This chapter describes the basic factors about arthropods: life cycles, life histories, and identification. It also discusses pesticide resistance and its management. Understanding the basics about arthropod life cycles and growth habits will help the grower grasp more complex and critical concepts of pest management.
Arthropods undergo changes as they grown and mature through a process known as metamorphosis. Some very primitive insects change so little, other than in size, as they develop from immatures into adults that they are said to exhibit ametabolous metamorphosis. This simply means that there is almost no change in form from one stage to the next. Most insects, however, exhibit either gradual (incomplete) or the classic complete metamorphosis that occurs among the butterflies and moths.
Incomplete or gradual metamorphosis is considered the more primitive of the two. It is characterized by immature stages called nymphs, which resemble the adult stage. The insect grows larger with each new growth stage or instar, but little external change is visible. The nymphs resemble the adults of these species and are found in the same habitat, usually feeding on the same food sources. The adult stage normally differs from the nymphal stage by the presence of wings and reproductive organs. Aphids, leafhoppers, plant bugs, and even mites are examples of arthropods that display this type of development.
The life cycle of insects exhibiting complete metamorphosis consists of four distinct stages: egg,
larva, pupa, and adult. The egg hatches into a larva which may pass through several instars. The
larva is usually wormlike in appearance, and its primary function is to eat and grow. It does not
resemble the adult, and many do not even resemble insects. The most recognizable larval forms
of insects are probably caterpillars, which eventually transform into moths or butterflies. The
larvae molt several times as they grow and, following the final larval stage, transform into a
pupa. The pupa is a resting stage during which the greatest transformation occurs. After this
resting stage, which can last from a few days to several months, the adult emerges.
The adult is the most recognizable form of those insects exhibiting complete metamorphosis. The adult serves primarily to mate and reproduce in most species. The feeding habits and host plants of the adult insects will usually be quite different from those of the immatures. Among apple pests exhibiting complete metamorphosis, the codling moth, leafrollers, plum curculio, and apple maggot are examples. Table 1.1 lists the arthropod pests of apples in the Southeast and their types of metamorphosis.
Some insect and mite pests invade orchards from outside while others remain in the orchard on a
continuous basis. In any case, pests are usually present in a life state susceptible to control
methods for only a short time. Therefore, producers have to be aware not only of the pest's
presence but also of the stage(s) present in order to make proper control decisions.
Most insect and mite pests of apples in the Southeast complete three or sewer generations each
year. A few, however, complete several generations and continue to produce offspring as long as
conditions are favorable. Because insects and mites are cold-blooded animals, they usually cease
activity during the winter months. Each species will have at least one life stage that is adapted to
survival during the winter. Table 1.2 lists the arthropods that attack apples in the Southeast, their
overwintering stages, the overwintering locations, and the number of generations that can
normally be expected.
The life histories of arthropods that affect apples range from the simple to the complex. Among
the simplest is that of the apple maggot which exhibits only a single generation per year and uses
few host plants. Among the most complex life histories are those of the aphids. Some, like rosy
apple aphid, complete three or four wingless generations on apple trees. Winged forms are then
produced. The winged aphids move to alternate summer hosts where several more wingless
generations are produced. Int eh fall, winged forms again appear. These winged aphids migrate
back to apples where mating takes place, and the eggs, which pass the winter, are laid.
First is the general size, shape, and color of the insect or mite in question. This is the
morphology of the species. Although it may not always appear so, all insects are similar in body
shape. Adults have three distinct body regions: head, thorax, and abdomen. The eyes, mouth,
and antennae are always located on the head; the wings and legs are attached to the thorax; and
the abdomen contains the reproductive organs. Immature insects exhibit the same three body
regions, but the regions may be more difficult to see. Immature insects may lack legs, may have
leglike structures on the abdomen and the head, and their sense organs may be reduced in size.
In mites, the head and thorax are fused together to form a cephalothorax, and the four sets of legs
(three sets in some immatures) are attached to it. The abdomen of mites is large in proportion to
the rest of the body and is usually not distinctly separated.
The type of metamorphosis also influences body shape. Most insects with gradual
metamorphosis will have nymphs or immatures that are very similar to adults but without wings.
In insects with complete metamorphosis, immatures may be grublike, caterpillar-like, or appear
as worms or maggots. The grublike larvae have well-formed heads, thoracic legs, and white or
creamy C-shaped bodies. They are usually found in plant tissue or in the soil. They are seldom
foliage feeders. Caterpillar-like larvae also have abdominal legs. The color is highly variable
and the body is not C-shaped. Larvae of this type may feed internally in fruit or plant structures
or externally on foliage. Wormlike or maggotlike larvae have no visible, well-defined head. The
body tapers toward the head and is usually white or cream colored. Most feed internally on the
fruit.
