Assessments of crop losses in rice ecosystems due to stem borer damage (Lepidoptera: Pyralidae)

Abstract

A database created from insecticide control experiments conducted under the All-India Coordinated Rice Improvement Project from 1965 to 1992 was used to derive empirical estimates of yield losses caused by stem borers. Each unit percent damage due to white earhead damage had a much greater impact on rice yield in the irrigated ecosystem than did damage due to dead heart. White earhead damage occurs later in the season and results in direct loss of a yielding panicle, and thus, no compensation (or very little) is possible. The grain yield loss from the two phases, dead heart and white earhead damage to rice, is more than additive. Based on 770 experimental units from 28 years data, our projections for damage over rice ecosystems due to 1% dead heart or white earhead, or to both phases of stem borer damage are 2.5%, 4.0%, and 6.4% yield loss, respectively. In terms of grain production loss over ecosystems, 1% dead heart, or white earhead, or both phases of stem borer damage would be 108, 174 and 278 kg/ha, respectively. In irrigated ecosystem, 1% dead heart resulted in 0.3% or 12 kg/ha loss whereas, 1% white earhead caused 4.2% or 183 kg/ha loss in grain yields; the loss due to 1% infestation in both phases of stem borer damage was 4.6% or 201 kg/ha. In rainfed lowlands, for 1% dead heart or dead heart and white earhead caused 2.3% or 76 kg/ha yield loss. Even at levels below the currently used economic threshold considerable losses can occur. This perception on losses assumes more importance because of the inadequate host-plant resistance to stem borer in rice. Although no insecticide gave total control of stem borer damage, many increased grain yields significantly. Emulsifiable concentrates of monocrotophos and chlorpyriphos appeared more economical for adoption by farmers as their application caused maximum mortality of larvae and unhatched eggs. Insecticide granules such as diazinon and carbofuran were equally efficient in preventing stem borer damage.

Introduction

Over 100 species of insects attack and damage rice (Pathak, 1968, Pathak, 1977; Grist and Lever, 1969). Many of them often appear sporadically but do not cause economic loss. A few species, however, do cause significant damage and are extremely important. The stem borers, gall midge (Orseolia oryzae Wood-Mason), brown planthopper (Nilaparvata lugens Stal), leaf folder (Cnaphalocrocis medinalis Gunee), and green leafhopper (Nephotettix virescens Dist.) are the major pests of economic importance in India and are among the production constraints consistently encountered in various rice growing environments (DRR, 1965–2003; DRR, 1975–2003).

Stem borers in the order Lepidoptera are widely prevalent and serious insect pests of rice. In India, 18 stem borer species in the family Pyralidae and three species in the family Noctuidae have been recorded (Banerjee, 1964; Kapur, 1967). Usually one to four species are important in any given area. The predominant species in India include yellow stem borer, Scirpophaga (Tryporyza) incertulas (Walker), striped stem borer, Chilo suppressalis (Walker), and pink stem borer, Sesamia inferens (Walker). Of these species, S.inferens is restricted primarily to hill regions in northern India and Bengal in eastern India. Occasionally, other species like the white rice borer, S.innotata (Walker), may be encountered. The yellow stem borer, S.incertulas is the most dominant species in India (DRR, 1965–2003; Kulshreshta et al., 1970).

Varietal resistance to yellow stem borer has been investigated (Israel and Abraham, 1967). Land races of rice such as TKM 6, CB1 and CB2 have been used as resistant donors since 1964 in India (Roy et al., 1971). The resistance of TKM 6 was reported to result partially from non-preference and antibiosis (DRR, 1969). Research elsewhere on varietal resistance to rice stem borer has received a low priority and none of the varieties developed so far have more than a moderate degree of stem borer resistance (DRR, 1965–2003; Chaudhary et al., 1984; Paroda and Siddiq, 1993).

Stem borer adults are moths and three or more generations occur in a season. Most borer species are capable of flying only a short distance; however, they can travel 8–16 km if carried by wind (Pathak, 1968). A single female can lay 100–200 eggs. An egg mass contains 50–80 eggs and is covered with pale brown hairs from the anal tufts of female moths. The larvae live and feed inside the stem or rice culm. Both traditional cultivars and the modern semi-dwarf indica varieties produce numerous tillers (15–20), and thus provide conditions conducive for stem borer infestation. The newly hatched larvae may feed externally for some time, bore into the stems, usually throughout the upper nodes, and eat their way down to the base of the plants (Pathak, 1968). Crop production practices such as clipping seedlings to remove eggs laid at the leaf tips, density of plant populations, and N fertilizer application are known to alter the pest incidence (Pathak, 1977; Singh et al., 1990). Rice plants are most prone to stem borer infestation at the tillering and flowering stages (Viajante and Heinrichs, 1987). In a transplanted crop, stem borer larvae cut off the growing points of tillers causing them to die, a condition commonly known as dead heart. When the plants are attacked later, during the flowering stage, larvae feed on the meristem and empty, whitish-looking panicles called white earheads appear. In the infested fields these white earheads stand erect and contain empty and unfilled glumes.

