Spider mites

Spider mites are tiny arachnids, not insects, but their outbreaks become a large-scale consequence of insecticide use. In the forests described here, chemicals intended to suppress spruce budworm also killed the mites’ predators, while disturbing the mites in ways that favored their spread and reproduction. The result was a striking reversal: the treatment aimed at one pest helped produce another, and the damage appeared across vast areas of forest.

Identity and feeding

Spider mites belong to the same broad group as spiders, scorpions, and ticks. Barely visible, they have mouthparts adapted to pierce plant tissue and suck out chlorophyll, the substance that makes foliage green. They feed on leaves and evergreen needles, so the effects of an infestation become visible in the plants they damage rather than in the mites themselves.

The signs vary with the severity of the infestation. A mild one leaves trees and shrubs mottled, with a salt-and-pepper appearance. When mite populations become heavy, foliage yellows and falls. In the forest outbreak that followed the 1956 spraying, Douglas firs browned and dropped their needles, leaving extensive blighted areas visible from the air.

The forest outbreak of 1957

In 1956, the United States Forest Service sprayed approximately 885,000 acres of forested land with DDT to control spruce budworm. The following summer, observers found a problem more extensive than the budworm damage the operation was meant to address: a spectacular spider-mite infestation. Almost all of the sprayed area was affected, while comparable damage was not evident elsewhere.

The blight was recorded in the Helena National Forest and on the western slopes of the Big Belt Mountains, then in other parts of Montana and south into Idaho. From the air, large tracts of Douglas fir appeared brown as they shed their needles. The scale and distribution of the damage made the outbreak notable even against earlier spider-mite scourges that foresters could recall.

Those comparisons also placed the 1957 episode in a longer history of outbreaks following forest spraying. One earlier episode occurred in 1929, before the DDT era, and involved lead arsenate; later outbreaks were noted in Colorado about twenty years afterward and in New Mexico in 1956. These precedents did not make the 1957 event identical to the earlier ones, but they connected mite surges to a recurring pattern: forest insecticide treatment could be followed by outbreaks of a pest it had not been intended to encourage.

Why insecticides favored the mites

The account identifies several connected reasons for the mites’ apparent success. They were relatively insensitive to insecticides, while many of the organisms that preyed on them were extremely sensitive. Their natural enemies included ladybugs, a gall midge, predaceous mites, and several pirate bugs. Spraying could therefore remove checks on the mite population without eliminating the mites themselves.

The chemicals also changed the organization of mite colonies. An undisturbed colony was densely settled beneath protective webbing, which concealed the mites from their enemies. When spraying irritated rather than killed them, the mites scattered in search of undisturbed places. In dispersing, they gained access to more space and food than the crowded colony had offered. With their enemies dead, they no longer needed to spend energy making protective webbing; that energy could instead go toward producing more mites.

The change in reproductive conditions could be substantial: egg production might increase threefold. The treatment thus had effects beyond the direct killing of susceptible organisms. It disrupted the existing predator-prey relationship and the mites’ crowded living arrangements, converting disturbance into an advantage for their population.

Ecological significance

The spider-mite outbreak demonstrates how a pesticide’s effects can extend beyond its named target. DDT was applied against spruce budworm, but the reported aftermath was a widespread mite infestation and damaged forest foliage. The sequence exposes a basic weakness in judging a control operation only by whether it attacks the intended pest: the same chemical can remove predators, alter competition and habitat within a pest population, and make a secondary outbreak more likely.

The mites’ transformation from a relatively inconspicuous organism into a major forest pest is presented not as an isolated accident but as part of a recurring ecological pattern. Earlier outbreaks following insecticide spraying, the predators’ sensitivity, and the mites’ increased dispersal and egg production together explain why broad chemical treatment could reverse its intended result. In this case, the forest’s response was legible in browning Douglas firs and falling needles across nearly the entire treated area—a consequence of intervention that reached well beyond the spruce budworm it was meant to control.