If I asked you to picture a ladybug in your mind, you would undoubtedly imagine a round, red beetle with black polka dots on its back. This is the charismatic insect we remember from our childhoods, and the ladybug we probably encounter most often in our gardens. Perhaps you've been asked by a child (or wondered yourself) – why do ladybugs have spots?
A ladybug's spots are actually a warning to predators. This color combination - black and red or orange - is known as aposematic coloration. Ladybugs aren't the only insects that use aposematic coloration to discourage predators. Just about any black and red/orange insect you can find is signaling the same thing to predators: "Stay away! I taste terrible!" The monarch butterfly is probably the best-known example of an insect using aposematic coloration. The spots are just part of the ladybug's clever color scheme.
Ladybugs produce alkaloids, toxic chemicals that make them unpalatable to hungry spiders, ants, or other predators. When threatened, ladybugs exude small droplets of hemolymph from their leg joints, an unusual response known as "reflex bleeding." The alkaloids in the blood produce a foul odor, another warning to the predator.
Recent research shows that a ladybug's colors are an indication of how toxic it is. Brighter ladybugs have higher levels of toxins than paler beetles do. Ladybugs with richer colors were also found to have better quality diets early in their lives. This correlation suggests that when resources are plentiful, the well-nourished ladybug can invest more energy in producing toxic defense chemicals and warning pigmentation.
Although the spots themselves are just part of the "warning" color scheme, the number of spots on a ladybug does have significance. Some people think they're age spots, and that counting them will tell you an individual ladybug's age. That's a common misconception, and is not true. The spots and other markings do help you identify the species of ladybug. Some species have no spots at all. The record-holder for most spots is the 24-spot ladybug ( Subcoccinella 24-punctata), which has 24 spots, of course. Ladybugs aren't always red with black spots, either. The twice-stabbed ladybug ( Chilocorus stigma) is black with two red spots.
People have long been fascinated by ladybugs, and there are many folk beliefs about the ladybug's spots. Some say the number of spots on a ladybug tells you how many children you will have, while others believe they portend how much money you will receive. A folk legend among farmers says that a ladybug with 7 or more spots predicts a coming famine. A ladybug with fewer than 7 spots is a sign of a good harvest.
- Insects: Their Natural History and Diversity, by Stephen A.Marshall.
- All About Ladybugs, Lost Ladybug Project, Cornell University (PDF). Accessed December 27, 2012.
- Redder ladybirds more deadly, say scientists, ScienceDaily, February 6, 2012.
- Ants, Bees and Ladybugs - Old Legends Die Hard, by Donald R. Lewis Extension Entomologist, Iowa State University Extension, May 1999. Accessed December 27, 2012.
- The Alkaloids: Chemistry and Pharmacology, edited by Arnold Brossi, Academic Press, Nov 1, 1987.