Monitoring and Management of Plum Curculio in Apple Using Odor-baited Trap Trees

The plum curculio (Conotrachelus nenuphar) is one of the most important insect pests of apples in the Northeastern United States. Adult female curculios lay eggs in developing fruit shortly after petal fall, which causes fruit blemishes and sometimes fruit loss if adults and/or larvae are not controlled by insecticide sprays. Most of the adult curculio beetles overwinter outside the orchard, and migrate in as fruit blossom, set, and begin to grow. The first flush of beetles are controlled by an insecticide shortly after petal fall that also helps control other insect pests, but additional immigration, feeding, and egglaying may occur for several weeks after that, meaning that multiple insecticide applications may be needed for effective control.

Monitoring and control of these insects is difficult – they are not very susceptible to biological control (though work on using entomopathic nematodes is in progress), and they do not respond well to trapping methods. It has been found, however, that using apple trees baited with synthetic fruit odor (benzaldehyde) and a pheromone (grandisoic acid) can intercept and collect immigrating curculio adults into a limited area, allowing much more efficient monitoring, and even the possibility of using insecticide only on the baited trap trees, instead of the entire block, or the entire border of the orchard. Finding a fresh curculio feeding or egglaying scar on a trap tree would be a trigger for a border or spot treatment; if no fresh injury is found, no insecticide should be needed. This has been shown to greatly reduce the number and volume of insecticide used for curculio control.

This method has been well established in small experimental plots and in a few small-scale cider orchards, but has yet to be adopted on a wider basis, largely due to cost ($4 per benzaldehyde lure, with 4 lures per tree needed, plus another $4 for a single grandisoic acid dispenser per tree = at least $20 per tree). In addition, growers and consultants have some uncertainty about scaling up from a few small blocks to an entire good-sized commercial orchard; how much additional time might be needed in setting up and continuing to monitor the trap trees. Technical Advisor Kathleen Leahy has been particularly eager to learn how well the trap tree approach might be integrated with the rest of the orchard scouting and pest management program.

Our proposal is to do the trap tree method across the entire orchard, demonstrating the effectiveness of this approach, identifying any shortfalls that might come along because of scaling up, and tracking the amount of additional consultant and grower time needed to set up and maintain the trap trees over the course of the curculio immigration period, from early May through late June. From earlier work done at UMass Amherst by Dr. Ronald Prokopy, it appears that trap trees spaced at about 60 meters along the border should be sufficient to intercept beetles flying in to the orchard. We will monitor all traps twice a week during that period, and compare the results of border insecticide applications, trap-tree-specific applications (sometimes called 'bomb trees'!), and a grower control.

We believe that using this approach successfully would encourage other growers to try it on a larger scale, even if the cost of the odor baits is several hundred dollars per orchard, given that it could reduce the need for one or more full-orchard or border applications. In addition, work currently under way under Dr. Jaime Pinero at UMass Amherst is showing promising results using apple trees grafted with varieties that seem to be especially attractive to curculio adults, instead of the synthetic odor baits; if this turns out to be a practical method, it could reduce costs. Finally, if use of the odor baits is adopted on a sufficiently wide scale, it's possible that the price could be reduced.