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The relationship between insects and humans is complicated. Many people initially react to the word “insect” with the image of household vermin, or a biting/stinging nuisance, or a disease transmitting vector, or a crop/garden destroying pest. This initial reaction might then give way to the recognition of all of the “good” that insects do (pollination, decomposition and functioning as a broad base for many important food chains). As I wrote a few years ago (Signs of Spring 13, May 19, 2016), even mosquitoes do some “good” in their ecosystems.
A one hundred year old, volunteer organization called the Entomological Society Krefeld has been keeping a close eye on the insects of west-central Germany. They just reported (in the October 18, 2017 edition of PLOS ONE) that over the past 27 years populations of flying insects in sixty-three nature preserves surrounding the city of Krefeld have declined 76%. These nature preserves are relatively small, well maintained “island” habitats (primarily grassland or heathland) that are located in between the extensive agricultural fields and cities and towns of west-central Germany. The purpose of these areas is to create habitats conducive to the maintenance of biodiversity. The observed precipitous drop in flying insects in these biodiversity refuges, then, is particularly disturbing.
The authors of Krefeld paper speculate that pesticide use in the surrounding agroecosystems, or the destruction and disruption of surrounding habitats by human activity or the environmental degradation from pollution may be causing the observed declines in the flying insects. Further studies are being proposed to look at the impacts of these declines on pollination, food chains (especially in insect-feeding birds) and nutrient cycling.
There are some insects and arachnids, though, that are thriving in our human-modified world. The Center for Disease Control (CDC) released a report on May 1, 2018 that describes a 300% increase in human diseases transmitted by mosquitoes, fleas and ticks in the United States over the past ten years. The report concludes that warmer weather is the most likely cause of this rapidly increasing number of vector-transmitted, human illnesses. Increases in mosquito transmission of the Zika virus and tick transmission of the bacterium that causes Lyme disease were major components in the overall rise in these statistics.
Gypsy moth infestations, though, are notably less severe than they were even five or ten years ago. In large part the decline in this alien invasive insect is due to the effectiveness of an introduced fungal pathogen that kills the gypsy moth caterpillars. The fungus (Entomophaga maimaiga) has been used since the early decades of the Twentieth Century as a biocontrol agent against the gypsy moth. Gypsy moths were accidentally released into North American forests in 1869 and have been spreading and wreaking havoc ever since (see Signs of Spring 13, May 21, 2015 for a review). The fungus has been used for the past eighteen years in coordination with a multi-state, integrated pest control program coordinated by the Federal government called “Slow the Spread of the Gypsy Moth” (STS), and researchers at Cornell University have developed ways to monitor the dispersal of the fungal spores from infected populations. They determined, in a 2017 paper in Applied and Environmental Microbiology, that a spore cloud can travel up to forty miles from its site of release. Further, only one spore is needed to fatally infect a gypsy moth caterpillar!
Another insect that is thriving in the human-modified world is the oak processionary moth. This moth is a native of southern Europe, but it has slipped off of the European Continent where it is well controlled by a variety of predaceous beetles, parasitic flies and wasps and fungal pathogens, and entered the predator and parasite free ecosystems of southern England. The transplanting of oak seedlings infected with oak processionary moth eggs from Europe to England in 2005 is the likely event that led to this invasion.
The caterpillars of the oak processionary moth makes extensive, white webbing on infected trees and are capable of stripping entire trees of their leaves. The denuded oaks are then extremely susceptible to further damage from drought and a wide variety of pests and pathogens. As we have learned here in North American in our long battle with the gypsy moth (another invasive that favors oak trees) a given oak tree can withstand a year or two of defoliation but then with its metabolic reserves exhausted will be unable to make new leaves and will die.
To add to all of these potential ecological problems associated with the oak processionary moth, there is also a serious human health hazard to consider: the hairs of the caterpillars contain the toxic protein thaumetopoein. Thaumetopoein acts as a protective chemical for the caterpillars to help them avoid predation. The tiny hairs containing the chemical, though, can be released into the caterpillars’ environment where they can persist for up to five years. Thaumetopoein is capable of causing serious allergic reactions ranging from contact dermatitis to anaphylactic shock in sensitized humans.
Great efforts are being made to contain the oak processionary moth in the southern regions of England. Research is also ongoing to try to find an effective biocontrol regime against it. We have just begun, after 150 years, to deal with the gypsy moth invasion. We don’t need another invasive insect attacking our oak forests.