This spring we have had some of the best weather in recent memory. In addition, we are pleased to report that the first major mosquito hatch is yet to occur! With this first issue for 2003, we provide a quick update of some key insect pests that are beginning to show activity.

Seedcorn maggot on snap bean
(Bob Koch and Eric Burkness, U of MN)

Snap Beans & Sweet Corn: In snap beans and sweet corn we have found reasonably high infestations of seedcorn maggot (SCM) in plots planted April 30th, Rosemount Agricultural Experiment Station (Dakota Co.). For example, untreated plots (fungicide only or untreated) in snap bean on May 20th averaged 1.13 maggots/5 seeds (3 replications) with 49% of the seeds showing obvious feeding damage. Most of this damage (40%) was from direct feeding to the emerging cotyledons (see photos). Untreated sweet corn seed averaged 2.0 maggots/5 seeds, with 23% of the seeds showing obvious damage (similar to corn seed photo). The SCM infestation likely developed form the FIRST generation flight (ca. 200 degree-days > 39F), which is typically a "low-level" generation flight in Minnesota and Wisconsin.

Seedcorn maggot on sweet corn
(M. Rice, ISU)

Seedcorn maggot and damage on hypocotyl and cotyledons of snap bean
(Bob Koch and Eric Burkness, U of MN)

These observations at Rosemount portend concerns for later planting dates, which will be exposed to the 2nd generation flight (600 DD), which is usually the most damaging flight in the upper Midwest. Maggot damage from the 2nd flight is evident by mid-June (at Rosemount). For both crops, these observations have been recorded from an insecticide seed treatment trial currently in progress. However, at this date it is too early to assess the total damage (including plant stands) to each crop or protection by each treatment. Future updates will be provided in the newsletter. In the meantime, commercial growers who still have a chance to have seed treated may still want to consider seed protection options this year. For snap bean, options include: Lorsban, Diazinon or Gaucho seed treatment, or a Thimet granule band treatment over the row at planting (do not place granules in direct contact with seed). For sweet corn, growers have more options, including: Lorsban, Gaucho or Cruiser seed treatments, or Thimet, Aztec, Force or Counter, applied as 7-inch bands over the row. Check the most recent labels and local dealers for availability, cost and logistics of each option.

The photographs above illustrate the specific locations of damage by SCM to snap bean seed. By contrast, we also show a photo for corn seed (Marlin Rice, Iowa State University).

Bean leaf beetle
(Bruce Potter, U of MN)

Snap Beans: Bean Leaf Beetles (BLB) are getting active. As noted by Bruce Potter, IPM Specialist in southwestern Minn., most of the BLB adults at Rosemount (Dakota Co.) have been found in soybean residue from 2002. The Iowa State BLB adult survivorship model also predicted >99% mortality based on the severe winter weather this year (low temperatures and minimal snow cover). Our first snap bean planting date (April 30th) is still emerging and has not yet received a high level of feeding pressure, compared to the previous two years.

Aster leafhopper

On May 20th, we captured the first potato leafhopper (PLH) in alfalfa, at 1 adult per 100 sweeps; on May 21st, we found 3/100 sweeps, still a low level. Given the low-pressure system we had this past week, this is not surprising. PLH overwinter in the southern U.S. and must migrate north each year to establish infestations in alfalfa, snap bean, potato, apples and many other crops. With over 200 suitable hosts, they have plenty to choose from. As noted last year, we also receive multiple migrating flights of aster leafhopper each year beginning in mid- to late-May, also from the southern states. ALH was detected in Rosemount alfalfa on May 7th, averaging 8-10/100 sweeps. This species is olive green with 6 black spots across the head and is a primary pest of carrot, lettuce and celery because of the phytoplasma disease it vectors to these crops. Unlike PLH, that ALH is NOT a pest of alfalfa.

Potato leafhoppers (left, nymph; right, adult)

For more information about the Aster Leafhopper see last year's MAY issue http://www.vegedge.umn.edu/mnvegnew/vol4/524pot.htm.

Sweet Corn: Black cutworm moth flights were detected in Minnesota during the past 2 weeks, via the annual pheromone trap network coordinated by Ken Ostlie. A few hot spots, with traps averaging approximately 8/trap/night have been confirmed in Watonwan, Steele, and Jackson counties. All sweet corn fields should be monitored for cutworm activity, including feeding by our locally overwintering dingy cutworm and other species. For black cutworm feeding that usually includes cutting the stem near the soil surface, we do not like to see more than 10% of the plants damaged (i.e., consider a foliar insecticide spray if >10% stand loss). However, if the damage is primarily by dingy cutworm, or other species that feed primarily on emerging leaf tissue, growers may be able to tolerate 25% or more damage, as plants will often recover and "grow through" the infestation.

For more information about all insect pests of vegetable crops, please view the VegEdge Fact Sheet site at: http://www.vegedge.umn.edu/intro/pestfact.htm
A new fact sheet on Bean Leaf Beetle will soon be posted (June 2003).