In tree fruit production, the go-to method for delivery of pesticides is via air blast, but it’s not necessarily the best way to go. Pesticide losses can be as high as 45% and this can result in the risk of non-target exposure to both the environment and workers.

Addressing this topic through a poster presentation at the most recent Great Lakes Expo were Celeste Wheeler and John Wise of Michigan State University, who wanted to get up close in personal in the fight against one particular pest. Their topic was the trunk injection of insecticides for pear psylla management over the course of two years.

Wheeler and Wise report that trunk injection doesn’t pose the same risk of non-target exposure – but it’s yet to be studied in pear trees. Pear psylla is a major pest of pear trees – their nymphs suck the trees’ sap, damaging the foliage, flowers and fruit and diminishing the crop. The research team believes psylla are good targets for trunk-injected compounds because trees’ xylem will deliver the materials primarily to leaf tissue.

Currently, azadirachtin (a botanical insecticide which is non-toxic to birds, mammals, bees and plants) and abamectin (an insecticide/miticide with low toxicity to mammals, already formulated for trunk injection) are used to control pear psylla populations. The objective of this study was to compare the efficacy of abamectin and azadirachtin when applied via injection vs. air blast sprayer.

Pear psylla. Photo courtesy of Whitney Cranshaw, Colorado State University, Bugwood.org

In their study, the highest labeled application rate for each treatment was used. Applications were made two weeks after petal fall. Trunk-injected treatments were made using an Arborjet Tree IV with four portals equally spaced around the circumference of the trunk. Each portal was plugged with watertight arbor plugs. Chemicals were diluted into 500 mL of water and pressure injected into the trees.

Foliar-applied treatments were also applied, for comparison, following conventional grower practices. Applications were made using an FMC 1029 air blast sprayer at 935.4 L/ha.

The research team’s field evaluations took place every two weeks to monitor pear psylla pressure, with sample leaves being examined for eggs and nymphs. At the end of the first year of study, a visual count was done to evaluate for the presence of sooty mold on the leaves. This evaluation was used to quantify the overall amount of psylla damage that occurred throughout the season.

In the second year of the study, the injected trees from the first year were evaluated for psylla eggs and nymphs to see if there was the possibility of a second year of control.

Researchers reported that all treatments significantly reduced the incidence of psylla nymphs for at least one of the evaluation dates. The abamectin and azadirachtin trunk-injected treatments performed equally well – or better – than the air blast-applied treatments. This is significant because with trunk injection, control was obtained with only one application, as compared with two applications with air blast. That means only half of the product was needed for season-long control in trunk injection.

Additionally, the trunk-injected trees from the first season provided a moderate level of control into the second season. This implies that trunk-injected trees could potentially provide control for a second season.

It was noted that abamectin rapidly degrades when exposed to light on the plant surface, and thus may provide longer residual control when delivered via xylem to the tree canopy. This may be why the injected treatment shows the best psylla control.

Azadirachtin features systemic activity and its metabolite is stored in the leaves without changing its biological effect. This explains why the injected treatment was able to provide the same level of control as air blast and why residues were detected longer than air blast. The systemic nature of azadirachtin also explains how it can reside in the tree longer, providing protection in the second season.

Wheeler and Wise concluded that trunk injection does effectively control pear psylla as well as conventional control practices with foliar application with only half the product and it does not pose the same risk of drift and non-target exposure as air blast application.

However, there are still improvements to be made on pear trunk injection before it can become a reliable alternative method for growers.