Entomology for Master Gardeners: Part 5
Understanding Insects in the Garden
How Insects Injure Plants
In many cases, plant injury is the first indication of an infestation. Knowing the types of injury caused by various kinds of insect mouthparts can be a clue to the identity of the pest causing the damage
and can also help determine appropriate control measures if needed. The time of year when the injury occurs and the species of plants being attacked also can be used in identification.
Following is a summary of injury types:
Types of insect injury to plants.
Chewers Chewing insects make holes in leaves, which in some cases are distinctive. Examples include the small, rounded holes of flea beetles and the skeleton-like holes of Japanese beetles. Some insects chew rounded holes in the middle of leaves, while others feed from the edges of the leaves toward the midrib. These “signatures” can change, however. Newly hatched caterpillars may only feed partway through the leaf, leaving a skeleton-like effect, while the larger stages chew completely through the leaf.
Because of limited mobility, caterpillars must remain near their feeding site, so they often leave large accumulations of dark particles of insect waste (frass). In contrast, beetles and katydids move around, leaving damage far behind as they forage over many plants. With a little practice, it usually is
possible to see the difference between fresh damage and injury that has begun to dry or form a callus.
Sap Feeders Noticing or recognizing damage by sap feeders is a challenge. Distinct symptoms (such as wilting, leaf yellowing, or distortion) or a buildup of honeydew or sooty mold are caused by
some insects. Many, however, don’t provide clues unless they have fed at the site for a long time in large numbers.
Frequently sap feeders (and mites) feed on the tender tip tissue or on the underside of leaves or needles. It takes careful, regular inspections of these areas to catch problems early.
Borers The larval stages of several species of beetles and moths tunnel inside plant stems, trunks, branches, twigs, or roots. Their activities disrupt water and nutrient flow; cause structural weakness; or allow entry for rots, pathogens, or other insects. Some attack specific plant species, while others are generalists. Examples include bark beetles, roundheaded and flatheaded borers, and some moth caterpillars. There are no “rescue” treatments for their injury, so controls are based on growing healthy plants and using preventive treatments applied as egglaying begins.
Leafminers Leafminers are very specialized larval stages of species of moths, beetles, and flies. They tunnel inside the leaf tissue as they feed on internal cells. The cavities they leave may be blotches, lobes, or narrow, winding passageways that gradually widen. One of the most familiar examples is holly leafminer. As with borers, a preventive strategy is needed because of the protection given to the larva by the plant.
Gall Makers Galls are irregular plant growths that can occur most anywhere on a plant. They may be caused by insects, mite, fungi, bacteria, or nematodes. Insects are responsible for most galls, and about 80% are produced by tiny wasps. However, aphids, maggots, and mites can be culprits, too. Over half of the known gall species occur on oaks. Thirty percent are associated with the daisy, rose, and willow families. Galls are unusual and striking, but some gardeners consider them unsightly. Galls usually are initiated about the time of bud break by chemicals injected into the leaf during egglaying or by the insect or mite as it develops. Galls provide food and protection for the inhabitant but other than early leaf drop, rarely affect tree health.
Disease Carriers More than 200 aphid species can carry more than 170 plant viruses. Planthoppers and more than 40 species of leafhoppers can carry pathogens, as can a few species of thrips, beetles, and mites. Insects have the following roles as disease carriers:
- Carrying of pathogens on or in their bodies from an infected plant to a healthy susceptible plant and injecting it as they feed
- Creating a wound on the plant through which a pathogen can enter. For example, disease in the soil or on plant tissue may enter at feeding wounds. In this case, the insect plays a very indirect role.
- Protection of plant pathogens with their bodies, with some pathogens even developing or multiplying within the arthropod.
Management of diseases carried by arthropods often is based on sanitation—removing infected plants that can serve as a source of the disease and avoiding the use of susceptible varieties. Usually, insecticide applications do not protect the plant adequately from arthropod-borne diseases.
Winter Survival of Insects
Winters in temperate areas such as Kentucky are a problem for insects. There is a long period with cold, sometimes freezing, temperatures and no food. Understanding the winter survival adaptations of pests can help with control strategies.
Diapause, a dormant period for the insect, provides a means of surviving this inhospitable time. Diapause can occur during any life stage, and varies with the species. The gradually shortening daylight of late summer and early fall cues insects to get ready to enter diapause. Their to-do list includes eating a lot to store up fat reserves, producing an antifreeze-like substance in the blood, and finding a sheltered place to settle down.
Diapause alone cannot guarantee survival in harsh conditions. Winter weather plays an important role, too, but it impacts some species more than others. Those species that spend the winter under surface leaf litter or relatively exposed places can be killed by extended periods or freezing temperatures. In contrast, the Japanese beetle passes the winter in the grub stage down in the soil, where it is protected from extremes at the surface. Sanitation, burying, or removing dead plants in the fall can reduce infestations of some insects by eliminating their overwintering sites or the insulation provided by surface residues.
A few insects are migrants, arriving in Kentucky in early summer and leaving in the fall. The Monarch butterfly is the most familiar example. Many of these orange and black butterflies can be seeing flying slowly but steadily to the southwest as they head to specific wintering sites in the mountains of Mexico.
Reducing Insect Problems in the Landscape
Following are a few keys to reducing problems with insect pests in the landscape
- Healthy, vigorously growing plants often are able to tolerate some feeding damage by insects. Sound cultural practices, including fertilizing and watering as needed, will allow many plants to compensate for light-to-moderate feeding damage.
- Select insect-resistant varieties if possible. For example, selecting plants that are not acceptable food for Japanese beetles will eliminate an annual battle with these hungry, persistent pests.
- Establish a diverse landscape with a variety of native or well-adapted species. This practice will foster a setting in which there are small numbers of many different kinds of insects instead of large numbers of just a few species. Insect problems are generally less severe in a balanced environment.
- Be willing to accept light-to-moderate levels of feeding damage or infestation. This tolerance will give a chance for natural enemies, such as lady beetles, to provide a degree of natural control and may reduce the need for insecticide applications.
- Choose a succession of flowering plants to provide the nectar and pollen that many beneficial insects need. This practice will increase the impact of natural control.
- Use selective insecticides as much as possible when pest outbreaks occur. This practice will minimize adverse effects on natural enemies and reduce the chance of problems with other pest species. For example, “Bt” insecticides only kill caterpillars; they will not kill insect predators and parasites that attack other pests.