PRIONUS APPLE ROOT BORER
This is a very large beetle and very common in apple orchards of Himachal Pradesh. This beetle does not attack above ground; it feeds on the roots of the apple tree. Severe infestation of this borer can cause the death of the tree. It is a root boring insect whose larva feed on the roots of a variety of trees—often killing them. Pesticides application has been found ineffective in reducing their populations. The adults of these borers are large beetles, dark brown to almost black. Larvae of these borers are the largest of the insects that attack apple tree. They are white to yellowish in color with three pair of small legs.
Young larvae can be as long as 6 inches which makes tunnels into the soil to find tree roots. Younger larvae feed on smaller diameter roots and finally reach the tree crown as mature larvae. During summer, larvae feed at depths around 6-18 inches and in winter they descend to about 36 inches down the soil surface. The adult of prionus apple tree borer is a large beetle, ranging in size from 1- 3 inches long. Adult beetles emerge from soil in late spring and early summer and are often attracted to lights at night. Adults are red to brown in color which flies at night in search of mates and eggs are laid in the soil thereafter. In day adults hides themselves under tree debris or some organic matter. Adult females live about 7-10 days and males about 5-7 days. In summer females lay eggs on roots or in soil adjacent to roots. A single female can lay 100-200 eggs during her 15-20 day lifespan.
Prionus root borers are exclusively root borers and feed only on roots of the plants. The damage is done by larval feeding on the roots of apple tree which injure the roots resulting in decreased water and nutrient uptake. Severe infestation can directly kill the apple tree. Sandy soils appear to favor borer infestations. Borers can completely destroy young trees and make older trees more susceptible to being blown over. Infected older trees will sometimes have only one or two roots holding them in ground until blown away by wind.
The adult beetles can be monitored with light traps. The beetles fly generally after the sunset from June to August. Larvae can be monitored by looking them in soil around a tree trunk 5-10 inches deep. The soil can be removed to search for infestations in roots. Usually, trees damaged by this borer shows signs of yellowed leaves and die-back during hot summers.
Prevention is the only way to control it. Keep your orchard clean free grass and other organic matter. This prevents the borer from hiding and making it visible to natural predators. Keep your trees in good health and prevent them from stress. Heavily infested trees that can not be recovered should be removed and burned. Adult beetles can be killed in summer by placing light traps in the orchards. Applying drench of biological insecticide like Beauveria bassiana has also been found effective.
There are no registered insecticides for prionus root borers. Systemic insecticides like Imidacloprid may help on younger larvae feeding on roots. It may not control older larvae positioned in lower trunk. Imidacloprid may suppress the larval population if used annually over years. Other insecticides like chlorpyrifos can prevent egg laying when fresh insecticidal residue is present on the lower trunk, but will not suppress larval populations on the roots. The only way is to use systemic insecticide over several years to suppress the larval population.