Effects of azadirachtin injection in litchi trees (Litchi chinensis Sonn.) on the litchi stink bug (Tessaratoma papillosa Drury) in northern Thailand

The litchi stink bug (Tessaratoma papillosa Drury) is one of the major pests of litchi (Litchi chinensis Sonn.) in tropical Asia. Systemic effects of azadirachtin (at a concentration of 0.17 g active ingredient per centimeter trunk diameter) on T. papillosa were examined by injecting the dissolved product (NeemAzal U) into the trunks of four caged litchi trees in an orchard in northern Thailand. Four untreated caged trees served as control. Mean weekly mortality rates of individuals released on the treatment date (initially 55 nymphs per cage) were significantly higher in the treatment cages (55%) than in the control cages (10%) between the second and the fourth week of the experiment. Mortality in the treatment cages was probably due to both toxic effects (ecdysis inhibition) and antifeedant effects (starvation). Weekly mortality rates of individuals released 2 weeks after tree injection (initially 20 nymphs per cage) were not significantly different between treatment and control cages, indicating a weakening effect of azadirachtin, 2-3 weeks after tree injection. Azadirachtin concentration in ripe fruits was less than 5 mg/kg fruit pulp 18 days after tree injection. Although the results of this study are not yet sufficient for practical recommendations, they provide indications of alternative options for integrated pest management approaches. The azadirachtin tree injection method should also be tested against other pests of litchi, and of fruit trees in general.