Michael J. Stout, William C. Rice and Dennis R. Ring
The rice water weevil is an important biological constraint on rice yields in the southern United States and has been recognized as such almost as long as rice has been grown in the South. Yield losses in Louisiana, where this insect is a particularly severe pest, typically exceed 10 percent and can approach 30 percent or more. In addition, this insect has been accidentally introduced into some of the major rice-producing regions of Asia and poses a global threat to rice production. For most of the past 30 years, this insect was controlled in Louisiana using granular Furadan (carbofuran). The registration for the use of Furadan in rice, however, was revoked in the late 1990s by the U.S. Environmental Protection Agency. Cooperative research conducted by scientists from the LSU AgCenter and the U.S. Department of Agriculture (USDA) has met the immediate need for alternatives to Furadan, and efforts to improve the management program continue.
The goal of ongoing research efforts is the development of a cost-effective and environmentally friendly program for management of the rice water weevil. Judicious use of insecticides will remain a large part of the management program for the foreseeable future. However, relying solely on insecticides to manage the rice water weevil sometimes results in unacceptable control and increases the likelihood of the development of resistance to insecticides. Sole reliance on insecticides also can result in toxicity to nontarget organisms. For these reasons, a major focus is the diversification of the management program through the incorporation of cultural practices and host-plant resistance. The foundation of this integrated management program is a thorough understanding of the biology of this important pest and its interaction with rice.
The seasonal history of this pest in Louisiana begins in early spring, when adult weevils emigrate from their overwintering sites to rice fields. Models based on more than 20 years of flight records and weather data predict initial flights of weevils in early April in southwestern Louisiana. Upon arrival in rice fields, adult weevils feed on leaves of rice plants, leaving lengthwise feeding scars. This type of injury, although evidence of the presence of weevils in a field, usually does not result in economic losses. Serious problems do not begin until the rice fields are flooded and the weevils begin to lay eggs. The newly hatched larvae feed externally on the rice plant roots. Root pruning by larvae can damage root systems extensively, ultimately reducing grain yields. The insect has two to three overlapping generations in southern Louisiana. In August, adults of the final generation fly to overwintering sites, primarily wooded areas, and live in bunch grasses and under leaf litter.
An important peculiarity of the biology of this insect is its dependence upon flooded conditions for egg laying and development. This peculiarity makes rice fields, which are flooded for a large part of the growing season, an ideal habitat. Recent studies conducted in a greenhouse have shown that both feeding by adult weevils and egg laying by adult females are markedly reduced when rice plants are not flooded. Rate of oviposition (egg laying) is influenced by the depth of flooding also. Moreover, development of larvae of the rice water weevil is impeded in dry soil. This dependence by the rice water weevil on flooding forms the biological rationale for the traditional cultural practice of draining rice fields when an infestation of weevils occurs. It also enables the manipulation of weevil populations in rice fields through water management practices.
Injury to rice plants results from chronic feeding by weevils on rice roots. Infestations begin when rice fields are flooded and continue at least through the early reproductive stages of rice. Feeding by large larvae in the later stages of their development is most injurious to root systems. Young plants are especially susceptible to the effects of feeding because their root systems are smaller. Removing large amounts of root tissue has several effects on the physiology of rice plants. The major physiological effects of root pruning appear to be a reduction in tillering of infested plants and a resultant reduction in panicle density at harvest. A reduction in average grain weight also contributes to yield losses.
Insecticides remain the most effective means of controlling weevils. Since the registration of Furadan was revoked, four insecticides have been registered for use against the rice water weevil: Icon (fipronil), Karate (lambda-cyhalothrin), Fury (zeta-cypermethrin) and Dimilin (diflubenzuron). Data on the effectiveness of these products generated by LSU AgCenter and USDA scientists were used by industry to support the registration of these insecticides. Use of these newly registered insecticides involves a considerable departure from past practices. Furadan was targeted against larvae, and applications were made into flooded soils when densities of rice water weevil larvae exceeded economic thresholds. In contrast, Icon is a prophylactic seed treatment, and the decision to use Icon must be made before rice is planted. Karate, Fury and Dimilin are applied to the foliage of rice plants, with Karate and Fury targeted at weevil adults and Dimilin targeted at eggs.
