Cutworms In Corn

Over the past ten years or so, we have seen a growing percentage of farmers applying pre-plant or pre-emergence insecticides to corn for soil cutworm control. The trend may be accelerating this year due to: 1) Higher crop prices, 2) Lower insecticide costs provided by the increase in the availability of effective generics, and 3) A "worry hangover" from the army cutworm problems seen earlier this year in alfalfa and wheat. The combination of these factors has led to an increase in these "insurance" type treatments.

There are several problems with the applications of pre-plant or pre-emergence insecticides for cutworm control. The primary reason is there is no reliable way to determine if there is a need for the pre treatments. Over the years, the vast majority of corn fields have not needed treatment for cutworms. This then means that the vast majority of the expense for the treatments, regardless of how cheap they are, is still wasted. Unlike seed attacking insect problems, such as wireworms, where there is no good rescue treatment, cutworms can be detected and treated to prevent damage if necessary. The cutworm complex in corn does not include the army cutworm, so despite the large amount of activity in alfalfa and wheat this spring, it has no direct correlation to what may happen in corn. The only possible connection to potential corn cutworm problems and army cutworms is the indication that the cutworm species that overwinter as larvae had good survival conditions.

Also, there has been aggressive selling of materials with the promise of very long residual activity. Under ideal conditions, some materials may give 14, 21 or even more days of residual activity. However, in Nebraska with varying temperatures and soil conditions, a 7 to 10 day residual is more realistic. Bottom line, even if you have treated for cutworms earlier, you still need to be scouting to make sure the black cutworms haven't come in after the earlier treatments have worn off. Even if light traps detect large flights of black cutworms, there is no way to know if they will choose your field to infest, or bypass it for "greener pastures".

However, if present, cutworms can cause serious damage to corn in the first couple of weeks after emergence, so it is important to scout early - emerging fields for damage. Some damage to field corn from cutworms occurs every year in Nebraska. Several different species of cutworms can attack corn. The severity and the area affected will vary greatly, and is dependent on species involved, previous crop history, and weather conditions.

Cutworms that attack corn can be divided into two general categories based on seasonal life cycles. Black cutworms do not overwinter in Nebraska. Dingy, claybacked, darksided, sandhills, pale western, and other species overwinter as partially grown larvae in the soil.

Since black cutworms do not overwinter in Nebraska, they are dependent on spring weather conditions, primarily prevailing southerly winds, to bring them into our state. Nebraska is on the western edge of the black cutworm's area of influence, and they are rarely found west of the 100th meridian. Because of their cutting habits and the possibility that large numbers can be transported to Nebraska if favorable weather conditions occur, they have the most potential to cause a widespread problem. Light traps and pheromone traps have been used to monitor the flight patterns and populations of black cutworms. Remember, the presence of moths in a trap only indicates potential problems and is no guarantee that extensive damage will occur. Trap counts are more useful in alerting growers and consultants as to when to begin scouting efforts.

The weather of the last few weeks has been dominated by cool fronts moving in from the north and northwest. This is NOT favorable for black cutworm establishment. The weather this week may be more favorable for black cutworms. From here on out, corn planted into heavy residue, or fields with winter annuals, or fields planted into spring killed alfalfa (that may attract egg laying moths) have a slightly higher potential for cutworms. When weeds are destroyed mechanically or by herbicides, they will feed on the newly emerging corn. Because most damage from cutworms occurs roughly in the first 7-10 days after emergence, growers should be on the lookout.

Cutworms that overwinter as larvae generally prefer to lay eggs in the fall in green vegetation such as small grain stubble, legumes, rye, and pasture. The eggs hatch and the larvae feed on the vegetation present before overwintering. In the spring, after the previous crop is removed and the corn emerges, the cutworms will transfer their feeding activity to the corn. Recent experience has been that corn planted into alfalfa that has been killed in the spring has a greater potential for cutworm problems.

Tillage has minimal effect on cutworm populations. If fields are tilled before black cutworm migration, it may limit egg laying in those fields. Cutworms already in the field may suffer some mortality by mechanical action, but there is no guarantee that tillage by itself will eliminate cutworm problems. Many cutworm problems have occurred in fields that have been tilled.

It is extremely rare to experience cutworm problems in continuous corn. Corn stubble is not a preferred egg-laying site. Potential problems in continuous corn may be the result of a previous year's late season flush of weeds, an interseeding of a fall cover crop such as rye, or a flush of winter annuals this spring, which possibly attract egg-laying moths.

Scouting Cutworms in Corn - Remember that early detection of a problem is essential because most of the cutting occurs within 7 days of plant emergence. Generally, a rescue treatment should be considered if 5% or more (an average of 1 plant or more in a set of 20) cutting is observed and the worms are one inch or less in length.

Rescue treatments are effective in controlling soil cutworms. Ambush 2E, Asana XL, Baythroid, Lorsban 4E, Mustang Max, Warrior and Pounce 3.2EC (or generics) will all give satisfactory control as post-emergence sprays. If the soil is dry or crusted, rotary hoeing immediately before or after Lorsban application may enhance control. The other insecticides are pyrethroids and should not be incorporated. See a full list of products and rates at our website http://entomology.unl.edu/instabls/cutworms.htm. For more information on cutworm biology and control see NebGuide G93-1153-A, Corn Cutworms. (KJ)

Soybean Row Spacing Affects Velvetleaf Competitiveness

Velvetleaf, also know as a button weed, is an important annual weed in row crops throughout the midwest. Its fast growth rate allows it to compete with crops for growth limiting resources such as light, nutrients and water. Velvetleaf competitiveness, however, can be reduced by modifying cultural practices, including planting crop in narrow rows. Narrowing row spacing allows the canopy to close earlier than in wide rows, resulting in higher shading effects on the weeds. Other factors, such as the time of velvetleaf emergence relative to the crop emergence, can also influence the outcome of crop and weed competition. It is logical to expect that the later emerging weeds will be less competitive than weeds emerging with the crop. This knowledge of relative emergence can be used when making management decisions.

We conducted field studies in eastern Nebraska at two locations in each of the 2002 and 2003 years to determine the influence of soybean row spacing and relative time of weed emergence on velvetleaf and soybean growth and competition. This study is an MS project for Shawn Hock. Soybean was planted in 7.5 and 30-inch rows. Velvetleaf was hand planted at soybean planting (VP), emergence (VE), and 1st trifoliate (V1) stage. Observations were made throughout the growing season to determine the effects of row spacing and relative emergence on growth and development of both the crop and the weed.

The results from this study showed that velvetleaf produced much less dry matter and less leaf area in narrow rows than in wide rows. By producing less dry matter and leaf area the weed takes less resources from the crop. Velvetleaf emerging at the cotyledon stage (VC) of soybean produced 60% more dry matter than the velvetleaf that emerged at the V1 (1st trifoliate) stage of crop.

Soybean yields were higher in the narrow rows, and when velvetleaf emerged later in the season. Soybean grown in 7.5-inch rows yielded 4 bushels more than soybeans in 30-inch rows. Soybean yields were also reduced by 47%, 38%, and 15% when grown with velvetleaf emerging at the VC, V1, and V3 soybean stages, respectively.

Practical implication of this study is that planting narrow row soybeans will increase the crop competitiveness against weeds, including velvetleaf. Also, an early season field scouting of weed emergence patterns relative to the crop growth stage can help determine management strategies. For example, earlier emerging weeds will require earlier weed removal. This study was partially funded by the North Central Regional Weed Science grant. (SK)