Pear psylla is an important pest of pear in Washington. Honeydew produced by pear psylla causes fruit russet, and serious infestations can stunt and defoliate trees.
Pear psylla likely arrived in the United States along with shipments of pear nursery stock from western Europe. It was first found in Connecticut in 1832 and spread to Washington State by 1939 (Westigard et al., 1979). Within a few years it became a serious pest throughout all pear-growing areas in the Pacific Northwest.
In the Pacific Northwest, pear psylla is a pest only of pear. Several other plants are transitory hosts and overwintering sites for winterform pear psylla adults. Pear psylla adults may feed on other deciduous fruit trees including apples, conifers, and shrubs as they disperse from pear orchards in the fall and return in the spring (Horton et al., 1994). However, pear psylla does not reproduce on these transitory hosts (Kaloostian, 1970; Cooper et al., 2019). In addition, pear psylla adults will colonize ornamental Bradford pears (Pyrus calleryana) and nymphs can develop, however, populations in Bradford pears are much lower than commercial cultivars and it is unclear if immatures can complete development.
Egg: The egg, shaped like a grain of rice, is attached to the host by a small protrusion extending from the rounded end. The egg is creamy white when laid but turns yellow to orange as it develops (Figure 1).
Nymph: The nymph passes through five instars each of which ends in a molt. The first instar is translucent yellow. It is long, cylindrical, and about the size of the egg. Each successive instar is larger, flatter, and more oval than the last. The fourth instar nymph is yellow-green to light tan. The fifth instar is dark green to dark brown. The third, fourth and fifth instars have progressively larger wing pads (Figure 1–3).
Adult: There are two adult forms: winterform and summerform. Both forms of adults hold their wings roof-like over the abdomen. Adults have reddish brown bodies with black markings, and winterforms may appear almost black. The winterform is larger (wing length 2.3 to 2.5 mm) than the summerform (wing length 1.6 to 1.8 mm) (Figure 4–5) (Slingerland, 1892; Wong and Madsen, 1967; Burckhardt and Hodkinson, 1986).
Pear psylla overwinter as winterform adults in a state of reproductive diapause. They begin laying eggs when pear buds begin to swell. First eggs are deposited on the wood, generally at the base of fruit and leaf buds (Horton, 1999). Offspring of the overwintered generation become summerform adults first appearing in mid-May. Pear psylla has two to four summerform generations in most pear growing regions, with generally two complete summerform generations occurring in Washington (Horton, 1999). Summerform adults tend to lay eggs on rapidly growing leaf tissues often placing eggs along the leaf mid-vein (Horton, 1990).
Fruit russet: Nymphs and adults are phloem feeders. Honeydew, produced by nymphs, drips or runs onto fruit, causing dark russet blotches or streaks and fruit downgrading (Figure 6). The damage may be exacerbated by a sooty mold fungus that colonizes the honeydew and also marks fruit (Burts, 1970).
Psylla shock: In large numbers, pear psylla can stunt and defoliate trees and cause fruit drop. A carry-over effect may reduce fruit set the following year. These symptoms, called psylla shock, are caused by toxic saliva injected into the tree by feeding nymphs (Westigard et al., 1979).
Decline: Pear psylla also transmit a mycoplasma disease organism (Pear decline phytoplasma) through its saliva. The disease damages sieve tubes in the phloem. This damage prevents nutrients from moving down the tree and results in root starvation. Trees grafted on Ussurian pear (P. ussuriensis) and Asian pear (P. pyrifolia, synonymous with P. serotina) rootstocks are the most susceptible. Trees grafted on P. communis, P. betulifolia, P. calleryana, and Cydonia oblongata (quince) rootstocks become infected but are tolerant and display reduced decline symptoms (Teng et al.; Blomquist and Kirkpatrick, 2002). Most pears in Washington and Oregon are grafted to tolerant P. communis (Elkins et al., 2012).
Adults: Monitor adults with beat tray sampling (Figure 7). Hold an 18-inch-square tray with a white cloth cover one foot below a 0.75 to 1.5-inch diameter limb with an average number of spurs and branches. Tap the limb firmly three times with a stiff rubber hose. Count adults jarred from the limb onto the tray. Thirty trays at random through the sampling area is standard for a pear block of ten to twenty acres.
Subsequent generations of eggs and nymphs should be sampled on new shoot growth. Collect a total of ten leaves from each of ten randomly selected trees. Select five leaves from the lower canopy with two in the center of the canopy near the crotch of the scaffold limbs and three in the middle of each of two scaffold limbs (Figure 8). Include 1-2 leaves that may not receive good coverage in the center of the tree. Use a telescopic pruner to collect five leaves across two clusters or shoots in areas that are difficult to spray such as the upper canopy and the backside of limbs.
Important biological control organisms in Washington pear orchards are the parasitic wasp Trechnites insidiosus; true bugs Deraeocoris brevis, Campylomma verbasci, and Anthocoris spp.; lacewings Chrysoperla carnea, Chrysopa nigricornis, Hemerobius spp.; and the earwig Forficula auricularia.
Trechnites insidiosus can parasitize pear psylla at rates that exceed 70% in unsprayed orchards and 50% in organic orchards (Beers et al., 1993). Maximum parasitism rates in a review of nineteen field studies range from 1.7% to 100% (Tougeron et al., 2021). Over half of studies report parasitism rates exceeding 40% (Tougeron et al., 2021). Trechnites insidiosus generally has four generations per year in Washington orchards. Many emerge from the overwintering generation of psylla hosts at approximately bloom time (Figures 9-10).