Invasive ants drain the energy of hard-working bumble bees

Bumble bees move through crowded landscapes shaped by survival, conflict, and delicate rewards. Their foraging journeys sustain entire ecosystems, yet many hidden pressures weaken these essential pollinators.

A recent study reveals how invasive ants alter bee behavior. The changes in behavior look small, but carry deep consequences for colonies and the environments they support.

Ant rivalry shapes foraging

Argentine ants gather in dense groups and display strong aggression at nectar. “They can dominate a food resource just by showing up en masse,” said Wilson Rankin, a researcher at UC Riverside.

The study describes these encounters as classic interference competition, where direct conflict reduces a species’ ability to feed. Many animals adjust foraging in response to risk, and bees show similar patterns when confronted with hostile ants .

Bees often avoid flowers with aggressive ants. The researchers found that two thirds of observed bee actions near nectar involved interactions with ants. Ant aggression shaped bee movement almost immediately.

Ant presence reduced feeding attempts and increased the chance of contact or biting. These patterns highlight how competition can shift behavior long before any physical injury occurs. Bees respond to risk with caution, hesitation, or retreat.

Bee choices shift under threat

The study showed that bees rarely fed when many ants surrounded nectar. Higher ant numbers strongly reduced the likelihood of feeding and also increased the chance of bees being bitten.

These bites carried no fatal effect but triggered meaningful changes. Bees bitten once often switched into defensive behaviors and avoided feeding for the remainder of the interaction .

Past experiences shaped later choices. Bees that received aggression early in an encounter showed far higher chances of responding aggressively themselves.

Altered behavior of bees

Behavioral sequences fell into two broad groups. One sequence reflected calm feeding and gentle actions.

The other sequence showed long chains of aggressive behaviors. These chains included biting, mandible gaping, and reactions to ant bites.

The study suggests that bees may learn to avoid future risks. Such memory aligns with theories of risk sensitive foraging.

Repeated aggression may teach bees to treat certain nectar sites as dangerous.

Aggressive clashes escalate quickly

Some bees responded with strong force. “We do see the aggression being bi directional,” said Rankin.

The bees used their mandibles in these clashes. “Mandibles are like teeth but not only used to chew,” said Michelle Miner, a former graduate student at UC Riverside.

Larger body size often gave bees an advantage in direct one on one fights. Yet the study showed a deeper pattern.

Aggression created long behavioral sequences filled with rapid transitions. One aggressive act often triggered another. Once a bee entered an aggressive sequence, stopping became difficult.

Intense interactions that drain energy

Aggression occurred more often and earlier when more ants crowded the feeder. Even non-aggressive encounters frequently shifted into escalating conflict. Bees bitten once often gaped their mandibles, then shifted into attacks.

Ants answered with more bites or chemical use. These chemical applications, while rare, further increased tension. The researchers concluded that aggressive interactions dominated most transitions between behaviors.

Calm actions rarely followed aggression. The bees stayed locked in intense exchanges that consumed time and energy.

Bees are weakened by ants

Aggressive behavior reduced feeding. Bees engaged in conflict fed far less often than bees who avoided conflict.

Some bees fed only before experiencing aggression, then spent the rest of the visit in defensive or offensive actions.

Prolonged aggression drained energy and slowed nectar collection. “The ant presence induced prolonged aggressive exchanges,” said Rankin. These prolonged exchanges limited the food delivered back to the hive .

Impacts on bee colonies

The researchers noted that colonies function as superorganisms. A single bee’s loss might seem small, but repeated losses reduce colony productivity. Fewer nectar returns may slow brood development or reduce stored resources.

The study suggests that colonies may adjust worker activity when foragers return with less food. Yet no data confirm such compensation.

It remains unclear whether colonies send extra foragers when conditions deteriorate. These unanswered questions shape future research.

Ripple effects of ant aggression

The study supports the idea that ants change how bees approach nectar. Aggression created long and diverse behavioral sequences that disrupted feeding.

Bees involved in aggression were five times more likely to continue aggressive actions than shift to calm actions.

The bees were also eight times more likely to stay aggressive than begin feeding again. Most aggressive transitions led to more aggression.

These patterns match known behavioral theories across many species, including other insects and even mammals .

Bees encountering ant aggression may carry physical or psychological stress into later foraging trips. Learned avoidance may reduce visits to otherwise valuable flowers.

Reduced visitation could affect plant reproduction and pollination networks. Risk shapes choices, and choices shape ecosystems.

Resilience in social species

We still lack clarity on colony level responses. “We do know that the youngest bees don’t leave the colony. Once they’re old enough to get their licenses they go out and forage,” said Rankin.

Whether colonies adjust worker deployment after disrupted foraging remains unknown. The study suggests that interference competition can influence long term decisions, not just momentary feeding. This raises important questions about resilience in social species.

More research will help clarify how colonies adapt and how landscapes change when invasive ants reshape the behavior of essential pollinators.

The study is published in the Journal of Insect Science.

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