Now that the first cutting of alfalfa is being taken in southern Iowa, it is time to think about insects that may prevent the stubble from regrowing quickly. Alfalfa weevils and clover leaf weevils are always potential problems in preventing regrowth, but this year variegated cutworms also may also pose a significant threat (see photo). Variegated cutworms have been found in corn in Marion County so they also may be abundant in alfalfa.
Variegated cutworms feed on new leaves of stubble and can substantially delay regrowth of new stems. This feeding reduces dry matter yields and forage quality in the second and possibly third cuttings. An economic threshold calculation was developed by Larry Pedigo at Iowa State University and is shown in the table below. Depending on the value of the crop and the cost of treatment, most fields could only tolerate a few days of defoliation before economic yield losses would occur.
If variegated cutworms are found defoliating alfalfa stubble and preventing regrowth, and most of the larvae are less than an inch in length, consider spraying the field if the economic threshold is exceeded. If most of the variegated cutworms are 1.5 inches in length, they have done most of their damage and should be finishing feeding; no control would be necessary in this situation.
Alfalfa stubble: economic threshold calculation (days of defoliation allowable).
||A. Insecticide plus application cost (dollars per acre)
|B. Value of hay (dollars per acre)
|C. Loss factor: (cutting at 1st bloom = 0.02); (cutting on 28-day interval = 0.035)
|D. Days of complete defoliation that can be tolerated
To estimate D, multiply B by C and divide into A. The example is calculated as follows: $7.00/($80.00 x 0.02) = 7/1.6 = 4.4 days.
This article originally appeared on pages 92-93 of the IC-486(11) -- May 28, 2001 issue.