Insects get sick too. We’re used to thinking about insects as vectors (transmitters) of human diseases. For example: Zika virus, malaria, and Lyme disease are all diseases carried by insects that affect humans. It is no wonder that this is what comes to mind when we think of insects and disease; diseases vectored by arthropods (insects along with spiders, millipedes, centipedes, and crustaceans) cause 1.5 million deaths every year . However, what we don’t typically think about is that, in many cases, these diseases also affect the insect itself. For example, carrying malaria increases mortality risk to the mosquito itself . Further, insects also suffer from their own diseases, which while devastating to their insect hosts, are harmless to humans. Insects can suffer from diseases caused by bacteria, fungi, viruses, parasitoids (insect parasites), protists, or nematodes . The study of such diseases and their insect hosts is called Insect Pathology. Seem obscure? While it is certainly specialized, it is a large and meaningful field of study.
Why study insect diseases?
With almost one million described species, insects make up the largest proportion of the world’s species . From the human perspective, these insect species are classified as beneficial, innocuous, or pests. The majority of insect species are innocuous (harmless) and many are directly beneficial (e.g. honeybees, silkworms, ladybugs), but pest species by far get the most attention.
Fig. 1. Estimated proportions of the different types of life on Earth today. Chordates, species with a spinal cord, includes many species we’re familiar with—fish, amphibians, reptiles, birds, and mammals. Graph from Fossil Focus by Ben J. Slater. Estimates based on data presented by Purvis and Hector 2000.
Diseases can help manage insect populations
We can use our knowledge of the insect pathogens to help aid in the management of these pest species. Pressure to minimize the use of chemical insecticides has led to increased interest in the application of pathogens to manage insect populations. This is called Microbial Control . Consider Bacillus thuringiensis, which is a particularly famous microbial control agent. You may have heard of it referred to as simply Bt. Bt is a bacterium commercially available for the control of insects, and is important for agriculture and public health. Safe for humans, Bt kills a wide spectrum of insects and other arthropods. Strains have been found to work against caterpillars, true bugs, ants, grasshoppers, lice, mites, nematodes, and more. Another famous microbial control story involves the fungal pathogen Entomophaga maimaiga. From Japan, the fungus can effectively control outbreak populations of Gypsy Moths . Gypsy moth is an invasive insect from Europe, which denudes trees. Microbial control can also be used for fruit pests. Codling moth is considered the most serious insect pest of the apple production worldwide  and apple production has a reputation of being heavily reliant on harmful pesticides. However, researchers discovered a virus beneficial in controlling the effect of codling moth on apple and pear production .
Fig. 2. Resting (overwintering) spores of Entomophaga maimaiga_, _a Japanese fungus used in the successful management of the invasive insect, Gypsy moth. Photo credit: Wikimedia Commons
More than just killing pests
Insect Pathology has implications for understanding and controlling pest species, but also has important applications for protecting beneficial insects. We use products from or supported by beneficial insects every day. About a third of our food crops depend on pollinators to reproduce; these pollinators include an array of bees, butterflies, moths, beetles, and other insects . Honey and beeswax comes from honey bees, silk is produced by silkworms, and in many countries insects are commonly eaten as a valuable source of protein. There is currently great concern about bee Colony Collapse Disorder, the rapid die off of overwintering bee colonies. Over the winter of 2006 to 2007, some beekeepers reported losses of 30-90% of their hives . While die off has been a bit lower the last few years and progress is being made to understand the cause, there is still much to know about this issue. Insect Pathology has been key to understanding and improving this situation to protect the insects that support important products and our food supply.
Fig. 3. A honeybee. Recently, there have been massive die off of honeybees. It appears that multiple causes are to blame, but collectively the die-off is referred to as Colony Collapse Disorder. Photo credit: Wikimedia Commons
Insect diseases teach us about human disease
If the two above reasons to study Insect Pathology—to manage pest insects and to protect beneficial insects—aren’t enough, another reason to study insect diseases is because they it can aid in our understanding of human diseases. Disease in insects exhibits similar interactions, complexities, and patterns as those found in mammalian hosts and thus can be used a model system . By studying insect pathology, we can learn a lot about human diseases.
In a nutshell: insect pathology is important for managing pest species, protecting beneficially species, and as a model system. Insect pathology may have seemed obscure at first, but hopefully I have shown a few of the very relevant applications.
Hill, C.A., F.C. Kafatos, S.K. Stansfield, and F.H. Collins, Arthropod-borne diseases: Vector control in the genomics era. . Microbiology, 2005. 3: p. 262-268.
Schwartz, A. and J.C. Koella, Trade-offs, conflicts of interest and manipulation in Plasmodium–mosquito interactions. Trends in Parasitology, 2001. 17(4): p. 189-194.
Vega, F.E. and H.K. Kaya. Insect Pathology. 2012; 2nd:[Available from: http://www.sciencedirect.com/science/book/9780123849847
Numbers of Insects (Species and Individuals). Encyclopedia Smithsonian [cited 18; Available from: https://www.si.edu/Encyclopedia_SI/nmnh/buginfo/bugnos.htm
Entomophaga maimaiga – The caterpillar killer Cornell Mushroom Blog 2009; Available from: https://blog.mycology.cornell.edu/2009/03/18/entomophaga-maimaiga-the-caterpillar-killer/
Lacey, L.A., D. Thomson, C. Vincent, and S.P. Arthurs. Codling moth granulovirus: a comprehensive review. Biocontrol Science and Technology 2008 [cited 18 7]; 639-663]. Available from: https://www.researchgate.net/publication/43272872_Codling_moth_granulovirus_A_comprehensive_review
Insects and Pollinators. [cited 2016 17 July 2016 ]; Available from: http://www.nrcs.usda.gov/wps/portal/nrcs/main/national/plantsanimals/pollinate/.
Colony Collapse Disorder. [cited 2016 17 July 2016]; Available from: https://www.epa.gov/pollinator-protection/colony-collapse-disorder.
Insect Pathology. Department of Entomology [cited 2016 17 July 2016 ]; Available from: http://entomology.umd.edu/insect-pathology.html.
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