Plan ahead to manage pear psylla
March 28, 2006 - Author: Michael Haas, and Larry Gut, Michigan State University Extension, Department of Entomology
Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.
Pear psylla management can be difficult, especially if you do not have a program outlined for the season. If control measures are not taken early in the spring, the summer may be spent trying to get a handle on this pest. A little planning can go a long way toward limiting your commitment of time and money over the course of the season. Knowing a little about pear psylla biology provides a foundation for sampling and timing of control measures.
Identification and biology
Psylla adults resemble tiny cicadas and are about 1/8 inch long when fully grown. They are yellowish orange to reddish brown and have four dark stripes across the back. The winter form of the psylla adult is darker and noticeably larger than the summer form. The young nymphs are pale yellow with red eyes, while older nymphs are darker brown. The nymphs go through five growth stages prior to becoming a winged adult. Stages 3, 4 and 5 can be distinguished from the earlier two by the presence of wing pads (undeveloped wings). The later nymphal stages are called “hard shells.” The oblong eggs are about 1/64 inch long and can be laid singly or in rows end to end. They are pale yellow when first laid and turn darker yellow to orange as they approach hatching.
Overwintering psylla adults become active early in the spring as temperatures rise above 45ºF. Adults can be monitored at this time by holding a cloth-covered catching frame under a limb and jarring the limb to knock adults onto the tray surface. After leaving their overwintering sites, females mate and lay eggs on pear trees. In the spring, eggs are most commonly found in creases in the bark, old leaf scars, at the bases of terminal buds or on newly emerging leaves and fruit spurs. To determine if eggs are being laid in your orchard at this time, simply pick fruit spurs as you wander through the orchard, and examine the woody portion and bud scales using a hand lens. Summer eggs are primarily laid along the midveins on the undersides of leaves. Eggs and young nymphs will primarily be found on new growth. Cutting down shoots and examining the young leaves near the shoot tips is a good sampling strategy for determining the densities of young nymphs and eggs in your orchard. Estimates of the total number of eggs laid per female range from 200 to 650. The eggs hatch in as few as 10 or as many as 30 days depending on the temperature. There are three generations of psylla in Michigan.
Types of injury
Psylla nymphs use their needlelike mouthparts to suck plant juices, discharging a waste product called honeydew. The primary injury is caused by black sooty mold growing in the honeydew deposits on the fruit, causing russeting. Other symptoms of psylla injury include sooty mold and tissue necrosis on the leaves. Affected trees may be stunted, lose leaves, set fewer fruit buds and produce undersized fruit which may drop off the tree before maturing. These symptoms are called “psylla shock” and are a result of a toxin in the insect’s saliva. Psylla may also spread disease while feeding. Fire blight and pear decline are thought to be spread in this manner. Prolonged exposure to psylla feeding can kill a tree. Asian pear varieties are less susceptible than European varieties to psylla injury. The variety “ Bartlett” is very susceptible, making it a good one to monitor for evidence of psylla problems.
Insecticide resistance in psylla is widely found; therefore careful attention must be paid to the class of insecticides used for control. Psylla management begins with a late dormant or delayed dormant oil application. An adulticide is sometimes mixed with the oil. An early oil spray helps deter egg laying for several weeks. This helps to synchronize the age of the following generations and makes management during the rest of the season a little easier.
A pesticide application at white bud stage targets the early egg hatch period. Insecticides in the pyrethroid class can be used against the overwintering psylla but are not recommended for summer applications. The insect growth regulator pyriproxifen (Esteem) stops egg hatch and prevents nymphs from reaching the adult stage, making it an option while eggs and nymphs are present.
A variety of pesticides representing several chemical classes are available for in-season psylla control. A threshold of one psylla nymph per three leaves provides a guideline for deciding when a treatment is needed in the summer. Sprays should be applied in a minimum of 100 gallons per acre to ensure thorough coverage. It is also very important that the entire orchard is covered at once, rather than making alternate row middle applications.
Cultural practices can help lessen the incidence of psylla infestations in the summer. Pruning of water sprouts, reducing nitrogen over fertilization and avoiding of aggressive dormant season pruning can all help decrease terminal shoot growth and reduce feeding sites.
Pear psylla can prove to be a tough pest to beat, but with a well thought-out management program that includes attentiveness early in the season, pesticide rotation and monitoring, this insect can be kept below damaging levels.
Dr. Gut's work is funded in part by MSU's AgBioResearch.