16.1 Pathogens of insects

Arthropods are the most diverse group of animals on Earth; insects occurring in most terrestrial environments, though only a few species are found in marine habitats, as these are dominated by the crustaceans. Estimates of the number of arthropod species vary between 1,170,000 and 10 million, accounting for over 80% of all known living animal species; with the insects as the most species-rich subgroup in land and freshwater environments.

As arthropods were emerging as the (numerically) dominant animals they are today, fungi were also colonising the land. Over the past 400 million years or so, fungi and insects have evolved together in a wide array of intimate associations; including mutualistic endosymbiosis; using fungi as obligate food sources, such as those found in fungus-gardening ants (Section 15.2); sexually and behaviourally transmitted parasites, such as Laboulbeniales (Section 16.4); and with fungi as the most common disease-causing agents of insects, and many entomologists believe that there may be more species of insect pathogens than there are species of insects (Lovett & St. Leger, 2017).

Insect species are infected by pathogenic viruses, bacteria, and protozoa, as well as two groups of organisms that have uncertain relationships, the microsporidia and the trichomycetes; all of these tend to cause infection after being ingested by the insect.

The true fungi that infect insects are invasive pathogens; that is, they can produce enzymes and hyphae that can penetrate the insect cuticle. These disease organisms often act as natural control agents by regulating the population size of insect pests. There is consequently considerable interest in harnessing this capability commercially to control insects that are pests because of their adverse effects on our agricultural activities or because they act as vectors of human diseases (Lacey et al., 2015; Butt et al., 2016; and see chapters 1 & 2 in Ecofriendly Pest Management for Food Security, Omkar, 2016; Blackwell, 2017; Lovett & St. Leger, 2017).

Updated July, 2019