Blackflies

Blackflies are presented as both a measure of chemical control’s ecological unpredictability and an exception in the widening history of insecticide resistance. In Ontario, spraying was followed by a blackfly population seventeen times larger than before; yet, when the book surveys resistance among insects of medical importance, blackflies are among the few groups for which resistance had apparently not emerged. The contrast captures a central tension: a species can escape one kind of chemical consequence while still becoming more abundant after the intervention meant to suppress it.

A population rebound after spraying

The book records blackflies in Ontario as an especially stark instance of pest resurgence. Spraying was followed not by a modest failure to control them, but by a population reported to be seventeen times more abundant than before treatment. The scale of the reversal makes the case more than a warning about an ineffective application: chemical treatment could be associated with an outcome opposite to its stated aim.

The Ontario example appears within a broader discussion of pesticide-induced disruptions in insect populations. Other cases in that account include an unprecedented outbreak of cabbage aphids in England after treatment with an organic phosphorus insecticide. Placed alongside that outbreak, the blackfly increase belongs to a recurring pattern in which spraying may be followed by an extraordinary resurgence of the target insect. The passage does not assign a specific biological mechanism to the Ontario increase, so the example’s force lies in the observed reversal rather than a proposed explanation.

An exception in the history of resistance

Elsewhere, the book considers how insecticide resistance threatens control programs for diseases including malaria, typhus, and plague. It describes the list of resistant species as encompassing nearly all insect groups of medical importance, but distinguishes blackflies, sand flies, and tsetse flies as groups that apparently had not yet become resistant to chemicals. This qualification marks the limits of the evidence at the time: it reports an apparent absence of resistance, not a guarantee that blackflies could never develop it.

That status contrasts with the already global resistance reported in houseflies and body lice. The comparison separates two consequences of chemical use that might otherwise be conflated. Ontario’s blackflies illustrate a population increase after spraying; the resistance discussion identifies a separate difficulty, the capacity of insects to withstand insecticides. The book presents blackflies as part of the first warning while, at that point in its account, leaving them outside the second.

Significance in the book’s argument

Together, these references make blackflies a compact but revealing case in the book’s challenge to confidence in broad chemical control. A treatment can be followed by a dramatic resurgence without the species being identified as insecticide-resistant, while the spread of resistance among other medically important insects threatens the continuing usefulness of chemical programs. The distinction matters: not every failure of control is evidence of resistance, and not every absence of known resistance means spraying has achieved its purpose.

The two appearances also widen the scale of the warning. The Ontario population change gives the argument a concrete regional instance; the later survey places blackflies within a much larger international problem of resistant disease vectors. The book’s account therefore resists a simple story of chemicals defeating pests. In one context, blackflies multiply after spraying; in another, they remain among the groups not yet reported resistant. Their significance lies in that unresolved contrast between immediate ecological outcome and the longer-term limits of chemical control.