Various plant extracts were widely used to control plant diseases and insect pests in organic farming system. This study was conducted to evaluate insecticidal activity of plant extracts against three moth in vitro. Insecticidal activity of 21 kinds of farm-made plant extracts including snowbell tree, sweet oleander leaf, sweet oleander and white cedar, collected from farmhouses were tested against diamondback moth (Plutella xypostella) and tobacco cutworm (Spodoptera litura) larvae. In addition, four plant extracts (pyrethrum flower, derris, neem and Sophora flavescens) were tested for insecticidal activity against Potato tuber moth (Phthouimaea operculella) larvae. As a result of insecticidal test on diamondback moth, insecticidal activity of pyrethrum flower extract was found to be 50% or more in diluted 100 times. The insecticidal activities against tobacco cutworm were 92.5% at snowbell tree extract, 77.5% at the sweet oleander leaf and white cedar extracts. Among the plant extracts, insecticidal activity of 300 times-diluted pyrethrum flower and derris extract was 85%, neem and Sophora flavescens extracts were similar to non-treatment.
Consequently, pyrethrum flower extract and snowbell tree, sweet oleander leaf, white cedar extracts and pyrethrum flower, derris extracts for controlling diamondback moth and tobacco cutworm and potato tuber moth were selected. We think the selected organic materials can be used to control diamondback moth and tobacco cutworm, potato tuber moth under organic farmhouse condition.