Forest Pests

animals that injure trees and shrubs in forests. The great majority of forest pests are insects. Less harmful are certain mite species and vertebrate animals, particularly rodents (Muridae) and double-toothed rodents (hares). Depending on their feeding habits, forests pests are subdivided into (1) coniferous-needle and leaf miners (primary), which attack healthy plants; (2) stem borers (secondary), which attack weakened trees; (3) root borers or soil dwellers; and (4) fruit and seed pests.

Coniferous-needle and leaf miners. Coniferous-needle and leaf miners are especially varied and numerous. They include forms of different orders of forest insects that feed on leaves (or coniferous needles). Since they live in the open in the larval and adult stages (only a few live inside of leaves in the larval phase), they are directly affected by all kinds of climatic factors. Some needle and leaf miners (Lepidoptera, sawflies, long-horned beetles) are characterized by substantial fluctuations in numbers, others (leaf beetles, weevils, and blister beetles, for example) by more moderate fluctuations. They form focuses chiefly in young stands, parks, and shelterbelts. Under favorable conditions forest pests periodically undergo sharp upswings of massive reproduction. Each rise usually covers seven generations and passes through four phases. In the initial phase the numbers of a pest increase slightly; in the second, the focuses of forest pests are formed; in the third phase, the rise, forest pests appear in swarms and consume the crowns of trees; in the last phase, the crisis, the rise subsides. During a rise in massive reproduction, needle and leaf miners can spread over hundreds of thousands of hectares in a comparatively short period of time and do severe damage to forests, destroying the current season’s growth and greatly weakening and causing subsequent desiccation of individual trees and entire stands.

Chemical methods as well as sanitary-prophylactic measures are used to control these pests. The stands are treated with insecticides, generally, while the insect population is increasing and the larvae are in the younger stages and less resistant to the chemicals, and when insecticides do the least damage to useful fauna.

Biological control measures include protecting insectivorous birds and attracting them to forests and protecting and dispersing forest ants. Methods are being worked out for the use of parasitic fungi, bacteria, viruses, and other causative agents of diseases.

Stem borers. Stem borers are very numerous. They include beetles (mainly bark beetles, long-horned beetles, metallic wood borers, weevils), Hymenoptera (horntails), and Lepidoptera (carpenter moths, clearwing moths). They maintain, as a rule, a concealed mode of life. Only the adults live openly (for example, bark beetles, who nevertheless spend most of their lives inside tissues). Digging holes in phloem, cambium, and xylem, they frequently cause the trees to dry out. Many burrow deeply into tree trunks, rendering the wood useless. Whether or not the insects reproduce on a massive scale depends on the viability of the trees and sanitary condition of the stands. Stem pests usually infest weakened trees. In stands in poor sanitary conditions or stands situated near focuses of massive reproduction of secondary pests, even completely healthy trees frequently be-come infested.

Control measures are mostly prophylactic. Two such measures are forest management actions that increase the biological resistance of the stands (creation of mixed stands with undergrowth, selection of species in keeping with local climatic and soil conditions and resistant to diseases and pests, selection of a proper felling system, and observance of sanitary regulations, among others) and prompt removal of slash from the cleared sites. Laying out trap trees in a stand is effective. Diseased and very weak trees felled by wind, storm, or snow are used for this purpose. These trees attract pests that fly in the spring (a month before the start of flight) and summer (just before the start of flight or when the first beetles appear). After the pests arrive, the trap trees are scraped while the insects are still under the bark and have not yet burrowed deeply into the xylem or mass of bark. The bark is burned off or scattered about on open places with the phloem exposed. Chemical control agents are beginning to be widely used.

Root borers. Root borers include the larvae of cockchafers and other lamellicorn beetles, click beetles, darkling beetles, and some other species that live and deposit eggs in soil where their entire development takes place. Preventive and exterminatory measures are used to control these pests, which are a great threat to nurseries, forests, and shelter-belts. Prevention includes proper forest management and the planting of suitable species. Extermination is achieved by the use of chemicals (mixing seeds before sowing with insecticides; adding insecticides to soil; treating seedlings, slips, and cuttings with them; spraying forests to combat adult cockchafers; and so on) and some physicomechanical methods. Systems of control measures based on the results of special studies are applied to specific cases.

Fruit and seed pests. Fruit and seed pests include a great many insect species from different families and orders that injure the reproductive organs of trees and often do considerable damage to forests. These pests are difficult to control because most of the time they maintain a closed mode of life inside seeds and fruits.