by Kevin Floate, Sunil Shivananjappa, Diana Wilches, Rob Laird, and Paul Fields
Larvae and adult khapra beetle on wheat grains. Photo: Ministry of Agriculture and Regional Development, bugwood.org
Less than 3-mm long (a dime is 1-mm thick), the khapra beetle, Trogoderma granarium (Coleoptera: Dermestidae), is a wee little villain with amazing abilities that have resulted in it being recognized as one of the world’s top 100 most-invasive pests. The khapra beetle infests stored grain and stored-grain products. Feeding by larvae causes product damage, and the feces produced by larvae alters product taste. Additionally, fine hairs that break off of the larvae can cause allergic reactions. Egg-to-adult development can occur in as little as 5 weeks, with up to 10 generations per year.
Adults cannot fly, so the khapra beetle is mainly spread in shipments of infested products. It is established in Asia, Africa, the Middle East, and parts of Europe; elsewhere, strict quarantine regulations are in place to prevent its establishment. The detection of infestations in quarantine countries can trigger eradication programs that can be expensive and time consuming. A 13-year (1953–1966) eradication program in the United States involved the inspection of about 97,000 properties and fumigation of more than 600 infestation sites at a total estimated cost of USD $102–138 million (2019 dollars).
Control of the khapra beetle is hindered by the larvae’s amazing ability to survive exposure to extreme temperatures, starvation, and insecticides. Although it is native to India, our research identifies the khapra beetle to be among the most cold tolerant of stored-product insects. To eradicate cold-acclimated larvae in diapause, they must be exposed to -5 °C for a year or -15 °C for more than 2 months. Why would insects from India be so cold hardy, you might ask? Our research provides evidence that this cold tolerance is cross-linked to the insect’s ability to survive desiccation (i.e., the same physiological mechanism confers tolerance to both cold and desiccation). This makes sense, given that khapra beetle infestations can persist in extremely dry conditions.
Equally amazing is the ability of the khapra beetle to survive conditions of extreme food deprivation for as long as 6 years in diapause. Diapause for many insect species involves the passage of time in an inactive, non-feeding life stage such an egg or pupa. In contrast, khapra beetles undergo diapause as larvae that continue to sporadically feed and intermittently shed their skin (i.e., molt). Each time the larva molts, it shrinks a little bit in size, a phenomenon termed “retrogressive molting” and one of few such examples known to occur. This ability to shrink reduces the larva’s food requirements until more food becomes available. When food again becomes plentiful, the larva has the ability to increase in size and complete its development to become an adult.
One of our more recent discoveries is that khapra beetles harbor infections of Spiroplasma bacteria. These bacteria form symbiotic relationships and are spread from one host generation to the next via eggs laid by infected females. The nature of the relationship varies with the insect species and Spiroplasma strain. In some cases, infections kill male host embryos (male-killing) to increase the prevalence of Spiroplasma in the next host generation. In other cases, infections help protect the host from attack by parasitic wasps and nematodes. Future research is required to determine the potentially important role that Spiroplasma plays on the survival of the khapra beetle.
Related research from our labs:
Shivananjappa, S., P. Fields, R.A. Laird and K.D. Floate. (in press). Contributions of diet quality and diapause duration to the termination of larval diapause in khapra beetle, Trogoderma granarium (Coleoptera: Dermestidae). Journal of Stored Products Research.
Shivananjappa, S., R.A. Laird, K.D. Floate and P. Fields. (in press) Cross tolerance to desiccation and cold in khapra beetle, Trogoderma granarium (Coleoptera: Dermestidae). Journal of Economic Entomology.
Wilches, D.M., Laird, R.A., Fields, P.G., Coghlin, P. and Floate, K.D. 2018. Spiroplasma dominates the microbiome of khapra beetle: comparison with a congener, effects of life stage and temperature. Symbiosis. 76: 277–291 (https://doi.org/10.1007/s13199-018-0560-5).
Wilches, D.M., Laird, R.A., Floate, K.D. and Fields, P.G. 2017. Effects of acclimation and diapause on the cold tolerance of Trogoderma granarium. Entomologia Experimentalis et Applicata. 165: 169–178 (https://doi.org/10.1111/eea.12632).
Wilches, D.M., R.A. Laird, K.D. Floate and P.G. Fields. 2019. Control of Trogoderma granarium using high temperatures. Journal of Economic Entomology. 112: 963–968 (https://doi.org/10.1093/jee/toy379).
Diana Wilches and Sunil Shivananjappa completed their MSc thesis research on khapra beetle in 2016 and 2019, respectively. KevinFloate (Lethbridge Research and Development Centre, Agriculture and Agri-Food Canada, Lethbridge, AB), Paul Fields (Morden Research and Development Centre, AAFC, Winnipeg, MB), and Rob Laird (University of Lethbridge, Lethbridge, AB) co-supervised their research.
Trogoderma granarium Everts, 1898
Order: Coleoptera (Beetles)
Family: Dermestidae (Carpet Beetles)
Hosts: stored grains and legumes, and their products
Range: established in Asia, Africa, the Middle East, and parts of Europe