Maryanne Duncan, of Poolburn, asks :-

I found an unusual spider inside our pump-house. It has a red spot on its back and two red spots on its belly. In its web was a large water spider. What sort of spider is it and is it dangerous?

Cor Vink, an entomologist at Canterbury Museum, responded.

The spider inside your pump-house is a female Australian redback spider (Latrodectus hasseltii). Redback populations were first recorded in New Zealand in the early 1980s near Wanaka and have since been found in Central Otago at Alexandra and Bannockburn, and also in New Plymouth. Up until now, redbacks had not been recorded from Poolburn. It seems likely that redbacks will continue to spread in Central Otago as they need a dry climate with hot summers so that their eggsacs can mature.

The cold does not bother them too much and they can survive temperatures as low as -10°C. Computer modelling of climate and habitat has revealed that they could potentially establish in many other parts of New Zealand.

Bites from redbacks are uncommon as they tend to keep to themselves and they are not aggressive. Their bites can produce a syndrome known as latrodectism, which is characterised principally by pain. Bites may not be initially painful, but extreme pain can develop later. Other symptoms can include nausea, vomiting and profuse sweating. Anyone bitten by a redback should seek medical help and capture the spider if possible, so that it can be positively identified by an expert.

The real threat of redbacks is to New Zealand’s native insects and spiders. They have been found in the Cromwell Chafer Reserve feeding on the endangered Cromwell Chafer (Prodontria lewsii). In Alexandra, redbacks feed on the chafer beetle Prodontria modesta, which is only found in that area. Redback webs are very strong and they have also been known to immobilise and feed on lizards, so they could pose the threat to some of the skinks in Central Otago. Redbacks are very closely related to the endemic katipo, Latrodectus katipo, which they could threaten by competitive displacement and/or interbreeding.