ISSUE 15 August 19, 2004
SOYBEAN APHID SITUATION
Soybean aphid infested plants in our fields are becoming easier to find. However, the numbers of aphids per infested plant continue to be low in the vast majority of the fields surveyed. The area where percent plants with aphids is the greatest continues to be in the area from Thompson to Northwood up to Highway 2 and beyond in Grand Forks County. Fortunately, in these fields the estimate for plants with greater than 250 aphids per plant is still quite low. In fact only two fields surveyed last week had low levels of plants with that many aphids present.
Watch this region closely for population increases. Smaller fields with abundant wooded area surrounding the edges are still most likely to have aphids. In addition, there are some late fields due to the cool temperatures. A little over half of the fields surveyed in Grand Forks and Nelson counties were described by the scouts as being in the R5 growth stage. Thirty percent of the fields were described as R3 to R4, stages that are still at some risk if populations decide to increase.
Eighty percent of the fields farther south were described as R5 growth stage. At this stage seed is 1/8 inch long in the pod of one of the four uppermost nodes on the main stem with a fully developed leaf. At this growth stage, treatment thresholds are not well understood. However , actively increasing populations exceeding 250 aphids/plant need monitoring with treatment at the grower’s discretion. It has been observed that yield response from treating at R5 will not be as significant or predictable as the response observed when treatable numbers of aphids are present during R1 to R4 stages.
In Minnesota, aphid populations are increasing in the southeast, but too late to contribute much in terms of winged, migrating aphids that can colonize North Dakota fields like they did last year. Populations in Polk County, MN continue to be the greatest found in the region through the MN Dept of Ag crop survey. These infestations are continuous with the areas of greatest concern in North Dakota. To view the Minnesota survey information, visit their web site at:
One final comment. Research plots established in Prosper and Casselton, ND for doing soybean aphid treatment threshold work continue to have low populations . . . including plots where aphids were "seeded" onto plants in late June and early July. Populations in the plots were increasing but have actually declined over the past two weeks. Environmental conditions do not appear to have been favorable for rapid increases in the population this year.
SUNFLOWER PROBLEMS: Deformed Flower Heads
There have been numerous reports of deformed sunflower heads coming to our attention from areas in South Dakota and southeastern North Dakota.
Unfortunately, diagnosing the problem has not been easy. Some of the blame for the problems has been placed on insect infestations. Other explanations address environmental conditions in June that may have affected normal development.
Some of the first heads submitted displayed odd symptoms where bracts and ray petal tissue was forming through the center of the buds. In the past, similar symptoms have been blamed on infection by Aster Yellows, however this does not appear to be the case this year. Another explanation has been cold temperature damage to bud tissue. Perhaps a better explanation based on the distribution of the problem throughout the fields affected.
Cold temperature or Aster Yellows?
A third suggestion has been the sunflower maggot, a fly whose maggot feeds in the developing bud. Often, we see the small brown pupa present where these have been feeding. Feeding damage to florets usually surrounds the pupa. It has been quite common to see two or more of these pupa in a flower head without symptoms currently being reported.
Sunflower maggot damage SF maggot pupa
Another possibility is sunflower midge. Infestations by this insect often result in distorted heads, sometimes completely destroyed buds. There is scarring of the bracts when midge have infested a flower bud and distortion of the head when damage is severe. Dr. Gary Brewer, NDSU entomologist, and others have reported an increase of midge damage in the region. These areas have included Jamestown area, areas of NE Benson county, and the south central Red River Valley. These infestations end to greater on field margins.
SF Midge damage SF Midge infested bud
As we continue to look into this problem, we will update people on additional explanations.