“Secondary kill” is a beneficial property of insecticide baits, in which active ingredients are spread by some insects to others. Baits lead to secondary kill in German cockroaches, for instance, when unaffected individuals feed on the feces or carcasses of affected cockroaches. A recent study, however, found that the such a dynamic does not lead to secondary kill in common bed bugs (Cimex lectularius), pictured above. (Photo by Barbara Bloetscher, Ohio State University, Bugwood.org.)
By Kelly McKinne
All across the world, bed bugs and other household pests have been making a noted resurgence. A preferred control method for these insects lately has been the application of baits. These baits have several advantages over the traditional sprays for household pests: Specifically, baits use smaller amounts of active ingredients, they typically last longer than conventional sprays, and baits can also deliver larger amounts of active ingredients through ingestion than by cuticular penetration. This also allows for the potential of secondary kills, as the active ingredients in baits are carried by foraging individuals and brought into contact with more sedentary individuals in the population.
The dynamics of secondary kills in the common bed bug (Cimex lectularius) are the focus of an article published in March in the Journal of Economic Entomology, reporting a study led by Yvonne Matos at the Department of Entomology and Plant Pathology at North Carolina State University.
Matos and her team knew that baits have proven to be an effective control option against the German cockroach (Blattella germanica), which is seen as a widespread urban pest and can often be found in hard-to-reach cracks and crevices. Matos notes that cockroaches are also ideal insects for the application of baits, because cockroaches often provide horizontal transfer of active ingredients to other members of their community through ingesting affected feces and other excretions or by feeding on dead insects. It is this horizontal communication in cockroaches that Matos wanted to investigate with bed bugs.
Like cockroaches, bed bugs are found in hard-to-reach places, making them difficult to affect with residual sprays. Their frequent proximity to mattresses and other areas commonly contacted by humans also make the use of sprays difficult. These two factors encourage the application of insecticide baits for bed bugs as well. Matos and her team studied the potential transfer of these active ingredients in insecticide baits to the first instar bed bugs cohabitating with insecticide-fed adults and their excretion.
In order to reach a determination on the potential for secondary kills in bed bugs in this scenario, Matos and her team needed to “determine if adult bed bugs affect the survivorship of first instars by assessing: one, the potential of adults to prolong survivorship by increasing relative humidity—i.e., blood-fed adult conditioning of the microhabitat—and, two, the effects of blood-fed or unfed adults on unfed first instar bed bugs (i.e., do nymphs gain nutritional benefits from adult excreta?).” Their team then “evaluated the direct effects of exposure of first instars to the excreta of adult male bed bugs fed insecticide.”
When Matos and her team studied the effects of relative humidity and fecal nutrients on nymph survivorship, they found that relative humidity did indeed affect the survivorship of first instar nymphs. They also found that the length of nymph survivorship was directly related to the level of relative humidity, with significant differences among treatments. Their results suggest that, under low environmental relative humidity conditions, with the introduction of fresh liquid excretion from recently-fed adults into the microhabitat, the relative humidity would increase and the survival of nymphs would also increase. To create a control for the study, they placed starved nymphs into environments with both blood-fed and unfed adults and set the relative humidity to 95 percent. This control study failed to show an increase in survivorship of the nymphs with the cohabitation of both blood-fed and unfed adults. These results suggest “unfed first instar bed bugs would not benefit significantly from either the water or nutrients in the excretions of recently blood-fed adults.” Their team believes that these results, in turn, reflect that secondary kill might be minimal in bed bugs.
Although this study suggests that current baits fail to produce a secondary kill in bed bugs, Matos says she and her team now realize what factors are most important when creating successful bed bug bait. They now know that bed bugs take larger meals than cockroaches (relative to body mass) and that their mouthparts limit them to ingestion of affected excrement only when in liquid form.
Kelly McKinne, MPA, has spent the last several years working and volunteering as a biologist/naturalist in the Midwest and on the east coast. He has served as a local guide for both insect and avian field trips. He currently explores the wilds of Pennsylvania. Twitter: @gonzonaturalist