Nymphs move to succulent stems and developing leaves to feed, with the heaviest concentration along the midveins of leaves and at the calyx end of fruit. Photo by G. Krawczyk.
Description and life cycle
Pear psylla adults look like small, dark reddish brown, 1∕10-inchlong cicadas. Eggs, just visible to the naked eye, are pear-shaped, yellowish, and are laid in cracks in the bark and around the buds. They become dark yellow before hatching. Nymphs have sucking mouthparts and feed on plant sap. The young nymphs are soft-bodied and creamy yellow. As they mature they become dark brown and more oval in shape, with distinct wing pads present on the late instars. These late instar nymphs are commonly referred to as “hard shells.”
There are generally four generations per year. The adults, which overwinter on trees or other sheltered places, become active anytime the temperature is above 40°F. Females begin laying eggs in late March and continue through the white bud stage. One female can produce as many as 650 eggs. The peak of egg laying is green tip to green cluster bud. Eggs hatch begins at the green cluster bud to white bud stage, with peak hatch occurring about petal fall. Nymphs move to succulent stems and developing leaves to feed, with the heaviest concentration along the midveins of leaves and at the calyx end of fruit. They pass through five instars, each subsequent stage becoming more difficult to control. The early nymphal stages produce more honeydew than the later, larger stages. The first summer adults mature about 20 to 25 days past full bloom. They begin laying eggs on growing shoots as the population shifts from spur leaves to the more succulent shoot leaves. Late season infestations are typically found on water sprouts.
The pear psylla secretes large amounts of honeydew, which runs down over foliage and fruit and in which a sooty fungus grows. This causes the skin of the fruit to become blackened and scarred and the foliage to develop brown spots. Heavy infestations may cause partial to complete defoliation of trees, reducing vitality and preventing the formation of fruit buds. Return bloom and fruit set are often reduced the following season. Overall tree growth can be stopped or stunted with heavy psylla injury. These combined effects are often termed “psylla shock.” There is also limited evidence that psylla inject some type of toxin into the tree, causing a disease known as pear decline. In addition, pear psylla have been implicated in the transmission of fire blight.
Growers should monitor for the presence of pear psylla using their most sensitive pear variety (e.g., Bartlett). To sample for pear psylla nymphs in the early season, examine at least 10 leaves (five spur and five recently expanded shoot leaves) per tree on a minimum of five trees per block. The action threshold at this time is 0.5 nymphs per leaf. For the summer generations again examine at least 10 leaves (recently expanded shoot leaves) per tree on a minimum of five trees per block. The action threshold now is 1.5 nymphs per leaf. When the psylla population is primarily in the adult stage, examine the leaves for the presence of adult activity and egg laying.
Several cultural control practices will reduce psylla populations and dependence on insecticidal control. First minimize heavy pruning, which encourages the proliferation of terminal shoot growth. An overabundance of terminals provides more feeding sites for the psylla. Second, pear trees should receive the minimum amount of nitrogen fertilization necessary for proper tree and fruit growth. Overfertilization can cause extended terminal growth and delay hardening off, allowing optimal feeding conditions for the psylla. Third, and most important, is to remove water sprouts during late June and early July. Because water sprouts provide one of the only sources of succulent leaves at this time of the year, this technique can eliminate a large portion of the psylla population.
In orchards with a history of psylla infestations, insecticidal control begins with a strong prebloom spray program designed to eliminate as many overwintering adults as possible before they have the opportunity to lay many eggs. As with all psylla sprays, good coverage is critical for control of this pest. All sprays should be applied to both sides of the trees and in a volume of water high enough to thoroughly wet and cover the entire tree. In most situations, this requires at least 100 gallons per acre. Surfactants may be added to achieve better coverage. Alternate row middle applications are not recommended unless tree size is quite small.
The first application should include oil, which has been shown to delay egg laying by over 95 percent for a 5-week period, plus an adulticide to eliminate adult psylla overwintering on the tree. This application also serves to concentrate in time a higher proportion of pesticide-vulnerable individuals later in the season. Typically, egg deposition and hatch occurs over a long period of time, making pesticide timing difficult. Early season oil sprays “bunch up” the population so sprays can be more easily targeted. The early season oil application must be applied prior to egg maturation in the female psylla. For growers in south-central Pennsylvania, this application should be made in most years by March 15–20.
With the oviposition period delayed, the delayed dormant spray (bud burst) becomes extremely important because additional adult control can be achieved by waiting until adult psylla that are living away from the pear tree return from their overwintering sites. This spray should again contain oil as well as an adulticide such as one of the synthetic pyrethroids.
The next prebloom spray should be applied between the green cluster bud stage and the white bud stage. This is the period when first-generation eggs begin to hatch. A number of very effective insecticides are available for this spray.
The next vulnerable period occurs at petal fall, when the first generation nymphal population is usually at its peak. Since petal fall is a key period for the activity of other pests that attack pears, it is usually necessary to add a broad-spectrum organophosphate insecticide at this time. If the four applications recommended thus far are carefully applied, psylla populations should be very low.
The next major period to control psylla is not until the second generation of nymphs begins hatching about mid-June. A second application should be repeated within 10 to 12 days of the first to control additional nymphs hatching from eggs. If the population warrants additional applications, these can be made against the third generation of nymphs, which usually begin hatching around mid- to late July. Since the second and third generations tend to overlap during the season, close attention should be given to determining which nymphal stages are present, since insecticides are most effective when directed against the early instars. Growers should rotate to different insecticidal chemistries for each spray. Specific chemical recommendations for home gardeners are in Fruit Production for the Home Gardener, and recommendations for commercial growers are in the Penn State Tree Fruit Production Guide.