A focus on blister beetles requires some comparison with, sometimes, similar looking longhorn (borer) beetles and a mention of the ground beetles, which can also emit toxic liquids. To gain a proper perspective, the various families of beetles discussed must be placed within the context of the whole order. In the class Insecta, there are 28 orders. The order COLEOPTERA, containing all the beetles, is the largest order in the entire animal kingdom. There are around 370,000 species already described world wide, with many more to be discovered.
Of this number around 18,000 species are known from the southern African region, ranging from half a millimetre to around 100mm in length. They are the most diverse order and occupy every imaginable habitat. The order Coleoptera is divided into over 45 families that occur within our region, each of which is sub-divided into genera of similarly classified species.
(As a point of interest, the weevil/snout beetle family - Curculionidae - is quoted as the largest family in the animal kingdom, with over 48,000 species). Like all other insects, most beetles are more active - and therefore more frequently encountered - during the warm, wet season.
Ground (Spitting) Beetles
Ground beetles are classified under the family Carabidae. Within this family the genus Anthia are probably the best known. There are several species of medium to large (20mm - 50mm) black beetles, often with white or cream warning markings - (see illustrations). They are flightless, fast running predators of any small animal they can overpower. The larger ones have powerful mandibles that can inflict a painful bite if handled.
More important to remember is their ability to squirt a fluid of formic acid and quinones in any direction up to 35 centimetres. In contact with the human skin, this fluid can cause severe pain and possible blistering - and more serious problems if it gets into the eyes. They are harmless to humans unless intentionally threatened and are beneficial in their predation of other potentially, crop-damaging insects.
Wash the affected area thoroughly with water (flush out eyes with an eyebath) and treat the area with cortisone cream or eyes with cortisone eye drops. Seek further medical treatment for affected eyes. Bear in mind that the irritant fluid from the beetle is likely to be acid and it should therefore be counteracted with a dilute alkaline solution.
The blister beetles belong to the family Meloidae, most of which feed on plant matter. Common throughout Africa are blister beetles of the genus Mylabris. They are generally sluggish, soft-bodied beetles, sometimes small (20 mm) and dull coloured, but often larger (up to 50 mm) and strikingly marked with black, yellow and red as a warning that they contain a powerful drug known as cantharidin.
Some species have long antennae, similar to longhorn (borer) beetles. (See accompanying photos). Species of Mylabris may damage crops as they feed on flowers but they compensate as parasites of many other crop-damaging insects, including grass-hoppers and swarm locusts.
Some of the smaller species of blister beetle are attracted to lights at night and may come into contact with humans. These beetles contain large amounts of cantharidin and if squashed or threatened they exude the cantharidin-containing liquid from their leg joints. In contact with the skin, cantharidin causes quite severe blistering and pain.
The reputed aphrodisiac, ‘Spanish Fly’, is manufactured from a species of Meloidae but its use is dangerous. Not only does cantharidin cause blisters but it is extremely poisonous and may cause kidney damage or death. Mylabris beetles have been used as a traditional poison. These beetles that happen to come into contact with humans are not likely to cause any problem if removed carefully as described below.
Prevention And Treatment
Insects are attracted to lights and illuminated, pale coloured objects at night. The best way to prevent contact with blister beetles (and other insects) is to sit away from lights. When outside, have a brighter light situated a short distance away, possibly with a white sheet hanging behind it, as a decoy to attract insects away from your position. (Insects attracted to the sheet can be of great interest to visiting entomologists and other guests).
If you get any unidentified insect on you, don’t slap it or squash it - being the instinctive reaction - but gently scoop it off sideways (with a piece of paper or knife blade). The blister beetles, as already mentioned, are rather sluggish and soft.
They squash easily and, most often, they emit cantharidin when caught between clothing and skin. Blisters and slight irritation will appear quite soon after contact with cantharidin. RESIST the temptation to rub or scratch AT ALL COSTS as this will spread the problem. Fullblown blisters will develop, accompanied by inflammation and an aching pain as the poison penetrates deeper.
