Specialists Speaking: April 2025

Insect Defense

Insect control in Arkansas rice starts up front before the seed hits the ground. Rice water weevil and grape colaspis are our worrisome early season insect pests, and the below-ground damage they cause is difficult to combat.

For years, we have recommended the use of an insecticide seed treatment to control rice water weevil and grape colaspis. This recommendation continues with a primary focus on a neonicotinoid such as Cruiser or NipsIt. These not only provide control of both pests but have also been documented to increase early season vigor and growth of rice.

A bonus is that they also seem to help plant growth in the face of herbicide drift events. The downside to the neonicotinoid seed treatments is that their efficacy against rice water weevil begins to lessen as we exceed 35 days after planting before going to flood.

In recent years, further testing of diamide seed treatments in rice (Dermacor and Fortenza) has shown the benefits of adding these products to a neonicotinoid treatment. The diamides have better efficacy against rice water weevil and the control lasts longer. While it is an extra cost to add to the seed, the data points to the treatments paying for themselves even when insect pressure is low.

Rice stink bugs are a persistent concern each growing season. While we do not always know to what degree they’ll show up, we ultimately know that they will be there at some point. Increased resistance to pyrethroid insecticides such as lambda-cyhalothrin has made these products limited in their utility and they should mostly be avoided.

Instead, we now only have one effective labeled insecticide for rice stink bug (RSB) control and that is Tenchu (a neonicotinoid). While the cost is greater for Tenchu than the pyrethroids, the residual control seems to be improved at recommended rates, which limits the need for multiple applications.

Also important to this conversation is a change to our RSB threshold recommendations. For many years, the threshold has been to treat when five RSB are found per 10 sweeps the first two weeks of heading, then increase to 10 RSB per 10 sweeps the second two weeks of heading. Research over the past 10 years has led us to change to a season-long threshold of 10 RSB per 10 sweeps. All this recent data points to no loss in grain yield or milling yield, and no increase in peck, with the new threshold compared to the old threshold.

Treat the seed up front to protect from tough-to-control, root-feeding insects and follow the new RSB threshold late in the season. These will be keys to maximizing our insect control in 2025 and minimizing our inputs.

Rice Water Weevil

The rice water weevil is the most important insect pest of rice in Louisiana and throughout the Mid-South. The pest is ubiquitous throughout Louisiana’s rice growing regions, and damaging infestations are common. Yield losses in excess of 25% can occur from severe infestations. A management program should be implemented in every rice field, every year to avoid losses.

Rice water weevil adults are grayish-brown weevils (beetles) about 1/8-inch long with a dark brown V-shaped area on their backs. Adults of this insect emerge from overwintering sites beginning in early April in southern Louisiana (later in northern Louisiana). Adults then 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.

Oviposition (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. White, legless, c-shaped larvae with small brown head capsules emerge from eggs in about seven days. After hatching from eggs, larvae 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-inch long. Peak larval density occurs three to five weeks after flooding. Mature larvae pupate and oval pupae remain attached to the roots until adult emergence. Pupae are covered with a compacted layer of mud and resemble small mud balls when roots are cleaned. Adults then either migrate to surrounding rice fields or overwintering sites.

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, elongate eggs in the leaf sheath at or below the waterline. In addition to rice, adult rice water weevils will oviposit in most aquatic grasses and sedges, including barnyard grass, fall panicum, red rice, yellow nutsedge, and broadleaf signalgrass. Thus, the presence of these weeds on levees surrounding rice fields may make the fields more susceptible to attack by rice water weevil adults.

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.

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. This feeding or root pruning results in a reduction in the number of tillers and in the amount of aboveground plant material produced by the damaged plant.

Root pruning may interfere with nutrient uptake by plants. Damage to roots ultimately can result in yield losses by decreasing panicle densities, numbers of grains per panicle, and grain weights. Plants with severely pruned root systems will appear stunted and may turn yellow and appear to be nitrogen deficient.

Infested stands are often thin in appearance and are more susceptible to lodging. At harvest, plants from heavily infested fields will be shorter than normal and have lower yields. Each larva found in a 4-inch (diameter) by 3-inch (deep) core sample is associated with an approximately 0.5% to 1.5% loss in yield. Yield losses tend to be higher in water-seeded rice fields. Losses are higher because these fields are usually flooded at an earlier stage of plant growth and thus are susceptible to oviposition and infestation by larvae earlier. Young rice plants are more susceptible to yield losses than older plants with more established root systems.

A variety of cultural and chemical controls can control rice water weevils in rice fields. Cultural strategies include planting rice early in the season rather than late, delaying the application of permanent flood and perhaps managing weeds in and around rice fields. However, insecticides are often needed in addition to cultural control tactics to prevent yield losses.

Insecticide management practices for the rice water weevil are evolving as pesticides are added to and removed from the integrated pest management plan. These insecticides fall into three general categories: (1) prophylactic seed treatments, (2) early post-flood adulticides, and (3) larvicides. For the most current list of registered pesticides, please consult LSU AgCenter publications 1838 (“Pest Management Guide”) and 2270 (“Rice Varieties and Management Tips”) found on the LSU AgCenter website:  www.LSUAgCenter.com.

The Usual Suspects

Time to round up the usual suspects! When talking about arthropod control in California, the usual suspects are rice seed midge, tadpole shrimp, and armyworms. I will focus on rice seed midge and tadpole shrimp in light of recent pyrethroid exceedances detected in the Sacramento Valley.

The challenge with both rice seed midge and tadpole shrimp is that they are difficult pests to scout for and their injury can go undetected and can happen quickly. Unlike shrimp, midge does not muddy the water, so its presence can go unnoticed. Midge larvae construct small silken tubes that get covered in soil and are attached to the seed. These tubes are about the length of a seed and visible with the naked eye. In fields that take long to flood or where there is a delay in seeding, the tubes can be seen even before seeding. Midge larvae hide in the tubes and feed on the seed, hollowing it.

Usually, we suspect a field may have shrimp when the water is muddy (and there’s no wind). However, small shrimp won’t muddy water but can injure rice. Once you can see shrimp with the naked eye, they can cause damage. Shimp grow very fast, and they can go from “invisible” to large in just a couple of days. Shrimp will feed on the shoots of the seed as it germinates; they will not hollow the seed like midge does.

Late planted fields, or seeding during periods of high temperature, increases the risk of damage by both pests. Seeding in fields that have been flooded for four days or longer also increases the risk.

Unfortunately, we do not have traditional thresholds for these pests. It is difficult to count midge or shrimp in the field. Instead, we rely on the number of healthy seedlings per square foot. Remember that at a seeding rate of 180 pounds per acre, you are broadcasting about 60 seeds per square foot. The optimum number of plants per square foot to maximize yields is 25, but rice is able to compensate as stands decrease. Many other factors will affect stand, so you want to be as close to the 25 as possible.

I use a small aquarium net to scoop seeds/seedlings and shrimp during windy days when the water is muddy. Inspect the seeds/seedlings and determine if they have been injured. If the number of seedlings per square foot is less than 25 and either midge or shrimp are present, an insecticide is needed. Rice is susceptible to both pests during the period of germination, but once the root is anchored in the soil and you have a well-developed spike, seedlings are less susceptible to damage. If you “leather” your field (drain very soon after seeding), then midge or shrimp won’t be an issue and no insecticide treatment will be needed.