When used correctly, these newly registered insecticides control rice weevils as well as or better than Furadan at the same or lower cost. Evaluation of the efficacy of additional insecticides against the rice water weevil continues, with registration of one and perhaps two new products possible within the next few years.
Karate, Fury and Dimilin pose a particular challenge to use. These insecticides are not effective against larvae once they have begun feeding on roots of rice plants. Thus, timing of application is critical and must be based on densities of adult weevils rather than on densities of larval weevils, as was the case with Furadan. Currently, applications of these products are recommended when adult weevils are found in a field and conditions for oviposition are present (fields are flooded or about to be flooded). The research necessary to develop more precise application thresholds is in progress. The most significant barrier to the development of such thresholds is the absence of a simple, inexpensive and effective monitoring tool for adult weevils.
Cultural practices for control
Although weevil management still depends heavily on insecticides, appropriate cultural practices can also contribute. Recent research has focused on the cultural strategies of delayed flooding and early planting.
Delayed flooding. One practice that can be used to reduce yield losses is to avoid applying floods to rice fields when rice is young. Historically, rice producers in many rice-growing areas of Louisiana have flooded fields within a few weeks of seeding, principally as a control for important weed pests such as red rice. Early flooding, although it effectively suppresses the germination of weeds, exacerbates the pest status of the rice water weevil by allowing infestation of young, vulnerable rice. Delaying permanent flood not only reduces the absolute number of larvae infesting plants in a season, because female weevils lay eggs only under flooded conditions, but also delays infestation until rice plants are older and more tolerant of injury. Delaying the application of permanent flood to rice fields by as little as two weeks (for example, flooding when rice plants have four to five leaves rather than two to three) can reduce the susceptibility of rice to yield losses dramatically (Figure 1).
Early planting. Another way in which losses from weevils can be reduced is to plant rice early in the growing season. Recommended dates for planting rice in southwestern Louisiana, where this research has been conducted, are from March 15 to April 15. Although rice water weevils begin to emerge from overwintering in early April, emergence is a protracted process, and populations do not build up to their highest levels until much later in the growing season. By planting early, producers can usually avoid exposing rice to high populations of weevils. Producers who plant later in the growing season can expect to encounter higher densities of weevils and have greater yield losses (Figure 2).
Host plant resistance
Investigation of host-plant resistance as a means of controlling the rice water weevil has a long history in Louisiana. A collaborative screening program, in which rice lines from various geographic sources are evaluated for resistance and tolerance to the rice water weevil, has been conducted by USDA and LSU AgCenter scientists for more than 30 years. A number of varieties and plant introductions with low levels of resistance or tolerance to the rice water weevil have been identified, but efforts to intentionally incorporate weevil resistance into commercial varieties have been limited. Analysis of screening experiments conducted over the past 10 years has shown, however, that many of the varieties developed and grown in the southern United States are more resistant to the rice water weevil than rice lines from other parts of the world. Furthermore, in a recent comparison of varieties commonly grown in Louisiana, some were found to be significantly less susceptible to the rice water weevil than others. Thus, it is possible that low levels of weevil resistance have been fortuitously incorporated into some varieties during the process of breeding.
Significant progress in the development of an integrated management program for the rice water weevil has been made since Furadan’s removal, but more research is needed to refine the management program. Improved methods for monitoring weevils and thresholds for the application of foliar insecticides are two areas of need. Probably the greatest need, however, is for the development of rice lines with high levels of resistance to the rice water weevil. Evaluation of rice lines from various parts of the world continues. If efforts to improve the resistance of rice to this insect using traditional breeding methods fail, the use of genetic engineering to increase the resistance of rice plants to the rice water weevil is being considered.
Rice culture is undergoing important changes because of new technologies (herbicide-tolerant and hybrid rice lines) and the increasing prominence of other practices such as conservation tillage. Continued study of the influence of these changes in rice culture on the rice water weevil is critical to an understanding of how to manage rice so as to minimize the destructive influence of this important pest.
Michael J. Stout, Assistant Professor, Department of Entomology, LSU AgCenter, Baton Rouge, La.; William C. Rice, USDA-ARS, formerly at the Rice Research Station, Crowley, La.; and Dennis R. Ring, Extension Entomologist, LSU AgCenter, Baton Rouge, La.
(This article was published in the winter 2002 issue of Louisiana Agriculture.)