Rice yield losses due to stem borers and the choice by farmers to predominantly use insecticides to control stem borers demonstrate a continuing need to develop sustainable stem borer management strategies.
Stem borers cause damage to cereal crops worldwide. A range of species that includes moths (Lepidoptera) and flies (Diptera) can cause yield losses of more than 70% to rice in some regions (e.g., Scirpophaga innotata in Java; Diatraea saccharalis in Brazil and southern USA; the stem borer complex in Nigeria) and are one of the principal targets of insecticide applications.
This is partly because the damage stem borers cause has a high visual impact (they cause rice panicles to turn white due to empty grains – known as ‘whitehead’ panicles) and because their damage is often most visible at about the time of rice harvest. In many cases, particularly when stem borers attack young, tillering rice plants, yield losses are minimal because the plants can compensate for damage by producing new tillers or overcompensate by redirecting resources to produce larger and more rice grain.
Often, these compensatory mechanisms do not reduce the signs of damage and are therefore unperceived by farmers. Furthermore, the death or reductions in biomass of rice tillers due to stem borer feeding, even in cases of compensation, will sometimes cause a delay in grain maturation that ultimately leads to losses during harvest (e.g., Diopsis longicornis and Maliarpha spp.).
Rice yield losses due to stem borers and the choice by farmers to predominantly use insecticides to control stem borers demonstrate a continuing need to develop sustainable stem borer management strategies. In recent years, there has been a noted decline in research on rice-stem borer interactions, including on conventional host-plant resistance for sustainable herbivore management. This has been partly due to the polyphagous nature of many stem borers and the difficulties in identifying stem borer-specific resistance genes in rice.
Research interest in host resistance to stemborers has also waned in the face of the tremendous attention paid to developing transgenic rice with Bt or other anti-herbivore toxins. However, there have been several complications with the development, deployment, and acceptance of transgenic crops. Therefore, the use of native sources of stem borer resistance deserves renewed attention, particularly in Asia.
Already, national and international agricultural research and advisory centers will often indicate categories of stem borer-rice interaction (e.g., susceptible [S], moderately susceptible [MS], or resistant [R]) for registered rice varieties, but without supporting information on screening methods or best crop management practices.
A number of authors and research centers have presented protocols to screen rice for resistance to stem borers. However, there is often little information to support the choice of different phenotyping methods, and a number of phenotyping methods seem more suited to the discovery of gene-for-gene resistance sources, such as those involved in resistance to sap sucking-insects, rather than the polygenic resistance and tolerance mechanisms that influence stem borer-rice interactions.
This study examines a range of rice varieties to assess the relative contributions of resistance, tolerance, and vulnerability to observed levels of field damage from two stem borer species. Furthermore, the study assessed changes in the nature of rice-herbivore interactions as rice plants grow and develop.
The main objective of the study was to determine the best phenotyping methods to distinguish categories of stem borer-rice interaction. In particular, by phenotyping for stem borer damage to long duration (≥125 days) and short duration (≤120 days) rice varieties under controlled conditions and under field conditions, we wished to assess the significance of vulnerability to field damage based on crop duration.
Our results highlight some of the challenges (i.e., several stem borer species, complex categories of interaction, and major ontogenetic shifts in interactions) for the effective phenotyping of crop varieties for their responses to stem borers prior to deployment in farmers’ fields. Importantly, phenotyping results from experiments conducted under controlled conditions with two stem borer species were only weakly related to results from field plot experiments.
In one case, a variety (T16) that appeared highly resistant in greenhouse and screen house experiments was heavily damaged in field plots likely because of interspecific interactions between two or three stem borer species. The two stem borer species showed marked differences in oviposition preferences and in their capacity to develop on rice at different growth stages. Varieties also differed in their capacities to compensate for damage from each of the stem borer species.
These results highlight the need to devise robust phenotyping techniques and to determine optimal breeding systems – including a pre-emptive choice of desired traits – based on a firm knowledge of regional stem borer complexes, of interspecific stem borer interactions, and of the diversity of stemborer interactions with rice plants.
Based on our results we make the following recommendations during the phenotyping of rice varieties for their interactions with stem borers. Categories of rice-stem borer interaction should encompass relative measures from susceptibility to resistance, from high to low vulnerability, and from high to low tolerance. Escape from stem borers due to early planting (e.g., Chilo plejadellus) cannot be regarded as a component of direct plant-based vulnerability, but crop duration is.
Field trials, particularly if carried out in a hotspot, will expose test materials to realistic conditions including complete stem borer assemblages and interspecific stem borer interactions. Efficient screening of varieties for regional deployment is best achieved through field trials under high nitrogen conditions because stem borers will respond to high nitrogen through increased oviposition and fitness, but relative interaction categories are largely maintained across varieties.
In effect, high fertilizer levels create local hotspots, particularly when poorly synchronized with surrounding rice fields. Finally, although high resistance and low vulnerability reduced stem borer damage by as much as 50% in our experiments, between 2% and 35% of tillers/plant were damaged even in varieties with a relatively low propensity for damage.
Therefore, the selection of rice varieties based on comparatively low damage must be combined with other integrated pest management strategies, such as pheromone trapping, biological control, or farm diversification to manage stem borers in farmers’ fields.
Read the full study:
Horgan F, Romena A, Bernal C, Almazan LP, Ramal AF. (2020) Stem borers revisited: Host resistance, tolerance, and vulnerability determine levels of field damage from a complex of Asian rice stem borers. Crop Protection. Vol. 142: 105513