As I write this, planting season in California is starting. Price of rice is still depressed, so it is important to keep costs down. This might be a good time to review some of the guidelines on how to scout for arthropod pests to help you decide if an insecticide application or other action is needed.
Tadpole shrimp is our most difficult pest to manage. They are difficult to see and grow very fast. The critical period for TPS damage is germination. If TPS are present during this time, they will feed on the emerging coleoptile and radicle, killing the seed. We do not have a threshold for number of TPS that would trigger an action during this time; if you can see TPS, even if they are small, the risk for stand reduction is high and an insecticide treatment is recommended. An alternative to insecticide application is to do a quick drain, but this has its own challenges and is not always possible. Typically, fields that do a “leathers” do not have TPS problems. In fields that take long to flood, TPS may be visible even before seeding. If this is the case, use an insecticide before or the day of seeding.
Once rice has a green spike and the roots are well established, the risk of TPS damage is much lower, even for large TPS. However, they can contribute to the uprooting of seedlings when it is windy or when herbicide injury occurs. At this stage of seedling development, check the stand often. If there are less than 25 seedlings per square foot and TPS are present, an insecticide treatment may be appropriate to protect the stand. Once the plants emerge from the water, there is very little risk of TPS injury.
While you are scouting for TPS, make sure you look for rice seed midge as well. Midge is typically not a problem, but there has been an increase in the number of affected fields in the past few years, especially late-planted fields. For midge, pick seeds a few days after seeding. Inspect them closely and look for midge larva inside the seed. Midge will attach small silken tubes to seeds they are feeding on; their presence is a sign that midge might be a problem. Once seedlings are past the germination stage, they are less susceptible to midge damage. Our current insecticides do not do a great job at controlling midge; however, draining is effective at killing the larvae.
Armyworms can be of concern in some years. As you know, I track armyworm adult (moths) populations with pheromone traps. This let us know when to expect the worm populations peak in the field. However, we can’t use the moth counts from the traps to decide if an insecticide is needed. For that, you have to get in the field (or at least look closely when driving around). Our threshold for insecticide use is 25% defoliation or 10% panicle injury. The 25% defoliation threshold can be a little subjective. A few years ago, I collected data that suggests that 25% defoliation is reached when there are six to 10 worms per square foot. You can use these numbers to decide if you need to use an insecticide for armyworm control. ∆
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