If you have an old pecan orchard in decline, with limbs that get weaker and weaker each year, you may be seeing the results of the Prionus root borer. These insects are large, destructive beetles whose feeding essentially destroys the tree’s root system over time.
Prionus root borers are particularly difficult to manage once they infest an orchard for a number of reasons. The woods are full of alternate tree hosts – Oaks, hickories, pines, grapes, poplars, etc. The females lay eggs in the soil near roots and the larvae feed upon and develop in the roots and soil until they pupate.
UGA entomologist Dr. Jim Dutcher has done a lot of work on this particular pest in recent years. While it is difficult to reach the larvae deep in the soil with insecticides, Dutcher’s work has shown that chlorpyrifos applied to the herbicide strip at a rate of 8 oz/50 gallons of water via a herbicide sprayer is an effective control method.
The adults’ peak emergence as measured by light traps occurs in June. Panel traps baited with prionic acid pheromone to attract and capture the male beetles helps to give a good idea of when beetle emergence is occurring. If traps are used, those placed along the periphery of the orchard tend to catch more beetles, presumably as the males fly into the orchard from surrounding woods. Panel traps can be obtained from https://www.alphascents.com/Traps/traps.html and pheromone can be obtained from www.contech-inc.com/products/insect-control. Prionus root borers have a long life cycle of 5 – 7 years, making this a long-term management problem. There is no quick fix to Prionus root borer elimination.
If the trees are already in decline, they may need to be cut back to help bring the top of the tree back in line with what the root system can support. In many cases, these trees also have crown gall and removal of the tree is the best option. However when re-planting a new tree, an application of chlorpyrifos as mentioned above will be necessary to give that tree its best opportunity in an infested orchard.