Strategies for Psylla
Pear growers are finding pest control more expensive and less effective.
A cold winter, a lack of rain, and pear psylla's resistance to pesticides combined to create the perfect storm for pear growers last season, Dr. John Dunley observed during the North Central Washington Pear Day in January.
Growers are spending more money—at least $700 per acre—to control pear pests and still have fruit that's sticky from pear psylla honeydew at harvest, said Dunley, an entomologist with Washington State University in Wenatchee. Sticky fruit is an increasing concern because pickers don't like to handle it. It's becoming more difficult, yet more important, to have clean fruit.
Many pear growers are using the neonicotinyl insecticides: Actara (thiamethoxam), Clutch (clothianidin), Assail (acetamiprid), Calypso (thiacloprid), and Provado (imidacloprid). A few years ago, growers could control pear psylla with two applications of an insecticide; now, they're struggling to control the pest with four or six applications.
Dunley said those products all have the same mode of action. This means, for example, that if Actara isn't working as well as it used to, switching to Clutch will not help. "Resistance to one means resistance to all of them."
Last year's moderately cold winter also led to greater pear psylla populations. Pear psylla does better in cold temperatures—except for periods of extreme cold, Dunley explained. It can survive temperatures of -20°F or lower.
In the fall, pear psylla develops into the winterform stage, which is bigger, darker, and harder than the summerform. It feeds in the fall, before the trees go dormant, and forms what is known as "fat body" that provides a reservoir of energy to help it survive through the winter and next spring's egg-laying phase. The more fat body left in the spring, the more eggs it will lay and the better able it will be to detoxify pesticides.
Pear psylla does not go into a true diapause, Dunley said. Rather than going to sleep the entire winter, like codling moth, it rests in a torpor. If disturbed, it will jump, and it will become active if the weather warms during the winter. When active, it uses its reserves, and the fat body becomes smaller. That's why it's tougher on pear psylla going into the spring if the winter has been warm. After a cold winter, they lay more eggs and are more likely to survive the sprays.
Dunley said hail damage in many pear orchards in 2006 resulted in less pear psylla control that season and a large population going into the 2007 season. The spring was windy, reducing the opportunities for spraying and making it difficult to get good coverage. There was also very little rain to wash off the honeydew in the summer. Washing the honeydew off psylla nymphs leaves them more exposed to heat and pesticides.
Pear growers need to adopt a good integrated pest management program using pesticides of different modes of action, Dunley said. "Resistance management is going to be key now and in the future, as it was in the past."
Several products are available, in addition to the neonicotinyls. They include Agri-Mek (avermectin), Centaur (buprofezin), Fujimite (fenpyroximate), Nexter (pyridaben), Esteem (pyriproxyfen), and Delegate (spinetoram). Of that list, only Fujimite and Nexter have the same mode of action. Delegate is a new codling moth control that appears to control adult pear psylla, also, Dunley said.
Rimon (novaluron), which is a very effective psyllacide and has a unique mode of action, will be available this season but can't be used after petal fall. It could be applied close to petal fall with Delegate applied later, Dunley suggested.
Typically, pear psylla develops though five to seven generations per year. Dunley recommends that when planning a pear psylla management program, growers consider three different periods: dormant though petal fall; mid-June through mid-July; and August through harvest. A product with a different mode of action should be used for each of the three periods. For example, products applied during the first period could be Surround, Centaur, Agri-Mek, or a neonicotinyl. During the second period, Delete or Esteem could be used to also pick up some codling moths and leafrollers. During the third period, Fujimite could be used preharvest.
"Don't use one mode of action throughout the year," Dunley stressed. "Don't keep coming back with the same mode of action and hit more than one generation of pear psylla or codling moth. If you use it against codling moth, you're also exposing one generation and possibly two of pear psylla. If you use Delegate for codling moth, don't use it for pear psylla control prebloom."
However, Dunley said a single generation of pear psylla can be hit several times with the same product, as long as the label allows it.
Growers don't usually target each generation of pear psylla, but if the different generations could be targeted with different products, it might help keep populations down, Dunley said. He is working to develop a degree-day model that would predict when the generations occur.
Sulfur or lime sulfur can be used to knock down populations of psylla after harvest, but Dunley said the psylla move around between orchards. A grower who treats them in the fall is likely to start the following season with the same population levels as other orchards in the area. If an orchard is isolated, a postharvest treatment is recommended. Alternatively, an areawide approach can be used with all growers in the area applying a sulfur treatment.
Oils can help suppress psylla. The addition of oil tends to make insecticides work better, and the newer oils tend not to be phytotoxic unless applied in hot weather, he said.
It's important to apply products when they're most effective and not target psylla when they are hard shells and protected from sprays. Dunley recommends applying pesticides in 200 to 400 gallons of water per acre for good coverage. Overhead sprinklers can be used to remove the honeydew and increase the efficacy of the insecticide.
Asked how long it takes for resistance to disappear after a product is no longer used, Dunley said it depends on the pest and product. Codling moth's resistance to Guthion (azinphos-methyl) goes away in about seven generations, but pear psylla's resistance to pyrethroids has never disappeared. For resistance to decline, there must be some susceptible insects in the population, but when all growers in an area spray at the same time, all the psylla are exposed, he explained.