Rootworm photos

Editor’s note: The following was written by Nick Seiter, University of Illinois field crop entomologist, and Kelly Estes and Joe Spencer with the university’s Illinois Natural History Survey for the farmdoc daily website.

Corn rootworm remains the greatest insect pest threat to corn in Illinois, despite low populations in recent years compared with historical averages.

Resistance to Bt traits and crop rotation throughout much of Illinois complicates the management decisions faced by corn growers.

Corn rootworms overwinter as eggs in the soil that generally hatch in late May to early June, well after corn has been planted in most years (2019 notwithstanding). Upon hatching, the larvae begin feeding on and ultimately pruning corn roots, causing reduced uptake of water and nutrients and increasing the potential for plant lodging.

Due to the timing and nature of rootworm damage, any controls (whether a Bt corn hybrid, soil insecticide or crop rotation) must be chosen prior to planting. Adult population monitoring from the previous season and field history are the primary sources of information available to estimate corn rootworm damage potential and determine whether a control is justified.

Corn root worm table

Populations of western and northern corn rootworm adults were low across most of Illinois in 2019. This continues a recent trend of relatively low population densities throughout Illinois and much of the Corn Belt over the past several years.

Annual statewide field surveys were conducted for western corn rootworms in Illinois back to 2011. The mean the number of corn rootworm beetles per plant ranged from a low of 0.01 in 2015, 2016 and 2019 to a high of 0.51 in 2017.

A mild winter followed by favorable conditions at egg hatch and adult emergence helped the western corn rootworm population to gain some traction from 2016 to 2017; however, compared to historical averages even these recent “peaks” were low.

Despite low averages, the variability observed between fields within counties and even counties within crop reporting districts indicates there are locations with higher populations of western corn rootworm.

While western corn rootworm is the most important species throughout most of Illinois, northern corn rootworm has gained some attention in northern portions of the state in recent years. Our colleagues recently confirmed the development of resistance in the northern corn rootworm to Cry3Bb1 and Cry34/35Ab1 in some localized areas of North Dakota — the first instance of field-evolved resistance to Bt traits in this species.

Northern corn rootworm numbers jumped dramatically in Illinois in 2018 based on the statewide survey for that insect, though they dropped considerably in 2019.

Despite ongoing resistance development to individual Bt proteins, pyramided trait packages have generally performed well in Illinois over the last several years. Where unexpected damage has occurred in these pyramided hybrids, it has primarily been in northern Illinois and associated with continuous corn. In east-central Illinois, these pyramided trait packages and a variety of soil insecticides continue to provide effective control, especially under the low to moderate pressure situations that have been typical in recent years.

However, the use of rootworm control in Illinois generally exceeds what is needed based on rootworm pressure due to difficulties in monitoring this insect and the potential for yield losses if a needed control is not applied.

Whatever control you implement, you should monitor its effectiveness by examining some roots for pruning sometime after pollination but before black layer.

Though crop rotation in Illinois does not necessarily protect first-year corn as it once did, planting soybean after corn does reduce the local population — any eggs that hatch in a soybean field will die.