Aphids are a familiar sight in the garden, sucking the juices out of your rose bushes. Luckily, so are ladybugs, which prey on aphids and keep them in check.
But the relationship between predator and prey is more complex than you might think. Aphids may be important to the survival of some ladybug species we have come to know and love by warding off another predator that has been moving in and feasting on them.
The arrival about 30 years ago in the United States of the multicolored Asian lady beetle, or Harmonia axyridis, which gleefully devours other ladybugs’ larvae, led to a drop in numbers of the seven-spotted lady beetle. Although the seven-spotted beetle is also an invasive species, it has been around longer than the Asian lady beetle, and exists alongside native ladybug species, which also take a beating.
However, some aphids contain a substance that’s much more toxic to this aggressive invader than to the other ladybugs. As a result, researchers show in a paper in the Proceedings of the Royal Society B, these aphids may provide refuge to the other ladybug species by killing off their common enemy.
The aphids in the study, which are themselves invasive, are all from the same species and all look exactly the same. But some are toxic to the Asian lady beetle and others are not. Although researchers still don’t know what the toxic substance is, whether aphids have it or not seems to depend on what plants they’ve been eating, as well as their parentage, says Paul Lenhart, an entomology researcher at the University of Kentucky.
Dr. Lenhart’s team studied the aphids and ladybugs in the laboratory as well as in a more realistic setting. In a field they put both toxic and nontoxic aphids on plants, and also added Asian lady beetle larvae to some to see what would occur over the course of ten days. In the final reckoning, nontoxic aphids fared very poorly when the Asian lady beetles were around, as did other ladybugs. But on plants with toxic aphids, there were not only more aphids, but other ladybug species were present in greater numbers.
“It’s kind of a refuge where there’s no Harmonia that’s going to eat them,” Dr. Lenhart said.
This suggests that there may be ways for prey to protect predators that are less damaging than the alternative. That may help explain why some native ladybug species have held on despite having many invasive species to compete with.
“They’ve declined, but they’re still around,” said Dr. Lenhart. “So there are probably very subtle ways that predators divide up food resources.”
Previous reproting on ladybugs and other beetles
Lest you assume that this is a cooperative relationship built over eons of evolution, remember that many of these species are from different parts of the world, and only began to co-habitate recently. While native ladybugs and other insects made appearances in the field experiment, many of the interactions here are relatively new. That makes it all the more fascinating to see what kind of détente has developed among these species.
“This is a hodgepodge of native and invasive species, which is becoming more and more common,” said Dr. Lenhart. “These aren’t natural systems, but these are the systems we have to deal with now.”