Emerging infectious diseases (EIDs) are often in the headlines. Typically insects only feature in these stories by virtue of their role as disease vectors, for example the transmission of Zica, Dengue and West Nile viruses by mosquitoes. Indeed mosquitoes are vectors for perhaps the biggest killer in human history – the parasitic organisms that cause Malaria. Of course insects are not only vectors for human diseases, but also those that impact many other animals and plants. As just one example, the peach-potato aphid (Myzus persicae) is one of the most economically important plant pests globally, infesting more than 700 plant species and transmitting well over 100 viruses to major crops.
But what impact do these viruses/diseases have on the insect? The answer is complex. For years it was thought little or no effect, because the insects typically showed no obvious symptoms. However, upon closer inspection it is apparent that insects do initiate an immune response (with many similarities to the human response), and there are several examples of viruses having negative effects, such as a reduced life span in mosquitoes carrying Malaria. However, some viruses are transmitted to eggs (i.e. can perpetuate across insect generations) with apparently no adverse effects. And some viruses can even be beneficial to the insect, for example plant viruses can alter insect biology allowing them to feed on diseased plants.
Emerging infectious diseases (EIDs) that directly kill insects have comparatively received very little attention. Yet we ignore these at our peril, given that insects represent well over 50% of all known species on earth and are critical to the functioning of all terrestrial and freshwater ecosystems. They provide fundamental services such as nutrient cycling, food (including for humans), control of pests (biocontrol), and of course pollination. It is this latter service that has received ever increasing attention over recent years because of continued concern about pollinator decline (especially bees), and the direct impact this has on our food security.
Insect diseases are certainly implicated in this decline, in combination with parasites (such as the Varroa mite), pesticide use, habitat loss and climate change.
We are right to be concerned, as insect pollination has an estimated value to global agriculture well in excess of £100 billion per year, not to mention an ecological value beyond agriculture which is far harder to quantify. This service is provided predominantly by ‘wild’ pollinator species (bees, hover flies, butterflies etc), although ‘managed’ pollinators (honey bees, bumble bees and several solitary bee species) also play a very important role. In fact, for many intensive crop systems managed pollinators are now essential, which, combined with honey production, means that pollinator husbandry is big business and undertaken on an industrial scale.
Just as factory farming in birds has been implicated in the spread of avian flu, the same is true of viruses in managed bees. The parallels are obvious, yet the lessons not learnt (or ignored) – i.e. that intensive production in crowded, stressful and unsanitary conditions provides the perfect breeding ground for a mutating virus (as well as other pathogens and parasites).
For several years it has been known that honey bee diseases can be transmitted to other species in the insect grouping, known as an Order, called Hymenoptera (this contains all bees, wasps and ants). However it wasn’t known if they could be transmitted to pollinators in other insect Orders such as Diptera (which includes all flies).
Very recent evidence has found three bee viruses in hover flies – and this should certainly raise alarm bells, as hover flies are very important pollinators (and also eat aphids). While it remains unclear whether these viruses can actively replicate in flies –the warnings from history are again there. Hymenoptera split from Diptera around 320 million years ago – coincidentally around the same time mammals and birds are thought to have last had a common ancestor. The ability of ‘bird’ flu to mutate and infect humans is all too clear – so it perhaps shouldn’t be surprising that bee viruses can also mutate to infect many other insect groups. Where this happens the viruses may well be lethal, especially in immune-compromised individuals (as with humans). This brings us back to the many other problems that insects face, all of which contribute to a reduced capacity to fight infection – pesticides, climate change and habitat loss etc.
Thus insects definitely do get sick, and emerging infectious diseases are clearly an increasing concern, however they shouldn’t be studied in isolation.