Ladybird washups: what causes them?

If you’ve come across a ladybug washup before, it’s likely made you worry, but this bizarre event can be explained.
By Australian Geographic December 9, 2019 Reading Time: 2 Minutes

Ladybird washups can be difficult to stomach. We’re accustomed to seeing a single ladybird on a leaf or crawling up our beach towels – not ladybirds en masse, collectively washed up on a seashore. 

The largest-known ladybird washup occurred in the Libyan Desert coast of Egypt in April 1939, with an estimated 4.5 billion ladybirds spread across 20km of shoreline.

Ladybirds have been known to congregate in large numbers, although it isn’t overly common. 

Take for instance when millions of ladybirds swarmed a South Australian radio tower. At the time, ladybird expert Adam Slipinski told Australian Geographic that it’s common for ladybirds to migrate towards prominent landscape features and eventually disperse.

The phenomenon of ladybird washups has been researched internationally for some time. Mainly, scientists are trying to understand how and why the ladybirds congregate in such large numbers, and how they end up in the water, to eventually drift to shore. 

According to the Lost Ladybug Project, the ladybirds may come together while gathering at or dispersing from hibernation locations. It may also be the result of rapid growth in ladybird populations and a subsequent depletion of resources.

For a long time, many researchers in the United States of America believed that the ladybirds were blown or knocked into the water to eventually drift ashore. They believed that such events correlated with what they described as “warm afternoons”. 

Image credit: Bonita Holmes Nuu

The most recent paper on the topic of ladybird washups by Eric Denemark of Cornell University, USA, lent some weight to this theory. Eric observed ladybird washups in the Finger Lakes Region of New York between May and August 2008.

“The frequency of washups reported in this and other studies and the observation that washups are localised and occasionally completely absent from a region can be explained by the dynamics of lake and sea breezes. 

“This explanation also accounts for the difference in size of washups compared to the size of the body of water, as larger bodies of water enhance coastal breezes. 

“…It seems likely that ladybugs are flying at times or altitudes that make them particularly vulnerable to being relocated by lake or sea breezes. Unseasonably warm temperatures are known to be a favourable condition for the production of lake or sea breezes.”

Ladybird washups tend to raise conservation concerns, but according to Eric’s study, “while a washup may cause massive mortality, it generally occurs to only one species. 

“Washups that contain the greatest diversity of species tend to be smaller. This phenomenon is therefore not likely to affect the long-term balance of species in a region.”