Cantharidin appears to contain alkaloids and therefore causes a caustic burn. The application of a dilute solution of vinegar or similar, dilute acidic liquid, may be of benefit. The liquid within these blisters, if burst, will run down and cause further blisters, making a complete mess of the afflicted person.
The blisters need draining BUT the only safe way to do this is to immerse the patient (or the limb) in a bath of warm water and drain the blisters under water, where the fluid will be washed away and diluted without further contact with the skin. Once drained, the blisters will partially fill with healing plasma from around the affected area.
The blisters should be gently pressdried with a piece of tissue and the area covered with an absorbent material. Paper tissue is preferable to cotton wool as it releases easier when dampened. As soon as possible, an application of cortisone cream is recommended. Antibiotic powders also help - but if in doubt, get medical treatment.
The trick is not to allow the liquid in the blisters to spread to other parts of the skin, where it will cause more blisters. Unsightly scarring can occur long after the blisters have vanished and repeat applications of Vitamin E oil may assist in the healing of these areas.
The larger, boldly marked blister beetles are normally diurnal and quite obvious, but keep a look out for the smaller, cylindrical, sluggish, khaki/light brown or greenish coloured species which tend to be active at night. They are about 20mm long and generally the culprits most commonly encountered after sunset.
Longhorn (Borer) Beetles
This is a very large group of beetles that belong to the family Cerambycidae. There are no known toxic species and they are characterised by very long antennae. They range in size from tiny (3mm) seed and leaf beetles to very large (100mm), grey/brown borers with formidable mandibles.
Some of the diurnal species are a bright, metallic green, with others boldly marked with black, red, orange and yellow warning colours. The latter appear to mimic the toxic, blister beetles as a measure of protection from birds and other predators. (Thus the inclusion of this group for comparison with similar looking blister beetles with which there may be confusion).
The species that dwell mainly on the ground, among dead wood and litter, generally have a cryptic (camouflaged) colour, while the nocturnal species are normally dark coloured. Compared to some blister beetles, the cylindrical, longhorn borers have a hard body with stout wing covers.
They are an attractive group - fast moving when disturbed - but safe to pick up and examine, as long as one keeps fingers away from the powerful mandibles. Most longhorn beetles lay their eggs in dead or dying trees and other wood sources. The whitish larvae that hatch out feed on wood and tend to diverge from each other as they grow.
The size of these larvae depends on the species but some of the largest are thumb sized. They are all very palatable and provide a popular source of food to many birds, mammals and other invertebrates. These larvae, which taste like custard, are highly nutritious and a good source of energy (raw or cooked) for humans under survival conditions - especially during the dry season.
Tell-tale saw dust below dead trees will often indicate their presence. Their gut contains only sawdust - the same as any stick one may happen to chew. Generally, all larvae found boring in wood are edible BUT never eat any larvae found in the soil, unless you are sure about them.
There are some other small, cylindrical wood-borers, with very reduced antennae - the auger or shot-hole borers of the family Bostrichidae. These are the species which leave piles of fine sawdust under cane furniture and other wooden items.
These small (13mm), brown beetles of the family Paussidae, can be identified by their flattened, segmented, plate-like antennae. They deserve mention as they are also capable of expelling bursts of caustic liquid when disturbed.
This liquid contains quinone and, being chemically heated, turns to gas, visible as little puffs of smoke. They live in association with ants, which feed on secretions from the beetle, which in turn feeds on the ant brood. They leave the nest of the host to reproduce, at which time they may be attracted to lights, when they can be observed.
They are only likely to cause discomfort to humans if trapped between clothing and skin and any effects should be treated as described above. NOTE Such a vast and diverse topic cannot be adequately covered in a short article.
For those interested in further investigation I recommend reference to - Skaife’s ‘African Insect Life’ by Ledger and Bannister, and also the - Field Guide to Insects of South Africa by Picker, Griffiths and Weaving. I would also like to thank Pieter Fourie of Hoedspruit Pharmacy for locating chemical information on cantharidin and for his comments on treatment.