Evaluation of action thresholds for chronic rice insect pests in the Philippines. IV. Stemborers

Abstract

Action thresholds (ATs) as insecticide application decision tools were developed and tested against mainly yellow Scirpophaga incertulas (Walker) and white S. innotata (Walker) rice stemborers in four sites in the Philippines over 68 crops. Damage incidence was low with a mean over all crops and sites of 2% deadhearts (DH) and 3% whiteheads (WH) based on weekly sampling. Highest incidence reached 19% WH as a mean of one crop and 31% WH in an individual field in 1 week. AT characters were based on deadhearts, egg masses, or flushed moths. A mean of 9% fields exceeded ATs in the vegetative, 5% in the reproductive, and 4% in the ripening stages. The most effective AT character in each of the three growth stages was percentage DH with 5% being optimal in the vegetative, 25% in the reproductive, and 10% in the ripening stages. These ATs resulted in 96 – 99% correct decisions based on criteria involving DH and yield loss benchmarks set from earlier studies on economic loss. Insecticide response to the highly accurate ATs was a disappointing <40% control, with chlorpyrifos + BPMC mixture being superior to chlorpyrifos or endosulfan alone. Despite this low level of control and the noted high tolerance of modern rices to stemborer damage, the AT treatments resulted in significant 0.2 – 0.3 t/ha yield gains over the untreated check in each growth stage. This yield response is explained in part by control of coterminous nontarget chronic pests and in part to a synergistic compensatory yield response when a crop under multiple stress is even partially released. It is hypothesized that under such conditions, even low levels of control allow greater compensatory capacity to tolerate stemborer injury but also from other causes, thus accentuating yield responses, particularly if the nutrient supply is adequate and the variety is longer maturing. Stemborer IPM is seen as a two-pronged strategy, the first is couched in integrated crop management as a preventative measure to bolster the crops' ability to compensate from stemborer injury or other crop stress and the second to regularly monitor the crop using ATs. Crop monitoring protocols were seen to be improved if adjustments were made for crop maturity and damage pattern. The typical damage pattern over a crop showed DH initially increasing in the vegetative stage, then leveling off during the reproductive stage (indicating a period of natural plant resistance) before a late peak of WH. Vigilance can be relaxed during the mid-growth stages and heightened during periods of tiller elongation and panicle exsertion. AT levels need to be adjusted for each location, but as a rule of thumb percentage DH could follow the ratio 1:5:3 and 2:3:1 in the three growth stages for longer and shorter maturing varieties, respectively.