Bryan Jensen, UW-Madison Dept. of Entomology and Integrated Pest Management Program
Now is that time of year when scouting adult corn rootworm beetles can pay dividends for next year. Beetle populations during the egg laying period (now and through early Sept.) will help predict if rootworm protection is needed next year and which practice will provide economic control and provide information that is useful to delay resistance to the Bt hybrids.
Beetle populations have been historically low the last 3 years according to statewide surveys conducted by DATCP staff. If populations remain the same, decrease or increase can only be determined by field scouting. I am a fan of the whole plant count method not because of reliability but because I get information the first time I am in the field. This method is simple but does take time and corn is not easy to walk in this time of year. However, the information gained will be extremely helpful. More detailed information can be found in the reference card Field Scouting for Corn Rootworms or if you prefer the YouTube version How to scout for Corn Rootworm Beetles
Briefly, count the number of beetles on 50 plants/field/sampling date. Keep samples distributed within a field. I would suggest counting beetles on 5 plants in each of 10 areas of a field. When approaching a plant, cover the silks with one hand before counting beetles on the rest of the plant. You will find them on the tassel, upper and lower leaves as well as in leaf axils. Remove your hand to count beetles on and in the silk. Don’t count beetles on adjacent plants because they are easily disturbed and will drop off or fly away.
If the field average is greater than 0.75 beetles/plant you will need root protection if you plant corn in the same field next year. One more count in 7-10 days will give you additional comfort in knowing if populations are increasing which help choose an effective, yet economical, management practice. If you stay below 0.75 beetles/plant for a minimum of two weekly counts during egg laying you will not need root protection next year.
An alternative method is to use the Pherocon AM unbaited yellow sticky traps. For every 10-50 acres, 6 traps should be deployed in two transects (12 total traps) across the field at least 330 feet apart. Each trap within a transect should be 165 feet apart. These traps are a visual attractant. I would suggest stripping leaves on adjacent plant to make sure the traps are visible. Check traps on a weekly schedule and count both northern and western adults. If an average of 2 or more beetles are caught/trap you should consider using a rootworm management practice on next year’s corn.
One final thought on rootworms. There have been several reports of lodged corn after strong storms moved through the state earlier this summer. Don’t assume lodged corn is the result of rootworm feeding. The window may be closing on assessing root damage. Late July is better only because corn can regenerate roots and mask earlier damage but it is still worth your time to check.