Parasitic robber fly (Mallophora ruficauda) larvae falling from a piece of grass before burrowing in the ground in search of grubs to infest. Photo credit: José Crespo.
Parasitic robber fly (Mallophora ruficauda) larvae falling from a piece of grass before burrowing in the ground in search of grubs to infest. Photo credit: José Crespo.
There are plenty of myths about animals foretelling the weather. Do moles really dig deeper to evade harsh winters? It's hard to tell. But one thing that definitely heralds a storm is low pressure. As bad weather sweeps in, the pressure drops below 1010 hPa. And a sudden drop in pressure can influence how insects behave. ‘Ants leave the nest more and bring in more leaves when the barometric pressure decreases’, says José Emilio Crespo, from Universidad de Buenos Aires, Argentina. But how might a drop in atmospheric pressure affect the choices of some parasitic insect larvae? Crespo explains that back in 2012, research suggested that the desire of parasitic robber fly (Mallophora ruficauda) larvae to sniff out their favourite hosts, Argentinian scarab (Cyclocephala signaticollis) grubs, declines when the atmospheric pressure plummets. But it wasn't clear how the speed at which the pressure falls might impact the larvae's choices, and how a fall in pressure might affect the choosiness of larvae to hunt out grubs for infestation. Curious to find out, Crespo built an Arduino-controlled pressure chamber to investigate the larvae's decisions as the pressure fell.
First, Jean-Noël Houchat, Marcela Castelo and Crespo checked out how choosy larvae (which are only prepared to infest pristine unparasitised grubs at a specific stage of life, to give themselves the best start) and easy-going larvae (which are happy to share their grub with other parasites and don't care about the grub's life stage, just to give themselves a chance of survival) responded to sudden, more gradual and longer pressure declines. Placing an individual larva in the middle of an atmospherically controlled chamber, Houchat offered it a choice of juicy grub on one side, or nothing on the other. Then he reduced the air pressure, either fast or more slowly, and gave the larva an hour to make up its mind: either stay put in the middle, wriggle over to the grub or wriggle in the opposite direction. Repeating this process many times, the team realized that the larvae were all motivated to go in search of a grub to parasitise as the pressure dropped. However, the choosier larvae gave up searching for a healthy grub faster than the easy-going larvae when the pressure fell. The atmospheric pressure did affect the parasitic larvae's motivation to find a host.
But how would the choosy larvae respond to changes in atmospheric pressure when they only had an already parasitised grub in which to set up home? Would they become less fussy? It turns out that the choosy larvae weren't prepared to go in search of substandard grubs when the pressure fell; they were still as particular as they had been when the pressure was normal. A drop in pressure didn't lead the picky larvae to drop their standards.
But why is it an advantage for these parasitic larvae, burrowing beneath ground in search of a grub, to sense and respond to changes in atmospheric pressure? It seems that sensing an incoming storm does motivate less picky larvae to keep searching for a grub to infest, giving them a better chance of survival. But more pernickety larvae are more likely to cut their losses as weather closes in; it's harder for them to find a pristine grub, so they might be more prepared to wait out a storm in the hope of finding a better grub later. So, changes in atmospheric pressure allow parasitic robber fly larvae to make decisions that could help them to weather incoming storms.
