Abstract
Biological invasions occur when a non-native species establishes and proliferates in an ecosystem, often resulting in detrimental impacts on the environment, local economy and in some cases human health. Islands such as New Zealand contain fewer competitors, predators, herbivores, parasites and pathogens for the invading species to adapt to, and are therefore particularly vulnerable to biological invasions. A high level of endemism and specialisation in New Zealand's invertebrates have left them vulnerable to aggressive and/or generalist species such as the Australian redback spider (Latrodectus hasseltii). Redback spiders are an internationally invasive species and one of the most medically-relevant (they present a threat to humans) spiders on record. They were initially observed in New Zealand in 1981, and have since established two separate populations in the North and South Islands. In 2013 redback spiders were recorded preying on numerous native and endemic New Zealand species, which drew attention to the need for a means to control redback spider populations.
In order to mate, male redback spiders abandon their webs to search for females, which are largely sedentary. Males are attracted to an airborne pheromone that virgin females place on their webs, and which ceases to be produced once a female has mated. Pheromone-dependent mating behaviour provides an opportunity to develop a species-specific biological control using a pheromone trap. A pheromone trap would eliminate the need for broadcast pesticides and their associated non-target mortality. It would prevent mating within a redback population by luring males away from females to a trap, where males could be destroyed.
The aim of this study was to investigate the feasibility of a pheromone trap using butyric acid, isovaleric acid, or a combination of both chemicals. These two acids had previously been identified as being unique to virgin female redback spider web, and thus were hypothesised to be the active chemicals in the airborne female sex pheromone. I recorded behavioural responses of redback males when they were presented with virgin female web in an olfactometer. I then compared these behaviours to responses elicited by a double control, butyric acid, isovaleric acid, and the two acids in combination. Finally I presented male spiders with the most attractive type and quantity of chemical, 3.5 µl of butyric acid, simultaneously with virgin female web to record any consistent preference between the two stimuli.
Male redback spiders showed positive taxis to sexual stimuli, although many did not approach the stimulus directly within 30 minutes. Males were attracted to butyric acid to a similar extent as they were to virgin female web. Males were not consistently attracted to isovaleric acid. Types of behaviour displayed that indicated sexual attraction were walking, climbing and hanging. Crouching and running were not associated with courtship and might be anti-predator behaviours. Grooming was also not associated with courtship and is thought to be a maintenance behaviour. The age of the male spider affected the proportion of time spent walking, grooming and climbing, while the drying time of the chemicals affected the proportion of time spent crouching and walking.
As redback spiders did not show any preference for butyric acid over virgin female web, a pheromone trap would not be feasible using this chemical. However, further research into the production of a pheromone trap should use 3.5 µl of butyric acid as a starting point, as this was found to be the most attractive chemical in this study. A high level of inter-individual variation in behavioural responses was found among redback spider males. This may be due to a high level of behavioural plasticity within the species, which would explain their success in invading novel and urban habitats. As a result, it is recommended that further research investigates behavioural plasticity and behavioural syndromes in redback spiders, and that these studies use larger sample sizes than those used here to compensate for inter-individual variation in behaviour.