Montana's gall-fly weed control linked to threefold rise in hantavirus-carrying mice
A 1973 biocontrol effort that introduced European gall flies to curb spotted knapweed unintentionally boosted deer-mouse populations, leading to over three times more hantavirus-positive rodents in infested areas.
Montana’s 1973 biological-control program introduced the European gall flies Urophora affinis and Urophora quadrifasciata to reduce the spread of invasive spotted knapweed. Although the flies successfully infested knapweed seed heads, they did not eradicate the weed and instead provided abundant winter prey for native deer mice. Ecologists discovered that in knapweed-dominated patches, gall-fly larvae could constitute up to 85 % of a mouse’s winter diet, leading to higher survival and larger spring breeding populations.
Consequently, field tests showed deer-mouse densities doubled and hantavirus-positive individuals rose to more than three times the rate found in native grasslands. The findings underscore the need for comprehensive food-web assessments before releasing non-native control agents. Current weed-management strategies now favor a mix of reseeding, competitive plants, and host-specific weevils to avoid similar side effects.
Why it matters
The case shows how well-intentioned pest control can unintentionally increase disease risk for humans.
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