In addition to reducing rice yields, stem borer damage may also make plants more prone to invasion by pathogens. The estimated loss from stem borer damage varies from 3–95% (Ghose et al., 1960). In areas where 2 to 3 crops of rice are grown every year, the first crop is damaged severely (Israel and Abraham, 1967). The annual loss from pests and diseases on paddy rice in India was projected to be around $ 100 million; the share of paddy stem borer alone was considered as $ 10 million (Mehta and Varma, 1968). The most commonly cited crop loss figures from rice are those of Cramer (1967), who estimated worldwide losses in rice production due to insect damage to be 34.4%. An appraisal team charged with assessing the nature and scope of pest problems affecting the food supply in Southeast Asia concluded that stem borers were among the insect pests deserving the highest research priority (Glass, 1971). Yet, all these reports were only subjective estimates at best with extremely limited data.

The magnitude of the loss caused by stem borers becomes apparent only when the grain yield harvested in an insecticide-protected plot is compared with that from an unprotected plot. Researchers at the Central Rice Research Institute in India estimated that for every 1% increase in white earheads, yields were reduced by 2.2% (Israel and Abraham, 1967). Stem borer attack was not completely prevented despite a regular treatment with insecticide (Israel and Abraham, 1967). However, a 48% yield increase in insecticide treated plots over untreated controls was found during the first two crop seasons in Tamil Nadu state in India (Ramakrishnan, 1972). Although stem borers are known to seriously affect crop yields in rice (Pathak and Dhaliwal, 1981; Litsinger et al., 1987; IRRI, 1990), few data are available on the actual yield losses derived for the different rice ecosystems. Savary et al. (1997) studied the relationship between rice cropping pattern, biotic constraints and yield levels in 251 farms. With principal component analysis of 14 injury variables, factors were identified for developing a multiple regression model in which the largest individual mean yield reduction (0.46 t/ha) was attributed to deadhearts. However, this model explained only 17.8% of yield variations.

There are numerous reports on the ability of young rice plants to compensate for the loss from deadhearts to some extent by producing new tillers. The estimates available are from comparison of yields in unprotected and insecticide protected plots. The data included for such analyses were mostly from only one site in a single type of ecosystem. Injury–yield relationships have been developed for stem borers (Israel and Abraham, 1967; Pathak, 1967; Pathak and Dyck, 1973; Gomez and Bernardo, 1974; Barr et al., 1981; Waibel, 1996). The relation between injury and yield is considered to be generally non-linear and to exhibit a high degree of tolerance to initial injury. Plants with as high as 30% dead hearts from stem borer attack may have no significant yield losses and as much as 10% white earheads can be tolerated (Teng et al., 1993). A higher degree of tolerance has been recorded under higher doses of fertilizer applications. Insect damage functions are thus speculated to depend on the crop age and nutritional status when the crop was infected, in addition to factors such as insect densities and feeding durations.

The use of inputs for plant protection was unimportant for rice prior to the mass introduction of modern varieties. Farmers had traditionally relied on host-plant tolerance, natural enemies, cultural practices and mechanical methods to contain the stem borer damage. In India, which has the world's largest area under rice production (44.6 m ha annually), losses increased in farmers’ fields as area planted to high yielding varieties increased. Most farmers protect their fields from the principal insect, stem borer, at least to a certain degree with insecticide application. Research and development in India is achieved mainly through the All-India Coordinated Rice Improvement Project (AICRIP). This project tests breeding material, and promising production and protection practices in various experiments by involving the available scientific force at different institutions to find solutions to problems through joint efforts (Muralidharan and Siddiq, 1997). Insecticide control experiments on rice stem borers study the impact of pest on yield and identify products for commercial use. Thus, the objective of this study was to use the database created from AICRIP insecticide control experiments in rice from 1965 to 1992 to derive empirical estimates of stem borer-induced yield losses.