How Systemic Pesticides Kill Pollinators

Understanding the Hidden Impacts of Modern Insecticides

Pollinator declines are often attributed to habitat loss, climate change, and direct pesticide spraying. While those factors matter, one of the most significant and least visible contributors to pollinator harm is the widespread use of systemic pesticides.

Unlike contact pesticides that remain on the surface of leaves, systemic insecticides are absorbed into plant tissue. They move through the plant’s vascular system and become present in nectar, pollen, leaves, and stems. For pollinators, this creates continuous exposure through normal feeding behavior.

Understanding how systemic pesticides function is critical to understanding how they affect bees, butterflies, and other beneficial insects.

What Are Systemic Pesticides?

Systemic pesticides are chemicals applied to seeds, soil, or plant tissue that are absorbed and distributed throughout the plant. Neonicotinoids, often referred to as neonics, are the most widely used systemic insecticides in the world.

These compounds are designed to target insect nervous systems. Once absorbed, they bind to receptors in nerve cells, causing overstimulation, disorientation, paralysis, and, at sufficient doses, death.

Because the pesticide becomes part of the plant’s internal structure, it cannot be washed off and may remain active for weeks or months.

Exposure Through Nectar and Pollen

Pollinators are exposed to systemic pesticides primarily through nectar and pollen. When a bee collects nectar to fuel flight or gathers pollen to feed larvae, trace amounts of pesticide may be ingested.

While acute poisoning can occur at high concentrations, much of the damage associated with systemic pesticides is sublethal. Sublethal exposure does not immediately kill the insect. Instead, it interferes with critical biological functions, including:

• Navigation and orientation
• Learning and memory
• Foraging efficiency
• Immune response
• Reproductive success

A bee exposed to sublethal levels may fail to return to its nest, gather less food, or produce fewer viable offspring. Over time, these impacts accumulate at the population level.

Effects on Reproduction and Future Generations

For solitary native bees, which make up the majority of bee species in Colorado, reproductive success is essential for population stability. A female bee typically provisions each nest cell with pollen and nectar before laying an egg.

If the pollen ball contains pesticide residues, larvae are exposed during development. Research has shown that such exposure can reduce survival rates, impair growth, and affect adult behavior after emergence.

Butterflies and moths may also be exposed during the caterpillar stage if host plants contain systemic residues. In these cases, larvae feeding on treated leaves can experience developmental disruption before reaching adulthood.

These generational effects are particularly concerning because they may not produce immediate visible die offs but instead result in gradual population declines.

Environmental Persistence and Movement

Systemic pesticides applied as seed coatings or soil drenches can persist in soil and migrate into water systems. Studies have detected neonicotinoids in groundwater, streams, and adjacent vegetation.

This mobility expands the zone of exposure beyond the treated plant. Wildflowers growing near agricultural fields may absorb residues from contaminated soil. Aquatic insects, which form the base of many food webs, can also be affected.

Pollinators do not operate within property boundaries. They forage across landscapes, meaning environmental contamination increases the likelihood of cumulative exposure.

Why Native Pollinators Are Vulnerable

Colorado is home to more than one thousand native bee species. Most are solitary and do not form large colonies like honey bees. As a result, they lack the buffering capacity that social species may have against losses.

Many native bees are specialists, relying on specific plant species for pollen. If those plants contain pesticide residues, exposure becomes unavoidable.

Because native bee populations are often small and localized, even modest reductions in reproductive success can have outsized ecological consequences.

The Difference Between Visible and Invisible Harm

Public concern about pesticides often focuses on visible spraying events. However, systemic pesticides represent a less obvious but more continuous form of exposure.

A flowering plant may appear vibrant and healthy while containing residues in its nectar and pollen. Consumers purchasing pollinator plants may not realize that systemic treatments were applied earlier in the production process.

This disconnect between appearance and ecological impact makes systemic pesticide exposure particularly difficult to detect and address without transparency.

Reducing Risk Through Informed Choices

Mitigating the impact of systemic pesticides does not require abandoning agriculture or gardening. It does require informed decision making and greater transparency.

Practical steps include:

• Supporting policies that reduce default pesticide seed treatments
• Purchasing plants grown without systemic insecticides
• Encouraging labeling transparency
• Expanding habitat to strengthen pollinator resilience

Pollinators are essential contributors to both ecosystems and food systems. Protecting them requires understanding how modern agricultural practices influence their survival.

Systemic pesticides do not always produce dramatic, immediate die offs. Instead, they undermine pollinator health gradually, through repeated exposure embedded in the very plants pollinators depend on.

Recognizing this mechanism is the first step toward creating landscapes that are not only attractive to pollinators, but safe for them.