Relative efficacy of insecticides against white grubs, Holotrichia spp. damaging potatoes in Himachal Pradesh (India)

Field experiments were conducted consecutively for three years (1990, 91 & 92) at the farm of Central Potato Research Institute, Shimla, for evaluating the relative efficacy of insecticides against white grubs, Holotrichia spp., a key-pest of potatoes in Himachal Pradesh. All the granular insecticidal treatments, viz., phorate 10 G, carhofuran 3 G and quinalphos 5 G applied separately at the base of plants @ 2.0, 2.5 and 3.0 kg a.i./ha were effective in suppressing the population of grubs and subsequently in reducing the damage (%) on potatoes hut quinalphos 5 G was the most effective insecticide at all the three dosages. However, its efficacy enhanced with the increase in dosages. Spraying of potato ridges with chlorpyrifos 20 EC twice i.e., initially @ 0.5 kg a.i./ha at earthing + once again at the same dose 21 days after the first spraying was next in order of efficacy followed by the same insecticide sprayed on ridges only once @ 0.5 kg a.i./ha at earthing time. Hence, granular application of quinalphos 5 G @ 2.0 kg a.i./ha near the base of plants, just before earthing or one spraying on ridges with chlorpyrifos 20 EC @ 0.5 kg a.i./ha, just after earthing is recommended for protecting potatoes from Holotrichia spp. In the case of white grubs prone areas, the dosages of quinalphos 5 G may be increased from 2.0 to 2.5–3.0 kg ailha. Likewise, two sprays with chlorpyrifos 20 EC may be given on ridges as suggested instead of one for achieving the desired protection.