Investigating Plant Tolerance for the Management of Insect Pests in Rice

The rice water weevil, Lissorhoptrus oryzophilus Kuschel (Coleoptera: Curculionidae), is the most destructive and widespread insect pest of rice (Oryza sativa L., Poaceae) in the USA. This insect poses a global threat to rice production, having invaded rice-producing regions of Asia and Europe. Moreover, lepidopteran stemborers, particularly the invasive Mexican rice borer, Eoreuma loftini (Dyar) (Lepidoptera: Crambidae), are increasingly becoming problematic in Louisiana rice. Although insecticidal seed treatments have been widely used against weevils and stemborers, alternative management strategies are needed. Plants possess defense-related traits that both reduce injury from herbivores (resistance) and the amount of yield loss per unit injury (tolerance). Tolerance is a type of plant resistance that allows crop plants to maintain yield in spite of injury by herbivores. Both plant resistance and tolerance can be utilized and integrated into pest management programs. Several experiments were conducted from 2016–2020 to (i) examine the susceptibility of rice cultivars to rice water weevil and stemborers and the corresponding impact of these insect pests on yields, (ii) investigate effects of cultural strategies and host plant resistance on weevil and stemborer infestations, and (iii) elucidate effects of chronic feeding by weevil larvae on rice plants. Results from the experiments in this study have shown that the rice cultivar ‘Jupiter’ consistently supported the highest numbers of immature weevils compared to other rice cultivars. Low levels of stemborer injury were observed in ‘Cheniere’ and ‘Jazzman-2’, which suggests that these cultivars express some levels of resistance to stemborers. Plant tolerance was assessed by evaluating differences in yields between insecticide-protected and unprotected plots. Weevil and stemborer infestations negatively affected rice yields, with losses among cultivars ranging from 4–49%. Comparisons of yields between insecticide-protected and unprotected plots indicated that hybrid cultivars exhibited higher tolerance to rice water weevil infestations than inbred cultivars. Delaying permanent flood application by two weeks reduced weevil densities and stemborer injury. Yield losses were also generally lower in plots subjected to delayed flood compared to normal flood timing. Our data suggest that the combination of cultivar resistance/tolerance and cultural tactics (e.g., delayed flooding) can serve as a valuable component of an integrated pest management program for both rice water weevil and stemborers. Furthermore, results from the experiments conducted in this study demonstrated that feeding by rice water weevil reduced plant growth, yields, and plant nutrient uptake. Rice cultivars expressing tolerance could be used to reduce crop damage in situations where the use of insecticides is not practical, too expensive, or only partially effective