Healthy Home Orchards (cont)
By: Whitney Cranshaw
Pears not only get hit by the codling moth, but can suffer a special plague: pear psylla (Psylla pyri). This small insect, a distant cousin of the aphids, sucks sap from pear leaves. It damages the tree in several ways. Most notably, the insect excretes a sticky honeydew that covers the leaves and fruit and promotes the growth of black sooty molds. Pear psylla infestation stunts the growth of leaves. It can even cause all the leaves to fall, a condition known as psylla shock. Finally, pear psylla can transmit phytoplasma, a type of bacteria that causes pear decline, a potentially lethal disease. This typically occurs on trees grafted to Pyrus pyrifolia or P. ussuriensis root stock.
Pear psylla spends the winter off the tree, but adults return in late winter or early spring as pear buds begin to swell. At first, the insect lays eggs on the bark, near buds; later in the season, eggs are laid directly on leaves. Nymphs, or immature psylla, spend much of their time immersed in a droplet of sticky honeydew and only move out of it to prepare for the adult stage. Typically, three generations are produced during the season.
Controlling pear psylla is a real challenge to commercial orchardists, mostly because the insect is quickly resistant to new insecticides. However, in backyard plantings (or in commercial orchards where spraying is minimal), a host of biological controls is available and can provide a high level of natural control.
Several cultural practices will help prevent pear psylla problems. Perhaps most important is limiting the amount of succulent new growth produced during the year. This denies pear psylla the tender shoots that allow them to increase their numbers. Limit new growth by using only modest amounts of water and fertilizer. Also, prune to avoid flushes of regrowth. Pull off -- don't cut -- water sprouts that grow from the base of the trunk in late spring. Pulling them off prevents regrowth. Finally, using two applications of horticultural oil prior to bloom will delay the pest's egg-laying cycle.
Aphids are abundant on all types of fruit trees. Usually they infest the shoots and the young leaves in spring. One species, the woolly apple aphid, is found on the bark of trunks and branches or on the roots, where it produces knotty growths. Another problematical aphid is the rosy apple aphid, which injects a toxic saliva into the plant, causing fruit damage.
Aphids found on fruit trees spend winter on the plant as eggs, usually laid around dormant buds. They hatch just as the buds break. After a spring feeding, the winged aphids move to a second host plant. Alternation of hosts is common among aphids. For example, the rosy apple aphid alternates between apple and narrow-leafed plantain; green peach aphid alternates between stone fruits such as peach, apricot or plum and summer hosts of various garden vegetables and weeds.
Late in the season, however, offspring return to the winter host plant--your fruit tree.
Horticultural oils should do the trick. One or two applications prior to bud break will cover and smother the overwintering eggs and prevent spring attacks. These treatments also help to control other pests that overwinter on the tree, such as leafrollers and mites.
A host of other insects visit backyard orchards -- various caterpillars, pear slugs, and leafhoppers to name a few. However, their needs are usually modest and don't seriously threaten your harvest. Even if a few insects do get into the fruit, the transformation of your fruit to jam or juice does wonders toward eliminating this worry.
Whitney Cranshaw is professor and extension entomologist at Colorado State University at Fort Collins.
Photography by National Gardening Association