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Rice Water Weevil or Lissorhoptrus oryzophilus is one of the most problematic insects in rice production today. Rice Water Weevil is a native to the Western United States and has sense then spread throughout the US as well as several countries. However, it was not discovered in California until 1959. Lissorhoptrus oryzophilus remained confined in the United States until its incidental spread to Japan in 1976 where it rapidly spread over Japan. The insect was also discovered in India, China, and North Korea in 1988 and was found in Europe for the first time in 2004. (Lupi et al.,2007)
Biology and Ecology
The Rice Water Weevil belongs to the Coloeptera order. It is holometabolous meaning it begins as an egg, grows to larvae, then to a pupa and finally an adult. The total life cycle of the insect takes anywhere from 35 to 45 days, depending on temperature (Texas A&M University,n.d.).
Adult Rice Water Weevils are small (1/8"-1/4") beetles with long snouts. They differ in color depending on their environment, dry weevils are brown with black spots on their backs, when they are wet they are solid brown. The adult weevils over winter in the bases of grass clumps, piles of plant material and other debris and emerge in the Spring when the temperatures reach around 70 deg F. Overwintering leaves the Rice Water Weevils wing muscles too weak to fly, they then feed on grasses until they have built their wing muscles back up enough to fly (Godfrey et al., 2009). The rate at which the muscle is rebuilt is strongly influenced by temperature. This process usually takes near a week. Once the insect has built up enough strength to fly it begins migrating towards open water in search of a place to feed and mate. This flight usually occurs from late April to late May, which is around the time rice fields are being flooded. The Rice Water Weevils attraction to open water makes a flooded rice field very appetizing. The adults usually move into the field soon after the permanent flood is established to begin feeding and oviposition. (Bernhardt et al., 2006)
The female water weevils normally lay their eggs around 2 weeks after emergence from overwintering; this however only takes place if the rice is flooded. The weevils normally feed on the crop for a few days then begin mating. The female, once fertilized, begins laying her eggs in the submerged leaf sheaths of the young rice plants. After about a week, the larvae emerge as small (1/32") white grub-looking worms, with brown heads. The larvae have two hook like appendages that help them move, as well as allow them to draw oxygen from the plant while submerged (Godfrey et al.,2009). Once the larvae have hatched in the sheath the feed on in for a short time then move their way down into the soil and begin feeding on the roots of the rice plant. The larvae feed on the roots of the rice plant for about 4 weeks until they are fully developed, they then enter pupal stage. Pupal stage, which will take around 7 days, is completed in a watertight cocoon made of mud. After the 7 days and adult Rice Water Weevil emerges from the cocoon and begins feeding on the foliage of the rice. The first generation rice water weevils that produced the newly hatched adults begin to die off around July. A small portion of the second generation weevils will mate and lay eggs and produce a smaller second hatch of Rice Water Weevils. The newly hatched adults will feed on the rice from June until around September or until they can find a place to overwinter. (Bernhardt,2006)
Pest Damage in the System
The only stage in which the Rice Water Weevil is an actual pest of rice is the larval stage. Once the larvae move from the sheath down into the soil they attach to the roots with a pair of hooks that allow them to draw oxygen from the plant to survive. They then use their chewing mouthparts to feed on the roots of the plant potentially causing sever pruning of the roots (Texas A&M Agrilife Extension,n.d.). The pruning of the roots causes slower plant growth which can lead to smaller plants which results in reduced yields (Stout et al.,2011).
Sampling Plans and Thresholds
The adult Rice Water Weevil feeds on the foliage of the rice plant for most of its life, leaving streaks down the leaves that run parallel to the margins and veins of the leaf. This feeding by the adult weevil has no effect on yield but it does leave identifiable marks on the plant that makes the presence of the insect known. The adult weevil is attracted to deep water and thin stand which makes them most prevalent near levees or on the sides of fields. These marks or "feeding scars" can be observed and used to make a decision using Sequential Sampling. The thresholds for sequential sampling from Bernhardt (2006) which are published in the University of Arkansas Rice Production Handbook are as follows:
Total Number of Plants With Feeding Scars on New Leaves2
Don't Treat Stop Scouting When Total Is Less Than
No Decision4 Keep Scouting When Total Is
Treat Stop Scouting When Total Is More Than
Each stop should consists of checking the youngest leaves on 40 plants and counting every plant with a scar. These checks should be done around 6 feet off the levee or a low spot in the field with deeper water and a thin stand. If the field is checked and numbers are between thresholds, come back to the field and scout again in 3-5 days. If the percentage of plants with scarred leaves exceeds 60% in drilled rice it should be treated. This percentage drops to 50% of plants scarred in aerial seeded rice. (Bernhardt,2006)
Another means of sampling for Rice Water Weevil is larval scouting. Larval Scouting consists of taking a sample of the soil and roots of a rice plant. This sample, called a core sample, is 4 inches in diameter and around 3 inches deep. The plant and soil sample is then placed in a screen bucket and shaken to dislodge the larvae from the roots and soil. Once the larvae are floating on top of the water they are counted. If there are 20 or more larvae and pruning has occurred yield loss can be expected and cultural control methods may be a viable option. Ten to twenty larvae in the core sample suggest yield loss will vary and cultural control could possibly be an option. Less than 10 larvae per core yield loss is minimal. Larval scouting is not as common as other sampling methods because it there is no chemical application that can be made to control Rice Water Weevil larvae. This type of sampling is used to make nonchemical decisions such as adding supplemental nitrogen or draining the water and reflooding. (Bernhardt, 2006)
Former and Current Management Techniques
PRE-1998 there was only one insecticide that was available for the control of the Rice Water Weevil. This one insecticide was called Furadan which was a dry derivative of carbofuran. Producers were very dependent on this product because of its ability to control the larvae of the Rice Water Weevil, which is the only damaging stage of the insect. However Furadan was banned in the mid-nineties by the EPA which drove up the demand for new technology. Icon, which also targeted the damaging stage of the insect, was introduced as a seed treatment not many years later, as a result of this demand for new technology. Icon however was also banned from use in rice because of its extremely high toxicity to crawfish. (Stout et al.,2011)
Current management strategies are fairly narrow compared to pests in other crops. The best preventative measure that can be taken is applying a seed treatment such as Cruiser Maxx or Dermacor. Cruiser Maxx effectively controls Rice Water Weevil by killing the adults that feed on the leaves as well as the larvae that feed on the roots of the plant. Dermacor only targets the larvae feeding on the roots of the plant and has no effect on the adults. A study done by Fortner et al. (2010) reviewed the efficacy of Cruiser Max, Dermacor X-100 and NipsIt Inside on CL 151 variety rice. It was concluded in this study that all of these seed treatments reduced pest populations. NipsIt Inside actually had the highest yield out of all three treatments. (Fortner et al.,2010)