Lina Bernaola, Maisarah Mohamad Saad, James Villegas, Emily Kraus and Michael Stout

With the human population expected to exceed 9 billion by the middle of the 21st century, there is a serious need to increase food production to meet global demand. The interactions of herbivorous (plant-eating) insects with their favored host plants are important determinants of plant productivity in agricultural ecosystems because foliage-, sap- and root-feeding herbivores consume approximately 20 percent of annual crop production worldwide. Increasing crop productivity by preventing crop losses from insect herbivores has become an urgent need. Currently, management of insect herbivores in crops relies heavily on the use of insecticides. Use of insecticides is accompanied by a number of serious problems, including high costs, nontarget effects on humans and beneficial organisms, and environmental contamination. In addition, insect pests reproduce quickly and have the ability to overcome, avoid or develop resistance to insecticides. For all of these reasons, there is a need to develop more sustainable and cost-effective approaches for pest control.

One alternative to insecticides for pest management is the use of crop varieties with inherent resistance to insect herbivores. Plant resistance can be defined as the heritable ability of a plant to escape, minimize, or tolerate feeding by herbivores and thereby minimize yield losses from herbivores. When available, resistant plant varieties offer an effective, economical and environmentally friendly management tactic. While considerable progress has been made in identifying and developing resistant varieties in some crops, plant resistance remains an underutilized tactic in other crops. One focus of the rice entomology program in the LSU AgCenter is to increase the use of plant resistance against the major insect pests of Louisiana rice.

In Louisiana rice, varietal resistance has the potential to be incorporated into integrated pest management programs for the rice water weevil and stem borers. The rice water weevil is a significant economic pest in Louisiana. This pest interferes with rice production by feeding on the roots of flooded rice plants. This insect is primarily controlled using insecticidal seed treatments or by application of foliar insecticides. Between 5 percent and 30 percent of the rice harvest can be lost in fields not treated for this pest. Stem borers are not yet considered a widespread or consistent problem in Louisiana, but their importance is expected to increase and the AgCenter entomology program is conducting research to incorporate plant resistance into management programs for this insect as well.

Evaluation of commercial rice varieties for resistance to these pests in Louisiana is an important first step in increasing the use of plant resistance. A recent five-year field study (2013-2017) investigated the susceptibility of a number of widely grown rice varieties against the rice water weevil. Unfortunately, all rice varieties studied were susceptible to rice water weevil infestation (Figure 1), although the medium-grain variety Jupiter was consistently found to be more susceptible than most of the common long-grain varieties. The long-grain variety Jefferson, which is no longer grown in Louisiana, was found to support lower larval densities than other varieties, and a cooperative breeding effort has been initiated to use Jefferson to develop high-yielding long-grain varieties with greater resistance to rice water weevils. Another more recent series of experiments has shown that hybrid rice may be more tolerant of weevil feeding than conventional varieties, meaning they are capable of maintaining high yields even under heavy attack by rice water weevils (Figure 2).

Similar studies have been conducted over the past two years to evaluate the resistance of rice varieties to stem borers. Preliminary results have revealed that the number of whiteheads, a symptom of borer infestation, varied significantly among rice varieties. CL151 showed the highest number of whiteheads, while LA110 did not show visual presence of whiteheads. Other varieties, like Cocodrie, showed contrasting results (Figure 3).

In cases like rice in which varietal resistance is not available for all pests, it is important to understand the environmental, agronomic and other factors that optimize the expression of resistance by plants. LSU AgCenter researchers in the rice entomology program are investigating the impact that nitrogen and silicon fertilization has on the expression of plant resistance. In addition, the researchers are attempting to stimulate the expression of plant resistance by applying plant hormones known to be involved in controlling the expression of resistance. For example, rice seeds treated with the plant hormone methyl jasmonate before planting have shown greater resistance than control plants against rice weevils and armyworms in greenhouse experiments, thus suggesting the possibility of applying plant hormones as elicitors in the field to increase the resistance of commercial rice varieties.

Finally, fundamental research in plant biology is vital to the development of future applications of plant resistance. Plants possess many defenses that confer resistance to pests, making them less detectable by or edible to herbivores. In collaboration with scientists from the USDA and elsewhere, AgCenter researchers are investigating the role of these potential defenses using rice plants with mutations in genes affecting the accumulation of specific defenses and using plants with mutations in the hormonal pathways governing the expression of these defenses. Using these mutant rice lines allows us to understand the role these defenses and hormonal pathways in plant-insect interactions with more precision. Greenhouse experiments were conducted in 2016 and 2017 to compare the resistance of mutants and wild-type rice plants against the rice water weevil and fall armyworm. Data show that numbers of larval weevils and weight gains of armyworms were significantly higher in mutants that produce lower amounts of specific defense-related compounds on the leaf surface (Figure 4 A and B). These data suggest that these defenses are important to rice resistance to weevils and armyworms, information that can eventually be exploited further to develop resistant varieties.

Developing and using plant resistance is critical not only for increasing the profitability of the Louisiana rice industry but also for ensuring global food security. The rice entomology program in the LSU AgCenter is pioneering efforts to increase the use of plant resistance by understanding the fundamental biology of plant-insect interactions.

Fall armyworm feeding on rice plants in the experiment conducted with mutant plants. Photo by Lina Bernaola

Rice water weevil adults feeding on rice leaves and producing feeding scars. Photo by Lina Bernaola

Figure 1. Rice water weevil larval and pupal densities per core sample in field evaluations at the H. Rouse Caffey Rice Research Station during 2013-2017. Larvae after emerging from eggs grow while feeding on the roots of rice plants. Then, larvae pupate in mud balloons and these attach to rice roots until they hatch.

Figure 2. Percentage of yield loss among rice varieties (hybrid and conventional) in a field experiment after weevil feeding, summer 2017.

Figure 3. Stem borer infestation based on the number of whiteheads per plot in field evaluations at the H. Rouse Caffey Rice Research Station during 2016-2017.

Figure 4 A. Number of 1^st instar larvae of rice water weevil on wild-type (Sabine) and mutant rice lines deficient in the production of specific chemical compounds on the leaf surface.

Figure 4 B. Larval weight gain of fall armyworm on wild-type (Sabine) and mutant rice lines deficient in the production of specific chemical compounds on the leaf surface.