The question of whether herbicides, chemicals designed to kill plants, also pose a threat to insects is important. While herbicides are not formulated to target insects, their widespread use in agriculture creates complex interactions within ecosystems. The relationship is not simple direct poisoning, but involves ecological disruption and unintended physiological effects. Understanding this duality requires examining how these products reshape the insect habitat.
Defining the Chemical Difference
Herbicides and insecticides are chemically distinct tools, designed to interfere with fundamentally different biological processes. Insecticides disrupt an insect’s nervous system or interfere with exoskeleton development, leading directly to death. In contrast, herbicides target mechanisms unique to plant life, such as photosynthesis or the production of specific growth hormones. For example, the herbicide glyphosate inhibits an enzyme in the shikimate pathway, which plants use to synthesize aromatic amino acids. Since this metabolic route is not present in insects, the active ingredient is often considered to have a low direct toxicity risk.
Indirect Ecological Damage
The most significant impact of herbicides on insect populations occurs indirectly, by fundamentally altering the environment they depend on for survival. By eliminating non-crop plants, herbicides remove shelter, nesting sites, and the diverse forage insects require. This widespread reduction of plant life leads to ecological consequences that can severely limit insect numbers.
Loss of Host Plants
One well-documented instance is the effect on the monarch butterfly, whose larvae feed exclusively on milkweed plants. Increased use of herbicides on herbicide-tolerant crops has nearly eradicated milkweed from agricultural fields. This loss of the monarch’s host plant is linked to a substantial decline in the butterfly population, causing starvation rather than chemical poisoning.
Reduction in Forage
Herbicide application also causes a general reduction in floral diversity, crucial for generalist pollinators like honeybees and bumblebees. Weeds often serve as important sources of nectar and pollen when crop plants are not flowering. Removing these resources leads to nutritional stress that can impair insect development, immune function, and reproduction. Furthermore, some herbicides can reduce the nutritional quality of remaining host plants, resulting in smaller, less vigorous adults.
Direct Physiological Harm
Despite their intended plant-specific action, herbicides can still cause direct physiological harm to insects, often through components other than the active ingredient. Commercial herbicide formulations include “inert” ingredients known as adjuvants, such as surfactants and solvents, which help the active chemical penetrate plant surfaces. These adjuvants can be acutely toxic upon contact. Surfactants reduce the surface tension of water, potentially causing drowning in aquatic invertebrates or terrestrial insects that land on the water. Research shows that organosilicone surfactants, a common adjuvant, can be toxic to honeybees, sometimes more so than the active herbicide ingredient itself. These effects are often magnified when adjuvants are tank-mixed with other pesticides, creating synergistic toxicity.
Beyond the adjuvants, the active ingredients themselves can have sub-lethal effects that compromise insect health. Glyphosate, while not directly poisoning an insect, can inhibit the shikimate pathway in the beneficial bacteria that make up the insect’s gut microbiome. This disruption of the gut flora, known as dysbiosis, increases the insect’s susceptibility to pathogens, impairs its metabolism, and reduces its lifespan. Studies have also indicated that exposure to glyphosate can impair the navigational abilities of insects like honeybees, making it harder for them to return to their hives.