I was asked the other day what do immature tadpole shrimp (TPS) look like. Young TPS look pretty much like the older ones, just smaller. The very first instar, which remains inside the egg for a few hours, looks somewhat different, but you are not going to see this instar in the field with the naked eye. A few hours later they molt and look more like regular TPSs. Here's a picture of a few of them a few days later swimming belly up. Notice the rice grain floating next to them for size comparison.

Winter is a good time to catch up on scientific reading. On a paper published in the Journal of Economic Entomology last year, Chinese scientists determined that certain insecticides used in rice (in China) are compatible with the wasp Trichogramma japonicum, a parasitoid of eggs of lepidopterans (moths). Among the insecticides classified as safe to the parasitoid were pyrethroids, insect growth regulators and neonicotinoids. Organophosphates and carbamates were classified as toxic and not very compatible with the wasp*.

Trichogramma wasps are very small, and present in many crops as parasitoids of insect eggs. I have collected them from lepidopteran (stem borers) eggs from rice in Texas. I don't know if they are present in California rice parasitizing our lepidopterans (armyworms); I'll have to collect some armyworm eggs and see if I can recover some.

The article made me think about how insecticides are used in California rice. We have two species of armyworms commonly found in rice late in the season. These two species can feed on rice foliage and panicles, but seldom injure rice to the point of producing yield losses. There might be several reasons for this. One is that at the time of infestation, rice has abundant foliage and the armyworms would have to consume a lot of it to cause a yield reduction. Another reason, and maybe the main one, is that there are several other organisms in a rice field that will feed on the armyworms and limit their population growth. There are two important wasps that parasitize these armyworm larvae. Their cocoons (which are formed after the wasp larvae have consumed the armyworm larvae) are commonly found in rice fields. 

Cocoon of the parasitic wasp Hyposoter exigua.

Coccons of the parasitic wasp Apanteles militaris.

Other organisms feeding on armyworms are spiders, beetles, bugs, even birds. There are probably egg natural enemies too, but I don't think they have been thoroughly documented yet. This abundance of natural enemies is in part the result of the judicious use of insecticides in California rice. Most insecticide applications are done early in the year, when the populations of natural enemies have not built up yet, but pest populations are starting to move in. Additionally, applications are mostly done to field borders, and not entire fields, leaving untreated areas that natural enemies may use as “refuges”. Also, the main type of insecticides currently used in California is the pyrethroids; these have replaced the organophosphates and carbamates that were commonly used in the past.

*Zhao, X. et al. 2012. Assessment of toxicity risk of insecticides used in rice ecosystems on Trichogramma japonicum, an egg parasitoid of rice lepidopterans. J. Econ. Entomol. 105: 92-101

While scouting for blast, a PCA noticed some yellow leaf tips. Close inspection revealed aphids. He got a little worried since he's never seen aphids on rice during his long career. They turned out to be yellow sugarcane aphid. These aphids are common in small grains like wheat and barley, and in corn, so it is not surprising that they can be found feeding on rice. Actually, they are considered a pest in southern rice, where they feed on rice during the seedling stage. 

Photo by Bart Drees, Texas A&M University

We went to the field to see how prevalent they were. There were some yellow leaves above the canopy level. These were mostly from bakanae infected plants. There were some yellow leaves here and there at the canopy level, on these we found aphids. 

Yellow sugarcane aphid injects a toxin into rice plants during feeding that causes the foliage to become reddish.

In California, aphids are not considered a pest of rice. Most likely, they are present in some rice fields some years at very low levels, and mostly go unnoticed. The infestation in this field was very low, and probably the aphids will disappear in a few days. Nothing to worry about - just something to be aware of.

While looking for the aphids, we noticed some feeding scars that, at first, looked like rice water weevil scars. After looking closely, we realized these were caused by something else. After some search, we found quite a few beetles feeding on the rice next to the road.

I haven't identified the beetles yet, but they look and jump like flea beetles. The field is next to some unmanaged vegetation, and most likely their food source is drying off, so they are looking for some green material. I think this is just an "incidental" infestation. Again, nothing to worry about - just something to be aware of.

Armyworms are common in rice fields from mid-July to August. Two species can infest rice, the armyworm and the western yellowstriped armyworm. These insects build up their populations in alfalfa, other grains and grasses, and invade rice late in the season. The adults are moths that lay their eggs either on rice leaves or on broadleaf weeds within rice fields. After the eggs hatch, the small larvae begin feeding on rice foliage and/or panicles. Young larvae are hard to see, they can be less than a quarter of an inch long and their color mimics older rice foliage. Larvae go through a series of molts, growing to 1.5 to 2 inches. When they are ready to pupate, they drop to the ground in search for a good pupation site in the soil. In rice, they usually drown. I have seen armyworms pupate in the leaf sheath of older leafs at the base of plants at the water level, but this is uncommon.

Foliage feeding does not affect yield unless the armyworms consume 25% or more of the crop's foliage. It's not uncommon to walk a rice field and find areas where feeding is noticeable. In those areas, open the canopy and look near the water level. You might find armyworms resting there during the day. At night or when it's cooler, they will climb up the plant and continue their feeding. Take notice of the size of armyworms you see – if most of them are small, they will continue their feeding and a treatment might be needed. If you can't find any armyworms, they probably have already completed their cycle and there is no need to do a treatment.

Armyworm damage can be more problematic during heading. Typical panicle injury consists of blank panicle branches, although sometimes the whole panicle can be blank. What happens is that the armyworm bites the rachis of the branch and the grains in that branch don't fill. Sometimes, armyworm injury can be confused with cold temperature blanking. However, if you look closely at the blank panicle or branches, you will see a chewing mark where the blank area begins. Inspect your fields as the panicles come out of the boot – I believe this is when most of the armyworm damage occurs. If after inspecting your field you find that 10% or more panicles are affected, and you can still find armyworms, a treatment might be needed.

For the most part, armyworms are not a major problem in California rice. Some growers experienced severe infestations last year; heavy infestations seem to be cyclical and return every few years. Luckily, armyworms have several natural enemies in rice fields that are very good at keeping their populations down. Birds are important armyworm predators and can function as a warning system for their presence in the field.