The Velvet Hammer

The Velvet Hammer

The destructive nature of the codling moth is well known. When it comes to tree fruit pests, the proverbial “worm in the apple” has gotten the lion’s share of attention, and rightly so. It can indeed wreak havoc. But when it comes to pears, there is one pest that can top even codling moth: pear psylla. “In most situations in central Washington, pear psylla is more difficult than codling moth, and more expensive to control,” says John Dunley, an entomologist at the Washington State University Tree Fruit Research Center in Wenatchee. “Pear psylla is basically a poster child for resistance management.”

While any pest can develop resistance to a certain pesticide over time, few have proven so resourceful as pear psylla. In the nearly 70 years since it first spread to Washington, growers thought they had KO’d pear psylla many times. But each time they would watch in disbelief as the pest got up from the mat, ready to fight some more. You don’t have to look further than that mainstay of growers’ arsenals in recent decades, Guthion, which, incidentally, has long been the top choice for codling moth. “The OPs (organophosphates) like Guthion smoked pear psylla in the mid 60s,” says Dunley. “But resistance developed in two years.”

But the OPs were hardly the last challenger. In the mid-80s came the pyrethroids, especially Asana. Again, growers thought they had the problem licked. Again, pear psylla came back, sucking the juice out of the trees and leaving messy honeydew behind. Fruit would russet, or worse, get sooty mold. Serious infestations would, and still can, stunt, defoliate, or kill the trees. Again, resistance had developed in just two years.
Other excellent products have been tried with success over the years: Agri-Mek, Comply, Assail, Actara, Clutch, etc. However, in each case, pear psylla would develop a degree of resistance. What’s clearly needed as we head into the 2008 crop year is not a reformulation, but a completely new chemical. That’s where Delegate insecticide comes in.

Biological Heritage

Most growers are already familiar with the class of products from which Delegate comes, the spinosyns. Developed by Dow AgroSciences, they are derived from the fermentation of Saccharopolyspora spinosa. The products with which growers are familiar include Success, and its organic formulation, Entrust, both of which have spinosad as the active ingredient. Dow AgroSciences has chemically modified two other spinosyns to come up with a unique product, spinetoram, the active ingredient in Delegate.

Avoid Resistance

Because pear psylla is, as entomologist John Dunley puts it, the “poster child for resistance management,” growers will have to exercise restraint in using Delegate. Dow’s Harvey Yoshida is recommending that the product be sprayed just twice a year in order to prevent the insect pest from becoming resistant to spinetoram. But just as important as the number of times it is used, is when it is used.

Pear psylla has a bad reputation for developing resistance in part because it has five to seven generations per year. If growers treat successive generations the pest will be able to develop resistance much more quickly, says Dunley, who explains the phenomenon thusly: Say a grower sprays a chemical and kills 95% of the pests. Of the surviving pests, 4% were susceptible to the insecticide, but for whatever reason were not killed.  The remaining 1% actually had resistance. In the following generation, 1 in 5 pests would have resistance, or 20%. In succeeding generations, resistance could build geometrically.  “That’s why, in theory, resistance can develop very quickly,” says Dunley.

Yoshida agrees, noting that though he recommends that growers spray Delegate twice to get excellent control of the targeted pest, both sprays should be in the same generation.  “Individuals that survive are predisposed to tolerance, and if you hit them again (in the following generation), you’re magnifying that resistance development,” he says.  “Spraying consecutive generations is definitely frowned upon – you certainly want to avoid that.”

Growers should rotate Delegate with other products. (In fact, growers should always rotate with products from other classifications. For example, if a neonicotinoid is used, another neonicotinoid should not be used on a following generation.) If pear psylla is the primary pest, growers can spray Delegate twice around bloom. If a grower is trying to get both pear psylla and codling moth, he can spray twice in late spring or summer. Because pear psylla is such a difficult pest, Dunley thinks most growers will want to spray early, once at cluster bud and once at petal-fall.  “Really try and hammer that first generation,” he says.

But no matter when a grower sprays, says Yoshida, both treatments should definitely be made within the same generation.  All pear growers will benefit from such an approach.  As Yoshida puts it:  “We want to keep Delegate around for as long as we can.”

Like some other products, Delegate affects the pest’s nervous system, only it does so with a completely different mode of action, quickly paralyzing and then killing insect pests. Insects are controlled two ways, through both contact and ingestion, providing both quick knockdown and residual control. Delegate also has translaminar ability, which is particularly important with a pest such as pear psylla, says Dunley, because in the nymph stage the pest is immobile, usually hunkered down on the undersides of the leaves. “You have to really get the material to them,” he says. “They won’t crawl across the tops of the leaves.”

Dunley says he’s optimistic about Delegate not only because of its unique new mode of action, but because it has the ability not only to neutralize per psylla, but the other major pest of pears, codling moth. There aren’t many products that take care of both, especially one to which pear psylla has no resistance. That makes it a lot easier on pear growers, notes Harvey Yoshida, the Product Technology Specialist for Dow AgroSciences in the Pacific Northwest. “In certain places, codling moth is worse than pear psylla,” he says. “With this, you can get two key pests with one spray of one product.”

Reduced Risk Registration

Besides the two key pests, Yoshida notes that Delegate also takes care of leafroller and thrips, pests which are also a problem on apples. (In addition to pears, the product is also labeled for apples, cherries, peaches, plums, nectarines, and apricots. See the label for a complete list.) But despite the wide spectrum of pests it controls, Delegate is generally soft on beneficial insects, such as lacewings and ladybugs, says Yoshida. Because of that, he says it fits nicely into growers’ Integrated Pest Management (IPM) programs.

Besides being soft on beneficials, Delegate is also registered under EPA’s Reduced Risk Pesticide Program. The re-entry interval (REI) is just four hours, whereas most other products used on pear psylla have an REI of 12 hours or more, says Yoshida. “That short re-entry lends a lot of flexibility to growers and consultants,” he says. “You can spray in the morning, and if you need to go back in and do any number of tasks, you’re free to do them that afternoon.”

Perhaps even more important to growers than the short REI is that in apple and pear orchards Delegate has a pre-harvest interval (PHI) of just seven days. That’s particularly important in Washington, says Dunley, because most pear plantings in the state are interplants of Bartlett and Anjou varieties. Bartlett harvest begins about three weeks before the Anjou harvest, which can lead to problems if pear psylla is present and the grower is using a product with a 28-day PHI. “At harvest, pear psylla is sometimes close to boiling,” he says. “Hopefully it has been suppressed by then, but if not, you need to get control.”

Another factor, which has become increasingly important in the past few years, is that pear psylla can produce copious amounts of sticky honeydew. Besides potentially leading to sooty mold, the honeydew is a real nuisance for fruit pickers. Growers can really use a product with a short PHI to get in and clean up pear psylla close to harvest. “Growers are becoming more and more interested in having a clean crop to harvest,” says Dunley, adding that if the fruit’s sticky, harvest crews might just opt to look for work elsewhere. “With the past two harvests, labor has become tight enough that growers need a clean crop.”

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