Male spider mites rip off females’ skin to be first to mate with them

Male spider mites closely guard juvenile females and tear off their outer skin as soon as they approach maturity so they can be the first to mate with them.

Two-spotted spider mites (Tetranychus urticae) are a common agricultural pest, feeding on a wide range of crops including beans and tomatoes. The mites shed their outer skin when they transition from juvenile nymphs to adults – a process known as moulting.

Adult females can have multiple sexual partners, but only the sperm of their first partner fertilises their eggs. As a result, there is strong competition between males for females that have just reached maturity.

To try to secure this access, male mites often guard juvenile females until they become fertile adults. This is a risky strategy because it costs energy and prevents the males from foraging for food – and rival males may still steal the females at the last minute.

Peter Schausberger at the University of Vienna in Austria and his colleagues found that male guards try to minimise this risk by pulling off females’ outer skin just as they near maturity so they can inseminate them before other males swoop in.

The researchers filmed juvenile female spider mites that were individually reared in cages with or without the presence of a male.

The females that were housed without a male naturally shed their outer skin at a leisurely pace when they reached sexual maturity.

In contrast, females housed with a male had their outer skin forcibly removed. As a female neared maturity, the male began drumming on her skin to encourage it to break open. Then the male used his mouthparts to pull off the female’s skin from behind so he could expose her genitals and immediately inseminate her.

This meant that females with a male emerged from their outer skin 5 minutes earlier on average than those on their own, says Schausberger.

“Five minutes is not long in absolute time, but it is in relative time because these spider mites often live in high-density colonies where other males are close by,” he says. “Every second pays when it comes to being first at the emerging female.”

The spider mites are the first species in which this skin-stripping behaviour has been experimentally documented, says Schausberger.