Displaying the cost of sex for a male locust, a digger wasp (Sphex cognatus) captures the mating pair of locusts (Chortoicetes terminifera) and drags them into its burrow.
Credit: D.Kemp
The digger wasp (Sphex cognatus) paralyses the female locust (Chortoicetes terminifera) for harvest of its larvae.
Credit: D.Kemp
SYDNEY: The cost of sex has been revealed through new research into how often locusts are attacked by predators while 'getting busy'.
Australian plague locusts (Chortoicetes terminifera) are a serious pest in Australia. They are preyed upon by species such as parasitoid digger wasps (Sphex cognatus), but in the summer can gather in dense populations, such as the swarms that plagued southeast Australia last December.
These swarms provided evolutionary biologists with a unique chance to look at the often deadly cost of sex.
"The wasps were capturing mating pairs during a time of maximum reproductive potential for the locusts, providing evidence of the cost of reproduction," said Darrell Kemp, a behavioural ecologist at Macquarie University in Sydney and author of the study in the current issue of the journal Behavioral Ecology.
Zombie parasites
The digger wasp paralyses its prey before burying and eating it alive. Usually the wasps target female locusts - seeking their larger form and abdominal space - in order to provide a living a food supply for its larvae.
"Male locusts are usually left alone," said Kemp, who saw the wasps pounce on male locusts and then disregard them. "When the wasp captured the locust mating pair, it would paralyse the female but was forced to bury the stunned male locusts [that were] unable to detach from the female during capture."
Biologists have speculated that there must be an increase in predation cost during sex, but the act of both reproduction and predation has been difficult to observe simultaneously in the wild.
The locust swarm event provided an opportunistic scientist with a view of mating behaviour at a time when population density was at its peak, thus providing many more pairings to observe. "Under normal conditions with decreased locust density, the number of copulating locusts would be at a low, and the behaviours too fleeting to document," said Kemp.
The cost of sex
In a grassland habitat near Horsham, Victoria, Kemp observed digger wasps dragging mating locust pairs into burrows. As the females locusts are usually under threat from predator attack, the males were stung by the cost of sex.
The females' risk of being attacked by a wasp when alone was found by Kemp to be one in 200, while solitary males were usually left alone. When mating, both the female and males' chances of being captured by the wasps increased to 10%.
Kemp observed that the mating pairs only accounted for 3% of the total observed locust population but surprisingly made up 30% of the wasps' total captures. "The question is if the wasps had evolved to strategically target mating pairs or whether this was just a chance event based on the dense population and increase in mating pairs," said Kemp. "This study cements the theoretical concept that reproduction is costly and increases vulnerability to attack."
The value in data
Evolutionary biologist Bob Newby from Central Queensland University in Rockhampton, who was not involved in the study, said it was an opportunity to gain valuable data but that the wasp predation isn't enough to put off mating locust pairs. "From an evolutionary point of view, the benefits of sexual reproduction must outweigh the costs associated with it."
He suggested that the copulating females represent "a higher energy source and are specifically targeted" or that they are "simply more exposed and vulnerable".
Evolutionary biologist Kate Umbers from the Australian National University in Canberra agreed, commenting, "These data are a compelling contribution to a classic field in evolutionary biology: the costs and benefits of mating."
Kemp hopes to be able to observe the same behaviour in a less dense population of wild locusts to determine if the wasps are purposely attacking mating pairs.
