Cool Spring Slows Development of Alfalfa Weevil, Bean Leaf Beetle

Cool Spring Slows Development of Alfalfa Weevil, Bean Leaf Beetle

Iowa's cold, wet spring has slowed development of crop pests such as alfalfa weevil and bean leaf beetle.

Iowa's unusually cool spring, capped by last week's cold and snow on May 2 in much of Iowa, has farmers wondering what these "March in May" weather events will mean for crop pests this spring -- such as bean leaf beetle and alfalfa weevil. Here's what you need to look for as temperatures warm.

BEAN LEAF BEETLE: Although overwintering beetles rarely cause economic damage to soybeans, their presence may be an indicator of a building up of the first and second generations of this pest later in the season. The cool spring has delayed all insect development, including bean leaf beetle and alfalfa weevil. But you still need to watch for them in fields as temperatures warm.

Bean Leaf Beetle: Survival over winter has been just above average. Although overwintering beetles rarely cause economic damage to soybean seedlings, the presence of this insect pest may be an indicator of first and second generations that will come along later in the growing season. That's the observation of Erin Hodgson, Iowa State University Extension entomologist, and Adam Sisson, an ISU integrated pest management specialist.

Bean leaf beetle adults are susceptible to cold weather and will die when the temperature falls below -10 degrees C (that's 14 degrees F). However, they have adapted to winter by protecting themselves in leaf litter. An overwintering survival model was developed by researchers at Iowa State University in 2000, and is helpful for predicting winter mortality based on accumulating subfreezing temperatures. Predicted mortality rates ranged from 40% to 90% for the 2012-2013 winter (See accompanying map, Figure 1). The northern third of Iowa did experience a colder winter, and more than 80% of the beetles were not predicted to survive the winter.

Figure 1. Predicted overwintering mortality of bean leaf beetle based on accumulated subfreezing temperatures during the winter (October 1, 2012 – April 15, 2013).

The average mortality rate over the last 24 years in central Iowa is 71%. The 2012-2013 winter had slightly better predicted survivorship than average (see graph, Figure 2). It is important to remember that the insulating snow cover can influence the survivorship of bean leaf beetle. The recent cold weather could also influence spring activity of this insect in alfalfa and later in soybeans.

Figure 2. Predicted bean leaf beetle mortality by year for central Iowa. The red line indicates the average mortality rate.~~~PAGE_BREAK_HERE~~~

Overwintering adults are strongly attracted to soybeans and will move into fields with newly emerging plants (Figure 3). First-emerging fields should be monitored this month, especially in southern Iowa. Other fields of concern include foodgrade soybean and seed fields where reductions in yield and seed quality can be significant. Bean leaf beetle is easily disturbed and will drop from plants and seek shelter in soil cracks or under debris.

Scouting and sampling this pest early in the season requires you to be sneaky to estimate actual densities that are present on the small soybean plants in the field. Although overwintering beetles rarely cause economic damage, their presence may be an indicator of building first and second generations later in the season.

To learn more about managing bean leaf beetle and bean pod mottle virus, visit this link.

Alfalfa weevil hatch has also been delayed in Iowa this spring

Alfalfa weevils develop based on temperature or accumulating degree days. This cool spring has delayed all insect development, including alfalfa weevil. Scouting in alfalfa should begin at approximately 200 degree days for areas south of Interstate 80, and 250 degree days north of Interstate 80. Based on accumulated temperatures since January, weevils are active in southern Iowa now (see map, figure 4).

Figure 4. Accumulated growing degree days (base 48°F) in Iowa from January 1 through April 30, 2013. Map is updated daily at this link. Map courtesy of Iowa Environmental Mesonet, ISU Department of Agronomy.

Alfalfa weevil is an important defoliating pest in alfalfa. Heavy infestations can reduce tonnage and forage quality. Adults can feed on plants, but the larvae typically cause the majority of damage. Newly hatched larvae can be found feeding on terminal leaves, leaving newly expanded leaves skeletonized. Gradually maturing larvae (Figure 5) move down the plant and begin feeding between leaf veins. Adults of alfalfa weevil (Figure 6) eat along the leaf margin, leaving irregular notches. A heavily infested field of alfalfa will look frosted or silver (Figure 7).

Figure 5. Alfalfa weevil larvae have a dark head and pale green body with a white stripe down the back. Fully grown larvae are about 5/16 inches long. Photo by Clemson Cooperative Extension Slide Series, available here.~~~PAGE_BREAK_HERE~~~

Figure 6. Alfalfa weevil adults have an elongated snout and elbowed antennae. Their wings and body are mottled or brown in color. Photo by Clemson University, available here.


Figure 7. Heavily defoliated alfalfa fields appear frosted from a distance. Photo by Whitney Cranshaw, Colorado State University, available here.

To initially detect alfalfa weevil larvae in the spring, use a sweep net to sample. After finding larvae, collect six alfalfa stems from five locations throughout the field. Take each stem and shake into a bucket to dislodge larvae from the plant. Average the number of larvae per stem and plant height to determine if a foliar insecticide is warranted (Table 1). Remember, cutting alfalfa is an effective management tool for alfalfa weevil larvae, and an insecticide application may be avoided if harvesting within a few days.

Table 1. Economic threshold of alfalfa weevil, based on the average number of larvae in a 30-stem sample.

TAGS: Extension
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