Pyrethroids in California rice
We are fortunate that in California the pressure from arthropod pests is not as severe as in other parts of the world. However, we do have arthropods that can be considered key or important. In some fields, tadpole shrimp is a problem that requires management every year. Armyworms are a problem some years but not always. Rice seed midge has not been a problem for some time, but in the past two years, there have been some fields affected. The rice water weevil, once a key pest, is found causing economic damage very rarely. In my 15 years working in California, I have not seen fields with economic damage caused by the rice leafminer or aster leafhopper.
After carbofuran use was phased out in the late ‘90s, pyrethroid insecticides took its place as the main active ingredient used in California rice. Overall, I believe this was a positive change. Pyrethroids are used at much lower rates and are less toxic to vertebrates via ingestion than carbofuran. But the increase in use also meant higher risk of contamination. Currently, irrigation drains are monitored to make sure pyrethroid insecticides residues are not leaving rice fields.
Pyrethroids have a place in rice, but not for all pests. They are very effective against tadpole shrimp, although in some areas, tolerance is building up. Similarly, they offer protection against rice water weevils. Recent trials by UC Davis Extension entomologist Ian Grettenberger have shown that pyrethroids do not do a good job against rice seed midge. He is studying what other products may be options. I have conducted several trials against armyworms, and pyrethroids only achieve 50% control; Intrepid or Dimilin are the products to use if armyworms are a problem. Consider these facts when deciding what insecticides to use, and always follow the label. Pyrethroids are a good tool for pest control in rice; let’s protect it by using it wisely.
Bugs in the System
Our entomologists Nick Bateman and Ben Thrash have been improving our rice insect control recommendations over the past few years. There are questions that we have needed answers to for both early season and late-season pests of rice. We’re at least a little closer to answers, but definitely could have more information to guide us.
For early season insect pests, we’re usually worried about rice water weevil and grape colaspis. While not all soils/fields have to worry about grape colaspis, all fields have to worry about rice water weevil. We consistently use the neonics CruiserMaxx or NipsIt on rice as they are excellent on grape colaspis and good on rice water weevil.
However, we’ve discussed for years that the neonics begin to run out around 35 days after planting, and we’re not always ready to flood then — meaning less control of rice water weevil. The answer now is to consider adding a diamide seed treatment (Dermacor or Fortenza) to prolong our control of rice water weevil (and some bonus residual against armyworms and other caterpillars).
The diamides certainly add another cost, but their improved rice water weevil control and their positive net return over cost have been apparent. This also applies to row rice fields where they seem to have activity on rice billbug. Notice I didn’t say control, but there have been positive yield benefits to the diamides in these scenarios.
As for the late season issue, it is of course the rice stink bug. Our pyrethroid insecticides such as lambda-cyhalothrin continue to provide less and less control each season. Our only answer is to begin shifting to other modes of action, especially if infestation levels are much above threshold.
Alternatives are currently limited, with Tenchu the best labeled option. The past two seasons, Endigo has received emergency exemptions for use, but it is unknown at this time whether that product will receive a full label or be allowed an exemption again for the 2023 season. Ultimately, we need help if we’re going to be able to manage stink bugs this year.
To sum things up, start off with an insecticide seed treatment, and consider stacking a neonic with a diamide to maximize control. Be diligent in your scouting for rice stink bug, and use the best product you can for the job that’s available. Finally, avoid “throwing in” insecticides with other applications that may flare stink bugs later.
Rice water weevil
The rice water weevil is the most important early season insect pest of rice in Louisiana. Adults of this insect emerge from overwintering sites beginning in early April in southern Louisiana (later in northern Louisiana) and fly to rice fields where they feed on young rice leaves. This form of injury is not economically important except under unusually heavy infestations or prolonged cold springs when rice grows slowly.
Egg-laying commences when standing water is present in a field that is infested with adults. This condition is usually met immediately after a permanent flood is applied to a field. Young rice is preferred for oviposition. After emerging from eggs, larvae feed under water on rice roots and pass through four larval instars and a pupal stage in approximately 30 days.
The rice water weevil is the most injurious insect pest in Louisiana rice production. Yield losses in excess of 25% can occur from severe infestations. Rice water weevil adults are grayish-brown weevils (beetles) about 1/8-inch long with a dark brown, V-shaped area on their backs. Rice water weevils overwinter as adults in grass clumps and ground debris near rice fields and in wooded areas. Emergence from overwintering sites usually begins in the first two weeks of April in southwestern Louisiana. Adults emerging from overwintering will invade either unflooded or flooded rice fields and begin feeding on the leaves of rice plants.
One key aspect of the biology of female rice water weevils is that females do not lay many eggs until fields are flooded. In unflooded fields, females may lay eggs in areas of fields that contain standing water, such as low spots, potholes, or tractor tire tracks. Application of the permanent flood is a trigger for females to lay numerous eggs in leaf sheaths of rice plants. Females deposit white, elongated eggs in the leaf sheath at or below the waterline. In addition to laying eggs in rice, adult rice water weevils will oviposit (lay eggs) in most aquatic grasses and sedges, including barnyard grass, fall panicum, red rice, yellow nutsedge, and broadleaf signalgrass. White, legless, C-shaped larvae with small brown head capsules emerge from eggs in about seven days.
