility. Draining a field to kill tadpole shrimp works if the field is dried to the point that no standing water or puddles remain.
Reseeding
To reseed or not to reseed: that is the question. In general, reseeding should be considered when the stand is reduced to 10 plants/ft2 or less. The current advice is that draining the field before reseeding increases the likelihood of establishment of the reseed, but it may not be possible in all cases. We are conducting research this summer to see how necessary draining the field may be. There is not much information available on the best practices for reseeding, but anecdotal recommendations are to use a higher seeding rate to increase the chances of establishment. In short, reseeding is a bit of a gamble.
If you have further questions about tadpole shrimp or rice seed midge, reach out to a UCCE advisor or specialist for more information.