Introduction

Many different insects may inhabit rice fields, but only few are considered pest species. In general, insect problems in Missouri rice fields are much less severe than those in other rice-producing areas of the United States. The most common insect pest of rice in Missouri is the rice water weevil followed by the rice stink bug and grasshoppers. Several other insects (ex. armyworms, chinch bugs) rarely build to economic infestations.

Management recommendations listed in this guide are based on work conducted in Missouri and other rice producing states (i.e. Arkansas, Louisiana, and Texas). These recommendations are designed to provide economical pest control, but minimize production costs, disruption of beneficial arthropod populations, and environmental pollution. Timely, regular scouting of fields will help determine when economic pest infestations are present to warrant insecticide applications. Insecticides recommended for controlling rice pests in Missouri are listed in this guide.

Insect Pests During the Vegetative Growth Stages (seedling to tillering)

Rice Seed Midge:
Adult midges frequently swarm over rice fields and levees and other bodies of water. This small insect resembles a mosquito but lacks wing margin scales and veins. Since the adults do not feed, only the larval stage damages rice. Adults prefer to deposit their eggs in masses on the water surface. Once the larvae hatch (1 to 2 days), they construct mud tubes on the bottom and feed on algae and organic matter. Generally, this insect is harmless in rice except in water-seeded fields. Damage: hollowed-out seeds, chewed off roots and shoots

Currently, seed treated with fipronil (Icon 6.2FS) is the only insecticide treatment available to combat seed midge infestations. Cultural means of avoiding midge infestations include: pre-germinated seed, time planting when conditions are present for rapid seedling growth, and seed fields within two days of the permanent flood.

Grape Colaspis:
This small, yellow-brown beetle has a wide host range that may include rice. Both the adult and larval (1/4-inch in length, off-white) stages feed on rice, but the larval damage to seedling shoots is the biggest concern. Generally, rice is most vulnerable to this insect when rice is rotated after lespedeza or soybean. After the female beetle lays her eggs in the soil near alternate host plants, the larvae hatch and feed on nearby roots. This insect overwinters as a larva in the soil. Currently, seed treated with fipronil (Icon 6.2FS) is the only insecticide treatment available to combat larval infestations. Damage: seedling root and shoot damage

Chinch Bugs:
These small, black and white insects may invade rice from adjacent grain sorghum or wheat fields or from border vegetation. They damage rice plants by extracting plant fluids. Generally, economic infestations are rare in Missouri. Damage: weakened seedlings

Rice Water Weevil:
An adult rice water weevil is small (1/8-inch in length) and gray. They overwinter in clumps of perennial grasses, leaf litter, etc. adjacent to rice fields. Once the low temperature exceeds 65 degrees F for three consecutive nights during the spring, adults take flight to feed, mate, and locate egg-laying sites. Female weevils prefer to lay their eggs in young grass plants (<2 weeks old) standing in water. This egg laying activity starts once a permanent flood is established (drilled-seeded) or when rice plants emerge above the water surface (water-seeded). After hatching the larvae feed for three to four weeks before pupating, and the next generation of adults emerges within five to six weeks. Generally, there is one generation per year in Missouri; however, another generation is possible in late-planted rice.

Adult foliar damage produces translucent, longitudinal scars, but this damage is not of economic importance. Rather, management programs are geared to prevent the white, legless larvae (i.e. root maggots) from causing severe root pruning damage. The progeny of the overwintering generation causes the most damage because the root systems of smaller rice plants may not compensate for the larval root damage. This root damage can eventually lead to lower yields. Also, weeds have less competition to become established because the weevil's root damage reduces the rice plant's tillering and growth. Damage: longitudinal scars on leaf, root pruning

Management:
The severity of rice water weevil infestations is dependent upon several different factors. These may include the type of cropping system (drilled-seeded versus water-seeded), length of time in rice production, weevil infestation levels during previous years, availability and proximity of overwintering sites, stand density, and environmental conditions. In particular, the type of cropping system can greatly influence the level of weevil infestations. Because of the smaller plant size and earlier flooding of the fields, water-seeded rice is more susceptible to rice water weevil damage than drilled-seeded rice. In water-seeded fields or drilled-seeded ones following many years of consecutive rice production, a preventative insecticide treatment is advisable.

Control:
A cultural approach to managing rice water weevil infestations is prematurely flood the rice paddies, remove the water after 10 to 20 days, allow the soil to dry and crack, and then reflood the field. This method is called the 'drain and dry' and can reduce rice water weevil populations by killing the soil-borne larvae; however, there are severe economic drawbacks involved with this system. These drawbacks include greater costs for fertilization, irrigation, disease and weed control, and labor. In addition, rainfall may never permit adequate drying of the field to kill the rice water weevil larvae.

The other control option for rice water weevils is to apply an insecticide. Before you elect to use an insecticide, you must first decide between an at-planting application or an in-season, rescue treatment. Formerly, carbofuran (Furadan 3G) was the standard material and was applied based on larval core counts. Now the options include a seed treatment with fipronil (Icon 6.2FS at 12-20 fl. oz. / 100 seed wt.) targeted at the larval stage, diflubenzuron (Dimilin 2L at 12-16 fl. oz. / acre) for the egg stage, and a pyrethroid insecticide to control the adult stage.

Each labeled insecticide has its own advantages and disadvantages in terms of controlling this pest as well as others. Since Icon is a seed treatment, application costs are eliminated when the rice is planted. In drilled-seeded rice with chronic rice water weevil infestations or in water-seeded rice fields, Icon provides good residual control of the weevils. Disadvantages associated with using Icon include: unknown extent of rice water weevil infestations before planting, and additional equipment and costs associated with treating the seed.

