Scientists examined milkweed flowers with dead honey bees stuck to them; visits dropped 37%, while ant numbers climbed 51% in a UC Davis field experiment

A milkweed flower can offer nectar to a bee, but it can also become a trap. Some honey bees fail to escape and hence remain trapped inside the flower and die on its surface. Their bodies remain attached to the flower, creating an unexpected ecological event. Scientists at the University of California, Davis, conducted a field experiment in 2024 to understand what happens next. They found that these dead bees changed the community of insects visiting the plant.

A dead bee on a flower may not only be the end of one insect's visit, but it can also alter the response of the other insects to the same flower. A field experiment indicated that killing honey bees resulted in a 37 percent decline in flower visits, while ant numbers around the flowers increased by 51 percent. What they found was an unanticipated domino effect: the flower's own reproduction can end up causing the insects around it to behave differently.

A flower with an unusual way of spreading pollen


Milkweed plants do not release loose pollen grains in the same way many other flowers do. Instead, they produce compact pollen packages called pollinia. When insects visit, these packages can attach to their bodies and later be moved to another flower. This system aids in milkweed's reproduction and also presents a challenge to some of its visitors. The narrow parts of the flower structure can hold honey bees in place, preventing them from escaping. Some bees collect pollen, and others get caught and die on the flower. For scientists, these dead insects presented an even bigger question: does a trapped bee simply disappear from the ecosystem, or does its presence influence what happens around the flower?

The unexpected effect of a dead bee

To answer that question, researchers made comparisons between the flowers with dead trapped honey bees and the flowers without honey bees and observed the number of insects visiting the flowers. The results suggested that the presence of dead bees could influence insect behavior around milkweed flowers. Flowers containing dead bees were visited less than those without dead bees. The decrease was particularly pronounced in the case of honey bees, which may be avoiding flowers where other bees have perished. Another change observed was that more ants were found close to the flowers with dead bees. The number of ants on the flower clusters with dead bees was 51 percent higher than the number on control flower clusters. Because ants can act as scavengers, they were likely attracted by the dead insects as a food source.

The uncomfortable part: the flower may lose visitors

Initially, the increase in ants seemed like the obvious explanation. Ants have been reported to have an impact on other insects and can also deter some visitors from approaching plants. Further analysis suggested that the decline in visits was linked more strongly to the presence of dead bees than to ants. Their analysis indicated that the reduction in flower visits was due more to the presence of dead bees and not just scavenging ants. This suggests that honey bees may recognize dead members of their own species as a warning sign. Scientists believe this behavior could be linked to avoiding possible risks such as disease or danger.

A small event that changes a larger ecosystem

The study demonstrates how one insect’s death can influence interactions among many species. A single bee trapped in the flower can influence the flower, the other bees, ants, and other pollinators that come to that flower. Milkweed is already important in an ecological context as it is the sole host plant for monarch butterfly caterpillars. Changes in the interaction of insects with the milkweed flowers could then affect not only honey bees. The researchers are not implying milkweed flowers are harmful plants. However, the trapping mechanism is part of an evolutionary strategy that is important in the successful transfer of pollen. The study instead emphasizes that a natural system will have trade-offs. A feature that benefits one species can create challenges for another.


How the findings fit into plant-insect relationships


The study adds to a growing understanding that plants are not always passive performers in ecosystems. Their physical structures and chemical signals can shape how insects respond to them. Researchers have long studied how plants attract pollinators, but this experiment examined a less explored question: what happens when a plant interaction has negative consequences for the visitor? The answer shows that even a small event on a single flower can create a wider ecological effect. A dead honey bee attached to a milkweed blossom becomes a signal that changed how other insects approached the plant.