Trombiculoidea are mites with a dubious distinction – most languages have one or more common names for them. In Australia we would tend to call the hexapod larval stage ’scrub itch mites’ for the intensely annoying rash-like erruptions they leave after biting – an all too common result of a pleasant stroll through the bush. In North America they are better known as chiggers, red mites, or harvest mites (i.e. common in the Fall). I imagine readers of this blog with a knowledge of non-English languages could supply a host of other sobriquets. Anyone with actual experience of scrub itch, or as it is technically known – trombiculosis, might be tempted to add some colourful modifiers.
Because a large wheal may develop at the spot that a chigger bites, some think that they are burrowing in the skin. Alas, no, they are ectoparasites that digest our skin and lymph for their feed. There’s nothing to dig out to reduce the itch and the best thing you can do is not scratch and help avoid secondary infections. The first scratch is probably good, since it will crush or dislodge the mite, but the mite leaves behind all the enzymes it has been injecting into your skin, and that can take a week or two for your body to deal with. The more you scratch, the longer it will take to heal.
If you live in an area where the rickettsial disease scrub typhus is endemic, then it is a good idea to watch out for any large black scabs that form on a chigger bite. A large black scab or eschar is a sign of infection and all it takes is one infected bite. In Queensland scrub typhus is a problem, but one good thing about Edmonton is that both chiggers and scrub typhus seem to be absent.
It may be of some comfort to know that only two families of chiggers feed on vertebrates – about a dozen other families of related mites feed on insects, millipedes, spiders, harvestmen, scorpions, and the like. The two-winged flies (Diptera) seem to be especially lucky when it comes to collecting chiggers. A possible example of one is above on the viewers right (Microtrombidiidae?).
Only the larval stage (i.e. the first active stage which has only 3 pairs of legs) is parasitic in the Cohort Parasitengonina (chiggers and their relatives and the water mites). The nymphs and adults are predators – and they may have common names too. In English, the terrestrial species are often called Red Velvet Mites and they can be quite large (up to 16 mm – the largest known mites outside the ticks) and are covered in a dense pelage of hairs, typically red but sometimes red & white patterned, orange, or another colour or combination. Alex Wild at MyrmecosBlog has posted a striking picture of one of the red species.
I don’t have any good SEMs of a red velvet mite – they are as hairy as a mammal and one would go bonkers trying to mask around all the hairs. However, I do have an adequate picture of a member of the Subcohort Erythraiae, a species of Calyptostoma (Calyptostomatidae) with short setae. These rather sluggish mites are thought to be predators of fly larvae as nymphs and adults. The mouthparts (capitulum or gnathosoma) are retracted into the body and can be shot out to impale a maggot. The mite larvae are parasites of adult Diptera (BugGuide has some good pictures of craneflies infested with the larvae). So, the next time you come home with a case of scrub itch, think about all those poor flies out there that couldn’t scratch even if they wanted to – and emulate them, don’t scratch.