Saskatchewan has about 20 species of blister beetles (aka oil beetles), ranging in body length from a few millimetres to about 2.5 centimetres. They are often conspicuously coloured and can be found on a range of plants though many have a fondness for solanaceous crops and legumes, especially caragana and alfalfa. Conspicuous body colour is a typical feature of many creatures (other insect examples include ladybird beetles, monarch butterflies and box elder [maple] bugs) and shows would-be predators that the owner is distasteful and to be eaten “at your own risk.”
The beetles get their common names from their ability if handled roughly, to exude an oily material, a process known as ‘reflex bleeding’, from limb joints and other body regions where their cuticle is thin. The material released is their blood which contains cantharidin, a nasty chemical that causes blistering of the skin. In mild cases, gentle washing with warm soapy water will provide relief; however, if blistering is more severe, medical advice should be sought.
Much worse, cantharidin is highly toxic and if ingested, even in quite small quantities, for example, by livestock, can cause severe illness and ultimately death. Though veterinarians in Saskatchewan and Alberta are not aware of cases in these provinces, in the USA there have been several reports of blister beetles causing horse deaths. The horses were fed hay containing alfalfa that had been crushed to accelerate drying, an action that had also squashed resident blister beetles, releasing cantharidin into the forage.
Adults generally emerge from their overwintering sites in May/June, living for several weeks, feeding and mating. Then, depending on the species, female beetles lay their eggs in the ground near a grasshopper egg bed or the entrance to the nest of a ground-dwelling solitary bee. A few species place their eggs in flowers. At this point, blister beetle development becomes particularly interesting (at least for entomologists!). Unlike the vast majority of insects whose juvenile stage is either a nymph (as in grasshoppers) or a larva (like caterpillars, maggots and other beetle grubs), blister beetle larvae undergo ‘hypermetamorphosis’, a reference to the fact that there are two distinct juvenile stages. From the eggs, hatch highly mobile, 6-legged creatures (the nymphal equivalent) called ‘triungulins’, a reference to the three claws at the tip of each leg that is used for digging or clinging on to the host bee. Depending on where their mother originally laid her eggs, the triungulins dig down into the soil till they find a grasshopper egg pod, or they wait at the bee’s nest entrance or in the flower for the host’s arrival. When a bee arrives, the triungulins climb onto her, to be transported to the nest housing the bee’s eggs and larvae.
From this point, the triungulins simply snack on the host’s eggs, and at their first moult change their form to become plump, immobile grubs (i.e., the typical larval form). With the onset of cold fall weather, feeding stops and the grubs remain dormant till spring. Then, depending on the species, they may continue to feed for a short time or immediately pupate, enabling metamorphosis to the adult form to occur.
Though as noted earlier, blister beetles show a preference for plants in the potato and pea/bean families, they will feed readily on many other plants. This may account for the rarity with which they reach pest proportions either agriculturally or in gardens. Should your veggie patch be invaded by a large enough number of blister beetles to warrant control, wear gloves and simply drop (or knock off) the beetles into a pail of soapy water where they will soon drown.
Cedric Gillott is a retired entomologist and professor emeritus in the Department of Biology at the University of Saskatchewan.
This column is provided courtesy of the Saskatchewan Perennial Society (SPS; [email protected]). Check our website (www.saskperennial.ca) or Facebook page (www.facebook.com/saskperennial) for a list of upcoming gardening events.