First instar larvae are about 1⁄32-inches long and feed in the leaf sheath for a short time before exiting the stem and falling through the water to the soil where they burrow into the mud and begin feeding on the roots of rice plants. The larvae continue to feed in or on the roots of rice plants and weeds in and around the field, developing through four instars in about 27 days. Larvae increase in size with each succeeding molt. Fourth instar larvae are about 3/16-inches long. Larvae pupate in oval, watertight cocoons attached to the roots of rice and weed plants. The cocoons are covered with a compacted layer of mud and resemble small mud balls. Peak larval density occurs three to five weeks after flooding. Adults emerge from the cocoons and are able to fly a short time after emerging and may return to overwintering sites or attack a different rice field. Newly emerged adult weevils usually do not re-infest the same field that they come from because they prefer to attack young plants.
The life cycle from egg to adult takes about 30 days. The length of the life cycle is temperature-dependent, however, and can vary from 25 to 45 days in warm and cool weather, respectively. The number of generations per year varies with latitude. As many as three to four generations can occur in the southern rice-growing areas of Louisiana. Fewer generations occur in the northern rice-growing areas.
Adult rice water weevils feed on the upper surface of rice leaves, leaving narrow, longitudinal scars that parallel the midrib. Adult feeding can kill plants when large numbers of weevils attack very young rice, but this is rare and is usually localized along the field borders. Most economic damage is caused by larvae feeding in or on rice roots. Under heavy infestation, the root systems of affected plants can be severely damaged.
All currently grown rice varieties are susceptible to the rice water weevil. Recent research, however, indicates some differences in varietal susceptibility. Medium-grain varieties appear to be more susceptible to infestation than long-grain varieties. Most producers are using prophylactic seed treatments. With this method of control, the insecticide is applied directly to the seed. Depending on the type of insecticide, either larval or adult control may occur. Scouting is not required with this method since it is used as a preventative treatment. Effectiveness of prophylactic seed treatment, however, should be assessed by monitoring larval populations. For other control methods, contact your local county agent.
Water management strategies
With input supply, commodity prices, and the need for rotation, many traditional Mississippi rice acres were planted to soybean during 2022. However, rice acres for 2023 are expected to increase. Additionally, with margins for corn looking tighter this year, more growers are beginning to evaluate rice as an option. With this new interest, questions have arisen about different water management strategies.
In Mississippi, I would say water is managed in one of two ways. The first of these is continuous flood or alternate wetting and drying. I consider this one management strategy because Mississippi State has generated a large amount of data showing that weed control, fertility, and insect management do not differ much. When rice is grown in this way, it is essential to have a plan to establish a four-inch-deep flood once rice begins tillering. From my time on the farm, I know it can be easy to get behind. For example, if we do not prepare to establish the flood prior to tillering, we can often run into situations where we have two or three tillers before actually establishing a flood.
The second is row rice or furrow-irrigated rice. This has gained popularity because of the ease of rotating between other row crops from year to year. Currently at Mississippi State, extensive research is being conducted on establishing irrigation timings, thanks to the Mississippi Rice Promotion Board. So far, it seems that the best frequency is every three to five days after we reach the tillering stage. Billbug, which tunnels into the base of the plant and results in blank heads, has arisen as an issue in row rice. So far, the best management strategy has been to include Dermacor or Fortenza with CruiserMaxx Rice or NipsIt seed treatments.
The last important insect issue I would like to point out for growers to consider for 2023 rice production is control failures with pyrethroids targeting rice stinkbug. With pyrethroids not being a viable option, growers may want to plan ahead for rice stinkbug sprays. In Mississippi, we recommend a spray when three rice stinkbugs are present per 10 sweeps until the soft dough stage, and then it becomes 10 bugs per 10 sweeps. One control option is Malathion which provides little to no residual control. The second option is Tenchu; however, due to supply issues, there has been little of this product available in past years. During 2022, a section 18 label was made available for Endigo due to lack of available options. Hopefully, it will be granted again in 2023 if needed.
Furrow-irrigated rice [slightly] exposed in 2022?
Every year around this time, everyone is getting sick of office and shop work. February tried to fool us into thinking we’d have an early spring, but here we are in late March seeing temperatures well below freezing between periods of rainfall. We were able to get our first planting date in at both trial sites on March 15 and 16, but to my knowledge, those are the only two acres of rice planted in the state of Missouri. After the dry fall, there is a lot of ground ready to go in the Bootheel. If we ever get a dry period leading into the first of April, rice planting will begin.
While we are currently worried about getting water off our fields, it won’t be long until we’re in the dry summer months and running wells nonstop. Last year tested the limits of some of our wells as well as the limitations on some of our rice ground, particularly furrow-irrigated rice (FIR).
It’s obvious that I like talking about furrow-irrigated rice, so why stop now? The large surge in FIR acres over the last five or six years has spanned over seasons where we’ve caught some tremendous “catch-up” rainfalls at key times on our FIR. However, 2022 was a different story, and most of us did not receive those “catch-up” rains, exposing a key weakness of furrow-irrigated rice — that being a dry summer.
With that weakness exposed, I’d expect us to better refine where FIR has a fit, as there are many farms still loving the benefits of the system.
For those of us growing FIR, another issue we have seen as acreage increases is rice billbug damage. Billbug larvae will burrow into the rice stem and can cause severe damage where infestations occur. The new MU Crop Protection Specialist Dr. Chase Floyd has and will continue to study the habits of the rice billbug in the Mid-South.
Currently, Floyd and others say the only effective way to minimize damage is with a diamide seed treatment. Consider adding either Dermacor or Fortenza to your seed treatment package if you expect to have billbug pressure or have seen severe damage in the past.
We may not ever know if a seed treatment truly paid off, but research data routinely shows an economic benefit to a diamide seed treatment regardless of billbug numbers. If you’ve got any questions, don’t hesitate to give me or Dr. Floyd a shout. As always, eat MO rice!