Evaluation of action thresholds for chronic rice insect pests in the Philippines. III. Leaffolders uri icon

abstract

  • Action thresholds (AT) as insecticide application decision tools were developed and tested against rice leaffolders Cnaphalocrocis medinalis (Guenee) and Marasmia patnalis Bradley (Lepidoptera: Pyralidae) in four sites and 68 crops over a 13-year period in the Philippines. Leaffolder damage levels were generally low with a mean over all sites and crops of 2 - 3% damaged leaves (DL) and 6 - 9 % in a given site as a per-crop average based on weekly samplings, with highest incidence per field on a given sampling date of 48%DL. The damage incidence over a typical crop cycle formed a bell-shaped curve with maximum numbers in the late reproductive and early ripening stages. Continuous population buildup over the planting cycle did not occur in any one site over the season, thus a strategy of monitoring earlier planted fields to forewarn against impending economic damage had no basis. ATs were based on different levels of percentage DL, number of larvae per hill (L), and density of flushed moths. Collectively ATs were surpassed in 1, 12, and 4% of fields in each of the three main growth stages. A scoring system was based on benchmark criteria including both leaffolder damage and yield loss. The 15%DL threshold performed best in all growth stages, only equaled by 1L in the vegetative stage, scoring 93 - 99% correct decisions in each of the growth stages. Flushed moths produced the most false positive decisions and the lowest performance scores. The best insecticide response gave only a disappointing 53% control and therefore was a significant weakness in using AT technology. BPMC, endosulfan, and monocrotophos performed the best and azinphos-ethyl and Bt the least. Sites with the highest densities had the most rapid rise in damage levels, thus requiring more frequent monitoring. Low leaffolder damage levels were most likely due to the activity of natural enemies. Given the ability of high tillering varieties to compensate from pest damage, farmers would be better off embracing integrated crop management as a preventative measure to bolster inherent tolerance levels combined with crop monitoring based on historical population levels. In sites with a record of high damage levels, monitoring should start 4 weeks after transplanting (WIT) on a weekly basis and more risk averse farmers may increase the interval to twice a week during flag leaf stage. In sites with lower observed levels, monitoring can begin 6 WT on a weekly basis and continue through the flag leaf stage. Significant yield increases were recorded with the best AT characters despite evidence of modem rices to tolerate high damage levels. Reasons for this apparent paradox are discussed.
  • 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 1:53 and 2:3:1 in the three growth stages for longer and shorter maturing varieties, respectively.

publication date

  • 2006
  • 2006
  • 2006
  • 2006
  • 2006