Once the insect's body type is correctly identified, the color becomes important in determining the species for field identification. This publication contains color photographs of all of the major apple pests and some minor pests, as well as beneficial arthropods. These photographs and the accompanying descriptions of each species should allow the producer or scout to identify insects or mites found in the orchard.
The second category of information that is important for field identification of arthropods in the
orchard is the ecology of the species. Ecology means what the insects' plant hosts are, where on
the apple tree the insects occur, and what type of damage they cause.
Many apple pests feed on a variety of host plants, while others are restricted to a few or one host.
The species descriptions appearing later in the publication deal primarily with the biology of the
species as related to the apple orchard and ignore other host plants.
The location of the insect or mite on the apple tree will provide clues to the pest's identity. For
example, green aphids feed almost exclusively on the tender foliage of the growing terminals,
while rosy apple aphis\d populations prefer spur leaves where they affect fruit formation.
Recognizing which pest is most likely to be found on each portion of the tree will streamline
scouting by reducing the areas to be examined.
Damage caused by each pest species is also indicative of the species. With a little experience,
the scout can easily tell the difference in internal feeding damage caused by the codling moth,
plum curculio, and apple maggot. The scout can use the descriptions of damage caused by apple
pests (presented in the following chapter) to help identify the responsible pest. Damage
sometimes may not be observed until the pest is no longer present, but it is still important to note
for future management decisions.
The feeding behavior of a pest is linked to the damage it causes and can help producers immediately identify the pest. For example, the codling moth usually enters the fruit through the side of the calyx end and burrows deeply to feed in the core of the apple. The plum curculio larva tunnels through the fruit, leaving large trails partially filled with brown frass (drippings). The apple maggot also tunnels through the fruit, but very erratically, leaving smaller brown trails and without the granular frass indicative of the curculio.
The third type of information useful to the identification of insect and mite pests, is related to
temporal (time) and environmental conditions. The time of year that the pest occurs will provide
clues as to when it should be monitored. In the Southeast, for example, scouting for the rosy
apple aphid should take place from the tight cluster stage of slower development until mid-June.
After mid-June, the aphids have usually left the apple orchard to feed on alternate hosts for the
remainder of the summer.
Environmental conditions are influenced by cultural practices within the orchard. Artificial
inputs, especially the use of pesticides, limit the number of species that will be found in the
orchard. Consequently, the ecosystem of an apple orchard is less diverse than that found in wild
habitats. Only those species most abundant in the region or those most tolerant of pesticides will
be commonly found in the orchard. Other species will be relegated to minor or occasional roles.
Thus, the diversity of pests in a managed orchard will be limited, making identification easier.
Resistance within an insect or mite population is usually genetic and develops as a result of
natural selection. For example, a few members of an insect species within a population may
possess a genetic makeup that for some reason makes them less susceptible to the toxic effects of
a pesticide. The continued use of that pesticide in the agricultural ecosystem where these genetic
mutants occur will insure that the ratio of susceptible individuals to resistant individuals will
increase in favor of the resistant strain. As the susceptible strains of the species continue to be
killed by the pesticide, those that are resistant will have less competition and will reproduce to
fill the niche left by those that were killed. Eventually, the resistant strain will comprise most of
the population.
Resistance can occur in many forms, from resistance to a single pesticide, to cross resistance, and
even multiple resistance. Cross resistance enables a pest to survive exposure not only to the
primary chemical to which the resistance was initially developed but also to related compounds.
Multiple resistance is the most serious type of resistance and extends to two or more classes of
pesticides with different modes of action and different detoxification reactions within a single
species. Because of heavy insecticide use in apple production over the past three or four decades,
several important apple pests have shown signs of resistance to a variety of insecticides.
There are methods that can be used to avoid, delay, or even reduce the incidence of pesticide
resistance among pest populations. To avoid or delay the onset of resistance, the producer should
employ two principles in pest management systems. First, apply insecticides in a spray-when-necessary regime rather than on a routine, calendar-driven basis. This will reduce the selection
pressure on both the target pests and on minor pests that can become major problems if they
develop resistance. Second, realize that 100-percent control of most pests species is not
necessary to prevent economic loss. The object is to maintain the pest population at low levels
which will not do enough damage to warrant additional controls. An added benefit of this
approach is that low levels of pest species will support populations of many natural enemies such
as predators and parasites, which aid in maintaining pest populations below damaging levels.
To reduce resistance if it is already a factor, use the following guidelines; they should help to
extend the life of available pesticides and decrease the levels of resistance already present in the
orchard. Totally eliminating resistance is almost impossible.
John R. McVay