Ground sprayers have been the delivery system of choice for pesticides over the last half century, but with the array of biopesticides and other new chemistries now becoming available, the time has come to start thinking about trunk injection, according to John Wise, professor of entomology in the Michigan State University Center for Integrated Plant Systems.
Wise presented the results of his group’s research into trunk injection for pear psylla management at the Great Lakes Fruit, Vegetable and Farm Market Expo in Grand Rapids, Michigan, in December.
“In my 25 years at MSU, I’m more excited about biopesticides now than I ever have been. But one of the characteristics often with biopesticides is that they tend to be sensitive to UV degradation, so putting them on the plant canopy surface can be a less-than-optimal way of delivering that material,” he said. “So, with that in mind, we began thinking about different ways of delivering our materials.”
That led to trunk injection, a method the ornamental tree industry had already begun using to control emerald ash borer and other pests in valuable trees, he said. It seemed well-suited to biopesticides because it would eliminate sun exposure and solve the problem with UV damage.
Trunk injection had another big benefit: no spray drift. Without drift, risks to beneficial organisms and the environment can be greatly limited.
Early results from his research show trunk-injected insecticides provide control longer than comparable spray applications, but there is still much to learn about how best to use the method in tree fruit orchards, economically and with best management practices, to prevent resistance.
Trunk injection success
To determine whether theory matches practice in an orchard, Wise and graduate student Celeste Wheeler tested trunk injection’s effectiveness on pear psylla. Psylla feed on the phloem in leaves, which is where trunk-injected products are delivered.
In their first experiment, Wise and Wheeler picked out a couple of products they knew had some level of activity on pear psylla and compared the effectiveness when applied via foliar spray or trunk injection. The two products were Agri-Mek (abamectin), which has been labeled for pear psylla control for many decades, and AzaSol (azadirachtin), a biopesticide based on neem extracts.
They sprayed the two products “just the same way as any pear grower would apply them: at the label rates at petal fall plus seven days,” Wise said.
For the injection, they took the labeled rates for the single foliar spray and converted them to calculate the equivalent dose for individual trees. They used the Arborjet TREE I.V. trunk injection system, which was developed for the ornamental tree industry and requires drilling a hole and then pumping in the pesticide solution.
Both the Agri-Mek and AzaSol foliar sprays provided control of pear psylla, but the trees needed second sprays in mid-June to keep the insect population down. In contrast, trunk injection of Agri-Mek and AzaSol provided control of pear psylla with no need for a second application, and those injected with AzaSol continued to show effectiveness into the following year, he said.
Wise decided to expand this study in 2018, after attending a pear meeting that spring at which Nestlé Gerber representatives mentioned that the company is considering increasing its pear sourcing from Michigan and is especially interested in organic production systems. In response, Wise and Wheeler expanded their trial to include the organic biopesticide Venerate (Burkholderia spp. strain A396) and the new pre-mix insecticide Cormoran (novaluron and acetamiprid).
Again, both the sprayed and injected products performed well against pear psylla, but the sprays required a second dose in midsummer, while the injected products did not.
That means at least two biopesticides — both AzaSol and Venerate — provided significant protection against pear psylla, and by using trunk-injection once instead of foliar sprays twice, growers could potentially get the same result with half the amount of product, he said.
“It was exciting because both as a foliar and an injection, we’ve now got more than one biopesticide that would be considered organic-acceptable, to bring to the table and start thinking about this challenge of how we can assist in seeing the pear production rebound in Michigan,” Wise said.
More work needed
Even with these results showing the benefits of trunk injection, fruit growers shouldn’t switch over quite yet, Wise said.
Engineers will likely need to help develop an injection tool specifically for fruit trees that affords “the speed to get across the orchard in a short amount of time,” he said.
In addition, researchers need to study the persistence of trunk-injected products, especially for insects, such as pear psylla, that have a propensity for developing resistance to pesticides.
“With foliar sprays, we often think about resistance management in a temporal point of view where, for a generation of a pest, we spray one mode of action, and then we switch to another to prevent the selection pressure from building resistance in the population,” he said.
He believes an option for trunk injection might be a spatial tactic, perhaps alternating pesticides on a row-by-row basis, so that the targeted pest “would not be exposed to the same mode of action consistently over multiple years.”
Perhaps the biggest hurdle will involve conducting the studies and collecting the necessary data to address the regulatory issues involved in making trunk injection a labeled use for these different products.
“All these things are possible,” he said. “We’ve just got to get from our big creative idea stage to really have the confidence that we’re ready to make some proposals to our regulatory officials to get these things done.
This research was funded mainly by the Michigan State Horticultural Society, with additional support from Nestlé Gerber. •
—by Leslie Mertz
Related: Let us (not) spray