David Owens, Ph.D. (back), extension specialist in entomology at the University of Delaware, and master’s student Ben Sammarco (right) scout for lesser mealworms (Alphitobius diaperinus) in the litter of a broiler chicken production house. Lesser mealworms tend to aggregate beneath shelter such as feed trays, packed litter, and objects placed on the litter such as the box pictured. (Photo by Michael Crossley, Ph.D.)

By Ben C. Sammarco and Michael S. Crossley, Ph.D.

Michael S. Crossley, Ph.D.

Ben C. Sammarco

It is no secret that anthropogenic systems radically change the dynamics between organisms. Often, organisms need to adapt to human-made systems, but sometimes an organism succeeds with almost no apparent adaptation. The lesser mealworm beetle (Alphitobius diaperinus) is one such organism, an omnivorous scavenger and nest symbiont of several African bird species.

In their natural setting they live in the material and lining of the nest in small numbers, feeding on scraps of food, feces, and shed skin or feathers. Lesser mealworm beetles are exceptionally adapted to living, growing, and reproducing on scarce supplies of food and water. Populations are even known to occur within caves, thriving on a food supply of bat guano.

Nancy Hinkle, Ph.D.

However, in the anthropogenic setting of a broiler chicken production house, these scavengers’ populations explode: A single house can contain more than 30 million larvae, pupae, and adult beetles. It is these populations of lesser mealworm that we featured in a pest profile published in January in the open-access Journal of Integrated Pest Management. We were joined in authoring the article by Nancy Hinkle, Ph.D., professor of entomology at the University of Georgia and extension veterinary entomologist for the state of Georgia.

It is important to note that the lesser mealworm has not undergone serious adaptation to succeed in broiler houses. Lesser mealworms are remarkable scavengers, putting the “omni” in omnivore: A captive colony of mealworms will utterly demolish anything from apple slices to chicken drumsticks. Broiler houses, meanwhile, are far from suitable habitat for most insect species. They are large, contained structures, with air maintained at a low humidity, and the sole substrate is a mixture of feathers, wood chips, and chicken manure. They are made further hazardous to insects by being almost constantly filled with thousands of voracious insectivorous predators: the chickens themselves.

In the anthropogenic setting of a broiler chicken production house, populations of lesser mealworms (Alphitobius diaperinus) explode: A single house can contain more than 30 million larvae, pupae, and adult beetles. As illustrated here, individuals grow and pupate rapidly and spend the majority of their lives as adults. Females can reproduce repeatedly during their lifetime, fueling rapid population growth. (Figure by Ben Sammarco, originally published in Sammarco, Hinkle, and Crossley 2023, Journal of Integrated Pest Management)

Never the less, to the mealworms, a broiler house is paradise. The floor of a broiler house essentially amounts to a massive bird’s nest, with chicken detritus, manure, and spilled feed abounding. These beetles, adapted to making the most out of the absolute minimum, find themselves literally burrowing through food. Even the dry, moisture-starved (for an insect) conditions of the house hardly pose a challenge, as lesser mealworms have evolved in equally dry, well-insulated nesting materials. Their populations explode, straining our ability to contain them.

This is where problems begin to arise and where our changes to this system have altered the dynamic between beetle and bird. In natural settings, the lesser mealworm is a benign symbiont, eking out a living on the periphery of the lives of birds and bats. But, when tens of millions of beetles make their home in one place, where their hosts are equally contained, conflict begins to arise.

The lesser mealworm serves as a reservoir for several avian bacterial and viral pathogens, including Salmonella, Escherichia coli, and the causative agents of Marek’s disease and turkey coronavirus, among many others. For many of these pathogens, the beetles also vector the diseases back to the chickens themselves when chickens eat them.

More directly, populations of the beetles that are numerous and hungry enough will begin to feed on vertebrates, transitioning from kleptoparasite to ectoparasite. This can result in severe discomfort and stress for the chickens, as the beetles chew away small portions of their feet, or disturb their rest by biting at their feathers and breasts.

The lesser mealworm has also grown to cause issues for the poultry growers themselves. In the course of burrowing to pupate, the beetles bore holes into broiler house insulation, rapidly degrading it and increasing costs of climate management. In addition to disease concerns, chickens that feed on lesser mealworms have reduced feed efficiency and growth rate, increasing feed costs.

One strategy for managing lesser mealworms (Alphitobius diaperinus) in broiler chicken production houses is “windrowing” between flocks. Mounding the litter stimulates microbial activity, causing the interiors of the piles to reach temperatures over 130 degrees F (54 C), sterilizing pathogens in the litter. (Photo by Michael Crossley, Ph.D.)

Use of pesticides within the houses is, understandably, highly restricted, with only a period of a couple weeks between 6-week flock cycles where it is considered safe to apply anything to the litter. As such, no house is ever free from the beetles. The populations are merely knocked down to less problematic levels for a while.

While ongoing research endeavors to find new and better ways to control lesser mealworms, a major obstacle to progress is that we lack understanding of the economic impacts of beetle populations. While we can observe reduced efficiencies of various aspects of chicken production, no one has determined what the dollar impact is for an increase of, say, a thousand beetles on the profits of a farm. Without this understanding, we cannot determine or implement proper integrated pest management, and controlling lesser mealworms remains a guessing game of switching between chemicals in an attempt to keep the population abstractly “low.”

We are not aware of any attempts to move toward something like an economic injury level for lesser mealworm in broiler houses, but it seems like a logical next step for improving management of this unwanted nest associate. Such an effort won’t be trivial, but the issue seems too important to be left in the dark.

Ben C. Sammarco is a master’s student in entomology in the Department of Entomology and Wildlife Ecology at the University of Delaware in Newark, Delaware. Email: bcsam@udel.edu. Michael S. Crossley, Ph.D., is an assistant professor and agricultural entomologist at UD. Email: crossley@udel.edu.