First of all, did the insecticide really fail? Was the site adequately sampled before the insecticide was applied so that you could truly recognize the difference in the insect population? For example, eight grubs identified before treatment and eight grubs found after treatment is much different than 30 before and eight after. Was the site adequately watered before the insecticide treatment to ensure that the insects were drawn to the surface, if grubs were the desired target pests? Was the water applied with enough time for the insects to travel to the surface? The chemical will not be as effective if it was applied while the insects were not as close to the surface as they could have been. Insects tend to move down in the soil during dry periods to avoid the hot temperatures. If the area to be treated is watered about 24 to 36 hours before the treatment, the insects will move toward the moisture.
Next, was the insect in question properly identified? Was the appropriate insecticide used? Was the application applied at the appropriate time in the insect's life cycle? Insecticides can be host specific. Insecticides need to relate to the designated insect and appropriate life cycle. Application of the insecticide at the wrong time in the phases of the insect's development may not kill the pest (e.g., eggs vs. small larvae or adults vs. larvae). Also, knowing that the life cycle can affect the effectiveness of the insecticide, a longer acting insecticide may need to be used. Last, for either surface insects or grubs, was the treated area correctly watered in after the treatments?