A foliar insecticide application may be preferable when rice fields are infrequently infested with the rice water weevil. A disadvantage with foliar applications is that timing these applications is absolutely crucial. Once the permanent flood is established and female weevils submerge to lay their eggs, these foliar treatments will no longer provide effective control of the adults. Dimilin is only recommended to control the egg stage. The University of Arkansas adult leaf scar sampling method (Table 1) is useful to determine when the severity of weevil infestations warrants a foliar insecticide application. Preliminary field research suggests that foliar insecticide applications are most effective when applied 7 to 10 days after permanent flood in drilled-seeded rice. Timing is even more crucial with water-seeded rice. In Arkansas, the recommended application timing is when approximately 50% of the plants are just above the water surface; whereas, in Louisiana, the recommended application timing is when adults, leaf damage, and favorable egg-laying conditions (water in the field) are present.

Sampling (Drilled-seeded):
The leaf scar scouting method is a sequential system that should begin within 3 days of the permanent flood and continue for 10 days. Starting 10 feet from the field border or a levee and avoiding thin stands, examine the youngest, unfurled leaf from 40 plants for signs of adult feeding scars. A minimum of two stops (40 plants per stop) is needed to determine if further scouting is needed or be discontinued and apply an insecticide. If an insecticide is not needed, inspect the field again after four to five days have passed.

Table 1. Treatment levels for rice water weevil using the leaf feeding scar method.¹

Armyworms:
Two armyworm species (true and fall) may attack rice but at different times of the season. True armyworms generally attack rice in May and June; whereas, fall armyworm infestations occur later in the season (July and August). True armyworm larvae are drab green to black with three longitudinal strips and a smooth appearance. Fall armyworm larvae are olive drab in coloration and have a more pronounced inverted yellow "Y" mark on their head. Rice most at risk to true armyworm infestations are fields adjacent to wheat. Fall armyworm infestations generally start along grassy field borders and levees. Early detection can help limit insecticide use to these areas. Damage: foliar damage

Maize Billbug:
The larval stage (legless, large, brownish) of this sporadic pest feeds on the roots and stems of rice. Adult weevils (large, black with long-snout) are attracted to poorly flooded rice fields. Flooding fields or spot treatments with insecticides are recommended control practices. Damage: seedlings lodge, white seed heads

Insect Pests During the Reproductive Growth Stages (panicle initiation to harvest)

Rice Stalk Borer:
This sporadic pest invades rice from levees and/or field border areas. The large (1 1/4-inches in length), white larvae typically penetrate large stems just above the waterline. As many as four to eight-inch long larvae can be found feeding within a single stem. Currently, there are no economic thresholds or insecticides available for this insect. Fall plowing can reduce overwintering larval populations, and early planting with a non-susceptible variety can help minimize late-season borer damage. Damage: stem tunneling, blank seed heads or "whiteheads"

Grasshoppers:
Many different grasshopper species attack rice; however, short-horned (with long antennae) species pose the biggest threat to rice. Generally, infestations start near field borders and levees where grasshopper populations buildup on the alternate host plants. Foliar damage has little economic importance but heavy damage to the seed heads can be a problem. Insecticide applications are recommended when populations are >1 per square foot during the seedling stage or >10 per 100 seed heads during the heading stage. Damage: holes in leaves, blank heads, mangled grain

Rice Stink Bug:
Adult rice stink bugs are approximately 3/8-inch in length, shield-shaped with a sharp spine at the widest point on their bodies. Its upper coloration is straw-brown to a bright yellow underneath. Adults overwinter near food sources within leaf debris. After mating, females lay their green, barrel-shaped eggs in masses on leaf blades. The nymphs hatch within five days and mature to adults within 18 days.

This stink bug species is a grass feeder and it prefers to feed on barnyardgrass or any other grass species. Populations usually increase in grassy field margins before movement into nearby rice fields. One preventative control approach is to avoid mowing these field margins once rice enters the heading stage. Both the adult and nymph stages can cause economic damage when they feed on developing rice kernels. Yield reductions are rare, but kernel quality can be affected in some years. To determine if an infestation is present, take several sweep-net samples at random locations in a field. Begin scouting when 75% of the panicles emerge and continue the weekly counts until harvest. An insecticide application is recommended when >5 stink bugs / 10 sweeps are collected during the first two weeks after 75% panicle emergence. For the remainder of the year, the threshold doubles to >10 stink bugs / 10 sweeps. Damage: "pecky" rice

Precautions:

All insecticides listed in the recommendation list (Table 2) are poisons. These products must be handled and applied with caution. Follow all clothing and safety precautions printed on the container label. Workers reentering treated fields should follow reentry procedures provided on the insecticide label.

Insecticides listed for rice insect control may interact with the herbicide propanil. This interaction can cause severe injury to rice unless applications are properly timed. Do not apply methyl parathion within five days before or after propanil applications. Do not apply malathion or carbaryl insecticides within 14 days before or after propanil application. If insecticides are necessary and the time frames suggested cannot be met, consider other herbicide options.

Do not apply long residual or highly toxic insecticides immediately next to or over fish-bearing waters. Leave an insecticide-free zone of 50-100 feet between any body of water and the treated rice field. Preharvest intervals for all recommended insecticides are listed in Table 2.

Table 2. Reentry and preharvest intervals for selected insecticides used in rice. This list may not include all insecticides labeled for rice. Reentry and preharvest intervals for insecticides not listed can be found on the insecticide label.