Predation risk in a migratory butterfly increases southward along a latitudinal gradient

Migratory insects provide a valuable ecosystem service by transporting
large amounts of organic matter across regions where they become
temporarily abundant prey. In species performing multigenerational
migration, such as the painted lady butterfly Vanessa cardui, successive
generations face a wide variety of predator communities and may be subject
to different predation risks. Here, we analyze the pattern of wing damage
of over 2,000 butterflies to investigate, for the first time, the risk of
predation of adult painted ladies across a latitudinal range of ca. 3,500
km extending from the northern Mediterranean through the Maghreb to
sub-Saharan West Africa. Large number of butterflies showed substantial
wing damage attributable to failed attacks, with birds, mantids and
lizards being the most likely predators. The risk of attack increased
towards the equator, even after controlling for wing wear. In addition,
there was a strong effect of butterfly size on predation risk, with larger
butterflies facing a higher risk compared to their smaller counterparts,
and clear evidence that females suffered more attacks than males. Although
size is a major factor, latitude was a stronger predictor of predation
risk across the migratory system, as evidenced by greater wing damage in
butterflies at lower latitudes, even though their size notably decreased.
These results raise an interesting evolutionary conflict, with a trade-off
between size and predation risk, as larger butterflies are likely to be
more fecund and efficient in migratory flight but, at the same time, more
vulnerable to predation.