ISSUE 8 June 20, 2002
SOYBEAN APHID INTEREST STARTING
Soybean plants in the oldest fields are reaching the V1 to V2 growth stage (numbered by fully-developed trifoliolates). Though there is no need to get excited at this point about aphids, it is time to start snooping around to see if aphids are showing up in fields. There was no evidence last year to suggest that injury occurs in these early growth stages.
Reports are starting to trickle in from areas where soybean aphid were present the past two seasons. Wisconsin reported on June 13 that small numbers of aphids were found in young soybeans (V0 - V1). In Minnesota, the first report of aphids on soybeans were some aphids found on potted SB plants placed near buckthorn.
Publication E-1232, Soybean aphid Management in North Dakota, has just been printed by NDSU Extension Service with the support of the North Dakota Soybean Council and the State Board of Agricultural Research and Education (SBARE). Plans are to distribute this circular to growers through the ND Soybean Council, and make it available through normal extension channels, including the internet.
Soybean Aphid Sampling Strategies
The following sampling procedure has been suggested by DiFonzo and Hines (2002) based on their field experience from the 2001 production year. As populations increased in 2001, they found the easiest way to sample for soybean aphid was to evaluate individual leaflets on a 0 to 4 scale. This sampling method proved useful when evaluating fields before and after spraying. Using this scale, leaflets can be quickly rated without counting individual aphids. The leaflet in the example is clearly a "4" on the scale. When the average leaf rating is 3 or greater, and other conditions are met, treatment would be advised.
Treatment Threshold and Spray Timing
Unfortunately, the treatment threshold is still vague, and future research and experience will better define it. Currently, the guidelines for making soybean aphid treatment decisions are:
Reference: DiFonzo, C, and R. Hines. 2002. Soybean Aphid in Michigan: Update from the 2001 season. Michigan State University Extension Bulletin E-2748.
BARLEY THRIPS CONCERNS
Barley thrips are causing concerns in the southwest ND counties. It is barley where we have the greatest concern. Damage potential from these insects is greater under dry conditions. Though barley thrips can be found in wheat, also, their numbers generally are low and the thrips migrate to barley before significant damage occurs.
In barley, a severe infestation will appear as bleached or whitened plants. This color is due to the feeding injury inflicted by the thrips. The adult thrips are only 1 to 2 mm. long, very slender and dark brown or black. The immature thrips resemble the adults but are smaller and white or green in color.
Sampling for barley thrips should be initiated when the flag leaf is first visible, and continue until the head is completely emerged from the boot. Most barley thrips can be found under the top two leaf sheaths. To count the number of thrips on a stem, first break off the plant at the second node from the top. Run your thumbnail between the two edges of the sheaths at the collar and slowly unroll the sheath away from the stem.
At present the economic threshold is 7 - 8 adult barley thrips per plant, before the crop is fully headed.
Treatments, when warranted, are only effective if applied before heading is complete. Treatment after heading has not demonstrated a yield increase according to NDSU trials. The only insecticides labeled on barley which list barley thrips is methyl parathion and ethyl parathion.
TROUBLESHOOTING PLANT INJURY
There have been numerous calls in the past week about the general poor appearance of some crop seedlings. In many cases the callers are trying to troubleshoot and identify whether insects are the cause.
In the case of soybeans, they have been described as having very ragged leaves, as if grasshoppers are chewing on them. However, grasshoppers havenít been found in most of these cases. Another possibility has been whether cutworms are present. The cutworm or pupa should be present in the soil at the base of damaged plants. If not found, then another reason needs to be explored.
It is possible that environmental conditions are contributing to most of the problem. Most accounts say the entire field is affected. Cool temperatures, wind and sand blasting, and/or hail all can contribute to mishapen, ragged leaves and would have a more uniform effect. One insect that can compound problems is thrips. They feed on leaf buds, destroying tissue at their feeding site. Leaves continue to grow but without these damaged areas being filled. Thrips injury is more common during slow plant growth. In this case, I would expect newer leaves to look much better with more favorable conditions.
SUGARBEET ROOT MAGGOT: FLY NUMBERS BUILDING, PEAK ACTIVITY IMMINENT
Sticky-stakes being maintained by the NDSU Sugarbeet Entomology project indicate that a significant amount of sugarbeet root maggot fly emergence occurred in previous-year fields during the past few days. As of June 18, our stakes in a couple of spring wheat fields (last-yearís beet fields) southwest of St. Thomas, ND were averaging over 100 flies daily per stake. Therefore, expect fly activity to increase sharply in the very near future. Peak activity in current-year fields is likely to occur by the weekend in the central and southern Red River Valley, and within the next 2-5 days in the northern portion. Remember, these are only projections: actual peaks will usually occur on the first warm (80 degrees Fahrenheit or above) day following the accumulation of sufficient heat units for overwintered larvae to pupate and subsequently emerge as adults. This will be further complicated if anticipated rain and thundershower activity occurs because fly activity is greatly suppressed by unfavorable weather conditions. Also, because of unusual spring developmental conditions and resulting delayed emergence, the peak fly activity period could be more extended than observed in more typical years. Finally, as mentioned in earlier articles, growers choosing to use granular materials for postemergence protection from sugarbeet root maggot injury are advised to apply them ahead of peak fly activity, whereas, liquid form. will work better if applied within 2-3 days of (before or after) peak.
Fly activity in current-year sugarbeet fields is being monitored throughout the Red River Valley during the 2002 season as a cooperative effort between NDSU, the U of MN Northwest Research & Outreach Center (Crookston, MN), and the MN Dept. of Ag. Please consult the following web site for the latest postings of sticky-stake capture results:
Research & Extension Entomologist