Integrated Crop Management News

Corn rootworm egg hatch in Iowa occurs from late May to the middle of June, with an average peak hatching date of June 6 in central Iowa. Even with recent warm temperatures, hatching is a bit delayed this year due to cool spring temperatures. Development is driven by soil temperature and measured by growing degree days (GDDs). Research suggests about 50% of egg hatch occurs between 684-767 accumulated GDDs (since January 1; base 52°F, soil).

The soybean cyst nematode (SCN) is the most damaging pathogen of soybeans in the United States. Farmers in Iowa successfully managed SCN for many years by growing SCN-resistant soybeans in rotation with corn. However, almost all resistant varieties contain SCN resistance genes from the same breeding line, named PI 88788. Not surprisingly, SCN populations in fields throughout the state are overcoming the PI 88788 resistance because of use of those resistance genes for more than 30 years.

Iowa State University (ISU) Extension and Outreach field agronomists have been receiving phone calls and texts about striped corn leaves this past week. There are many reasons for striped corn, and the cause of light green, whitish, or yellowish stripes sometimes is not obvious. Here is a review of some of the reasons for striped corn and also methods to distinguish what might be causing the stripes. 

Genetic Striping

This spring, we have had several reports of fields with high numbers of grubs from field agronomists in central Iowa. There are multiple species of white grubs in Iowa, including the Japanese beetle. Recent warm temperatures are accelerating insect development, and with forecasted temperatures, Japanese beetle emergence could begin in southern Iowa counties this week (Figure 1).

Stalk borer is an occasional pest of corn, but it can be persistent in some fields, especially those fields near fence rows, terraces, and waterways that serve as overwintering sites. Tracking degree days is a useful tool to estimate when common stalk borer larvae begin moving into cornfields from their overwintering hosts. Foliar insecticide applications are only effective when larvae are migrating and exposed to the insecticide.

ISU Extension and Outreach Field Agronomists recently highlighted a significant number of phone calls related to fomesafen carryover injury to corn rotated from 2020 soybeans.

“How much nitrogen (N) has been lost?” is a question we get this time of year, particularly in areas that have received more rainfall. Although most of the state is dry, the southeast part of the state has received above normal rainfall, and there are valid concerns that some N has been lost and additional N could be needed.

Canada thistle has been a consistent threat to crop production and perennial habitats since its introduction to the state in the late 1800s. Canada thistle is commonly found in crop fields, pastures, hayfields, CRP, and other full sun habitats. While fall is typically the best time to manage this weed species, late spring, when Canada thistle plants are in the bud or early bloom stage, is a close second for providing consistent herbicidal control.

Multiflora rose (MFR) is a common weed in pastures, CRP, timber and other areas that are not annually disturbed. Now is an appropriate time for treatment of these weeds in pastures and other areas they invade.

Iowa’s most significant soybean insect pest, soybean aphid, has host-alternating biology. Its primary host is buckthorn, an invasive shrub often found in hedgerows and roadside ditches, and its secondary host is soybean. For the majority of the year, soybean aphids exist as cold-hardy eggs on buckthorn branches near leaf buds. As spring temperatures increase, the eggs hatch and a few generations are produced on buckthorn before moving to soybean.

Black cutworm (BCW) is a migratory pest that arrives in Iowa with spring storms each year. It is sporadic and unpredictable, making it essential to scout to determine whether BCW larvae are present in a field and management is required. Since much of the corn planting around the state is complete and many fields have emerged, now is a good time to begin scouting for larvae.

Performance of preemergence herbicides in many areas of the state may be less effective than normal due to limited rain following application. Although weed emergence following planting was reduced by dry soils and cool temperatures, it is likely that most fields will have some weeds that emerged during this period. Systematic scouting of fields, beginning shortly after crop emergence, will be essential to determine how best to manage weeds throughout the remainder of the season.

Pesticide applicators and handlers need to wear, at a minimum, the Personal Protective Equipment (PPE) specified on pesticide product labels. Most pesticide labels require a long-sleeved shirt and long pants. Proper laundering of work clothes that may be contaminated with pesticide residues is essential to reduce pesticide handlers’ short- and long-term exposure to pesticides and prevent the potential of residue cross-contamination onto other clothing. 

The growth and development of alfalfa is affected by many factors, including temperature, soil moisture, stand age and even cultivar. Alfalfa growth has been slower than normal due to cool conditions in April. This is a good reminder that using a calendar date to determine when to harvest the first crop of alfalfa may not the best method.

Bean leaf beetle adults (Photo 1) are susceptible to cold weather and most will die when air temperatures fall below 14°F (-10°C). However, they have adapted to winter by protecting themselves from harsh temperatures under plant debris and loose soil. Each spring, adult beetles emerge from their overwintering habitat and migrate to available hosts, such as alfalfa, tick trefoil, and various clovers. As the season progresses, bean leaf beetles move to preferred hosts, like soybean.

Current cooler weather conditions are posing challenges for growers with a cereal rye cover crop to get it terminated. However, for growers planning to plant soybeans into the rye cover crop, there may be a perk to letting the rye cover crop accumulate more biomass. That perk being the cover crop biomass can help with weed suppression. The ability of cereal rye to suppress weeds is directly related to the biomass accumulation at the time of termination.

Pasture and forages stands have really started to green-up. Here are some fertilizer and weed management considerations and recommendations to help make the most out of pasture and forage stands this year.  

Fertilization is just as important for forages as it is for row crops to maximize productivity. However, current fertilizer prices may have you seeing a lot of dollar signs.

Current cool temperatures increase the risk of failures in terminating cover crops. While cereal rye continues to grow during these conditions, activity of herbicides is reduced. Translocation of glyphosate to growing points is reduced under cool temperatures, slowing activity and increasing the potential for control failures.

Adult alfalfa weevils become active and start laying eggs as soon as temperatures exceed 48°F. Like other insects, the development of alfalfa weevil depends on temperature, and we can use accumulation of growing degree days (GDD) to predict activity. Alfalfa weevil egg hatching begins when 200-300 GDD (base 48°F) have accumulated since January 1.

Seedcorn maggot is a seed and seedling pest of corn and soybean. The larvae, or maggots, feed on germinating corn and soybean seeds or seedlings (Photo 1). Feeding can delay development or kill the plant. Plant injury is especially prevalent during cool and wet springs when plants are growing slowly. Infestations tend to be field-wide instead of in patches like for many other pests.