Metallic blue mud-dauber wasps turning aggressive?

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Nandkumar M Kamat

Beware of metallic blue mud-dauber wasps belonging to Sphecidae, a diverse group of solitary wasps, especially inside your house or office or school premises. Once known as non-aggressive or non-stinging they still have a sting and now this sting is used to injure humans instead of the spiders, their common prey.

Under climate change conditions and heavy urbanisation, these wasps can no longer be seen as harmless and beneficial insects, because now in urbanised Goa, their ecological adaptation to human habitat is perfect and they are turning aggressive. We ignore them outside where they feed on nectar, pollen and sugar-rich juice. Their population is now dangerously on the rise and people must protect themselves and especially vulnerable children, from these wasps because there is zero knowledge in Goa about their venom and its effects.

At the time of writing this article I have not fully recovered from the sting of such a female metallic blue mud-dauber wasp inside my University residence. My right palm was swollen within hours and the swelling then increased and spread within four days now reaching the arm joint. The right arm had to be put in a sling. Dermatologist Deshpande from Taleigao couldn’t find any evidence of the sting or the precise area of the bite, but fortunately, by providing him specific information with photographs every day on my condition I could get some relief.

But this episode showed me that citizens who consider these wasps friendly or harmless need to be henceforth careful because the female wasp without any reason stung me after hovering around for some time. Citizens having large gardens or residing near vegetated areas and who keep their windows open must have seen these blue metallic wasps entering and flying from room to room. Basically, the females are now so adapted to our machines that their white nests would be found behind electrical and electronic equipment which have some holes or cavities. If you find such wasp hovering behind a computer, pedestal fan, television or any other machine or gadget not being moved for days, then expel it immediately. Do it every time till it gives up. If you see the same wasp return then just kill it. Nothing is going to change in the ecosystem because we are now helping them to breed faster. The new generation of aggressive females would be attacking more humans, making the use of television, laptops, computers, pedestal fans difficult. These wasps are now producing more offspring from several nests which they build inside human habitats and various machines.

The metallic blue mud-dauber wasps found in Goa could belong to Chalybion bengalense common in India. Vietanmese scientist Phong mentions that copulation of this wasp takes place at the nesting site. For nesting, the female uses old nests of Sceliphron and various eumenides, pre existing holes, and trap nests. The wasp cleans the nest cell and inserts paralysed spider prey. The female lays an egg on one of her prey, frequently the second or the third brought to the nest. She closes the nest cells with mud and on the cell cap adds an extra layer of lime, mud, cow dung, resinous material, or rotting plant. The total time from egg to emergence of the adult wasp is 27–34 days. The longevity of adults is 15–47 days for males and 21–49 days for females. Chalybion bengalense overwinters as prepupa from mid November to early April of the following year. This wasp belongs to Sphecidae. It is a cosmopolitan family of wasps with about 10 thousand species of different shape, size, and colour.

C bengalense female digs its nest in the soil, sand or wood. Then it collects and deposits food in each nest cell. The food is a paralysed prey like spiders. The female then lays her egg on it. The mouth of the nests is sealed with a whitish substance. The wasp larva feeds on the readymade dead prey and then grows and exits by drilling a hole through the soft nest.

Few weeks ago, I noticed tremendous wasp activity near a pedestal fan kept near a window open to wild herbaceous vegetation outdoors. Large number of wasps were bothering us and we couldn’t understand their hovering over the fan. Then it was noticed that they had built several mud nests using the holes in the machine. After close inspection of several other electrical and electronic gadgets more such nests were located. That showed how quickly these wasps adapt to the human built environment for mating and breeding. The holes inside the machines were safe and perfect places identified by these wasp females.

In this same column years ago, I had written about ants and the same species of wasps adapting to computers and television sets. But there was no incident of aggressive behaviour or stinging. Even after I was stung, I noticed other female wasps trying to target me inside the house. At the time of writing this article I am wearing surgical gloves to protect myself while my wife is maintaining a vigil on these flying females. The pest controlling sprays are not effective against these rapidly flying wasps. It was wrong on the part of western scientists to certify these as non-aggressive and harmless. Definitely not inside houses during their mating and breeding time and nesting. Keep them outside your premises.