Chapter 22

Greenhouse Insect and Mite Pest Management

Plant Symptoms of Insect and Mite Feeding

Plant injury symptoms caused by incest and mite feeding are rarely mistaken for contagious diseases, since insect or mite bodies, cast exoskeletons, frass, or characteristic feeding injury are usually visible to the careful observer. The following is a description of some common symptom of insect and mite feeding that are known to cause damage to greenhouse crops.

Leaf Spots

Leaf spots are most frequently caused by plant pathogens (See Figure 22.1). Sucking insects, such as leafhoppers, may also cause leaf spots. When an insect's saliva is toxic to a plant, a dead spot may develop around the feeding site and holes may develop when the damaged tissue becomes brittle and falls out.

Leaf Curling, Puckering or Rolling

The saliva of some sucking insects, particularly aphids, may cause plant leaves to curl, fold or pucker (See Figure 22.2). These symptoms can be confused with plant diseases that cause similar symptoms.


From a distance, plants heavily infested with spider mites appear discolored (See Figure 22.3). Close examination of infested foliage reveals a bronze discoloration of the leaves.

Premature Leaf Drop

Plant pathogens or environmental problems generally cause premature leaf drop (See Figure 22.4). However, heavy infestations of aphids, mites or scale insects can also cause leaf drop.

Stem and Leaf Galls

A gall is an irregular growth of tissue by the plant in response to wounding caused by pathogens or insects (See Figure 22.5). The shape of the gall formed is often characteristic of the causative organism. Several insects and mites form galls, including gall wasps, gall midges, aphids, adelgids, eriophyid mites and sawflies.

Cankers and Swelling

Many beetle larvae and caterpillars bore into tree or vine trunks or limbs, causing the infested tissues to swell or form cankers (See Figure 22.6).

Branch Dieback

Wood-boring insects damage plant vascular tissues, resulting in dieback of the infested limbs or branches (See Figure 22.7). Branches damaged by other causes and weakened trees or vines in general are particularly susceptible to insect borers.

Root Damage

Nematodes and the larvae of some insects feed on and can seriously damage roots (See Figure 22.8). Because roots are not readily visible, diagnosing insect injury to roots is difficult. The primary symptom is a gradual decline in plant vigor.

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