Firefly, Corn Rootworm Populations Both Declining
Published: Friday, June 27, 2025
The following is from Christian Krupke of Purdue University University.
Most readers have probably noticed a sharp increase in firefly activity lately. Long-time readers of this newsletter will know that the emergence of adult fireflies in the genus Photinus are an indicator that rootworm eggs are hatching. It just so happens that the growing degree days (GDD) requirements for both rootworm egg hatch and adult firefly emergence coincide with one another.
Our most common species, and one of the largest, is Photinus pyralis. Commonly called the eastern firefly or the "big dipper" firefly (because of it's habit of flashing while flying in a J-pattern), this species is approaching peak emergence. A species of firefly, Say's Firefly, Pyractomena angulata, became Indiana's state insect in 2018.
Rootworms and fireflies have more in common than just their emergence dates. Despite having "worms" and "flies" in the common names, both are beetles, and both spend their larval life stage in the soil. Rootworm larvae are herbivores and feed almost exclusively on corn. They complete development from egg-adult in a few weeks during the summer (late May through mid July).
In contrast, fireflies are carnivorous as larvae and the lifecycle may take two years to complete. They feed primarily on soft-bodied invertebrates including slugs, snails and earthworms.
Rootworms and fireflies are also both declining in abundance in Indiana. The decline of rootworm abundance over that last 20 years is happy news to most readers, and a product of both long-term, widespread use of Bt corn hybrids and heavy late spring precipitation events. This also means that Bt corn for rootworm control is no longer a yearly necessity for most Indiana farmers.
Firefly numbers are also in decline. The reasons are not as clear as they are for the well-studied rootworm pest, but likely a combination of overlapping factors (including light pollution), which is usually the case in biology.
All in all, a reminder that while some things in biology (like the relationships between temperature and insect development) don't really change, some other aspects of biology that we take for granted—like the constant nature of rootworm infestations and the presence of fireflies during late spring and summer—can change quite rapidly!
Adult of Photinus pyralis, one of the most common Indiana fireflies, on a corn leaf. Adults flash while flying in an ascending J-pattern, giving it the common name of the "big dipper" firefly.
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