**3. Conclusion**

When trees are young, usually until one and a half years after planting in southern Vietnam, neonicotinoids by soil drench are expected to control psyllids effectively for two months after the application. Methidathion by spray can keep high residue effects for a half month for the control of psyllids on seedlings. This insecticide can quickly kill psyllids in 12 hr after their infestation on plants, and neonicotinoids did not attain such quick effect on the psyllid. This is the advantage in the use of methidathion. Dimethoate showed similar lethal effects on the psyllids as methidathion, although both its quickness and residue effects were inferior to those of methidathion. Field application of imidacloprid, methidathion, dimethoate, and the mixture of imidacloprid with any of the two organophosphates showed that these insecticides showed similar effects on the control of psyllids: their residue effect was maintained for less than two weeks and no insecticides succeeded in attained high lethal effect to eradicate psyllids on trees even in a couple of days after the spray. It should be noted that dimethoate could not control aphids as much as imidacloprid or its mixture with dimethoate but imidacloprid would lead the increase of scale insects after the application. The application of any insecticide examined in this study did not lead these pests to be eradicated for even one week after the application. These results indicate that the application of insecticides cannot be expected to attain perfect protection of citrus trees from CG infection once the tree grew. Furthermore, since nymphs could increase in two weeks after the application without elimination, even secondary infection would not be avoided when citrus trees have grown to a stage of fruit yielding. The application of insecticide would only reduce more or less the probability of the second infection of trees by CG in the orchard.
