Horticulture Principles and Practices

sucking insects (also found with some chewing insects) is an abnormal growth called
a gall (Figure 7–8).
Identifying the kind of insect is a critical first step in their control. Insects, like
other living things, have a preference for feeding time. Some avoid the bright daylight,
feeding only when it is dark. As such, some insect pests hide on the undersides of
leaves or even retreat and hide in the soil at the base of the plant during the day. Being
aware of such habits not only helps identify insect pests but also aids in their control.
Mealybugs and scales often dwell in colonies and at some stage may secrete a waxy
layer over themselves for additional protection; that layer is difficult to penetrate by
many insecticides.
7.4.4 CLOSE RELATIVES OF INSECTS
Mites
Mites are insectlike organisms. They belong to the order Acrina, of which the family
Tetranychidae (spiders and mites) is very important in horticultural plant production.
They have four pairs of legs (not three) and have no antennae or wings. They are among
the most widely distributed of pests, affecting a variety of plant species including ornamentals,
fruits, vegetables, and field crops. They occur in the field and greenhouse. Mites
are sucking insects that reproduce rapidly and frequently. Where infestation is high, they
form fine webs on plant parts (Figure 7–9). Spider mites (Bryobia praetiosa, Tetranychus
urticae, and Panomychus ulmi) are economically important horticultural pests.
Spiders
Spiders are closely related to true insects. They belong to the class of arthropods (jointed
legs) called Arachnida, which also includes mites.
Millipedes and Centipedes
Millipedes (Diplopoda) and centipedes (Chilopoda) occur most commonly in damp
areas such as beneath stones, piles of leaves, and logs. They rarely damage plants directly
and prey on small insects and spiders.
Galls
FIGURE 7–8 Galls can develop on both
the above- and belowground parts of plants.
FIGURE 7–9 Typical spider mite damage. (Source: Photo provided
courtesy of Oklahoma Cooperative Extension Service, Oklahoma State University)
7.4 Insects 227