Earlier this month, I wrote an ICM News article showing peak corn rootworm egg hatch in Iowa. That means at least 50% of viable eggs have likely hatched and larvae will start feeding on corn roots. To assess larval activity, some try using the float test (that’s just what I call it). Tom Hillyer in southeastern Iowa found rootworm larvae using the float test last week.
Sample at least ten locations per field and collect one corn plant at each location. Use a flat shovel to cut a large square around the corn plant to preserve the roots. Soak the plant in a bucket of salty water, and watch for corn rootworm larvae to rise to the surface and float. Or you could simply dig up plants, break apart the soil around the root system, and look for larvae.
It is difficult to distinguish northern and western corn rootworms by looking at larvae. Both species have larvae that are white, nearly translucent, and slender. Corn rootworm larvae have dark heads and the end of the abdomen is dark. Fully-developed larvae (third instars) are about ½ inches long.
What can be learned from doing the float test? There are no effective rescue treatments for corn rootworm larvae, so nothing can be done to protect roots this growing season. But it does satisfy a curiosity about rootworm activity, especially if soybean was planted in that field last year. It could help answer questions like:
- Have the eggs hatched yet?
- Did the larvae survive saturated soils?
- How big are the larvae and how much more feeding will they do?
- Is the Bt trait package working?
While digging up plants, it is also a good time to start assess for root injury. First instars feed on root hairs and on the outside of the roots. Root tips can become discolored after feeding. Older larvae can burrow into the roots and consume them entirely. Assess rootworm feeding and adjust management strategies if the average root injury is about 0.5 on a 0-3 rating scale. Ultimately, larval injury can help make management decisions for the following growing season. The float test is a complementary tool that can provide a bit more information but management decisions should not be made from this kind of sample alone.