ISSUE 14 August 3, 2000
RED SEED WEEVIL AND BANDED SUNFLOWER MOTH UPDATES
Field populations of these two pests are quite variable around the state. Spot checks from southeast ND found seed weevil numbers ranging from less than 1 per bud or head up to 20 to 30 adults per flower when checking field margins. Fields with the lower numbers were younger fields, primarily in the late bud stage with maybe 1% of the plants with ray petal. The higher numbers were fields well into bloom (90% ray petals, 60+% pollen shed). The blooming fields with the greater populations were averaging 4 to 5 weevils per head as scouting moved to the interior of the field. Reports from other areas of the state suggest that the greatest seed weevil activity may be a few days away. In general, adult numbers were below treatable levels
At some locations in southeast ND, adult Banded sunflower moth were easily detected in the grassy borders around fields. Last week we reported that moths were easily found in Benson County. Some small larvae (1/8 inch) were found in flowering heads. The more advanced the flowering, the more likely larvae were found at 1 to 2 per head. In central ND (McLean Co.), several people are also reporting finding BSM larvae in heads.
One other insect to note in sunflower heads is the sunflower maggot. Many buds or flowers have 1 to 2 pupae of the maggot present. The pupa is light brown to yellow, looking like a small bean seed sitting on the surface of the bud/flower. A small number of maggot damaged florets surround the pupa.
(Note: In last weeks newsletter, I mentioned that the industry standard for Red seed weevil damage tolerance in confection sunflower was 3 to 4%. It has been suggested that I use 1 to 2% as a better indication of tolerance by the industry. Thank you PAG)
Grasshopper activity has been limited to isolated hot spots around the region. As small grains ripen, adult grasshoppers begin moving in search of green plant material so they can continue to feed. This movement will begin to concentrate grasshoppers into later season crops such as corn, soybean, dry bean, sunflower, flax, and others. In places where grasshoppers have not been a problem, you may suddenly observe feeding damage.
Reports of grasshopper "hot spots" have come in from Dale Siebert, extension agent, Richland Co., Minnesota Dept of Agriculture which is reporting high numbers in eastern Wilkin County, and various fieldmen from some of the central counties in ND. None of these reports seem to be extensive. It does emphasize the need to watch grasshopper movement during this period of migration.
EUROPEAN CORN BORER UPDATE
There was a brief increase in moth captures late last week in black light trap. This increase is probably due to emerging or migrating bivoltine moths that produce the second generation. It appears the numbers have already declined, though.
Inspections of corn fields in SE North Dakota found tolerable levels of corn borer. The largest infestation found was 1 larva per plant on 25% of the plants. Larvae were in the late 2nd or 3rd instar and were beginning to tunnel at the base of leaves. If infestations have not developed in fields by now, the likelihood of future problems due to infestations by the second generation should be small.
WHITE AND YELLOW BUTTERFLIES IN THE FIELDS
The white butterflies that become so abundant in August are composed of three different species. From our canola fields, there are large numbers of imported cabbage butterflies emerging. They lay their eggs in mustards and the larvae feed on the foliage. Canola growers may still be able to find some of the green caterpillars feeding on leaves. The butterflies will be attracted to flowering plants where they feed on nectar.
Other of the white butterflies are either alfalfa butterfly or common sulpher butterfly. These butterflies have both a white and yellow form. Alfalfa fields in bloom attract very large numbers of these butterflies. The caterpillar of the alfalfa butterfly can be a pest of the crop, but rarely is abundant enough in ND to cause concern.
U OF MINNESOTA: APHID ALERT
Captures of winged aphids in the Aphid Alert network traps increased substantially this week. Most of the aphids captured this week are those associated with small grains but green peach aphids were found at most locations. As small grains mature throughout the Valley a greater proportion of aphids will develop wings. Winged aphids leave small grains in search of a suitable host. None of the grain aphids can colonize potato, but they are capable of spreading potato virus Y (PVY).
Minnesota and North Dakota locations averaged 8.45 aphids per trap of all species and 0.17 green peach aphids per trap. Total aphid captures in the sixth week of 2000 are 1/2 to 1/3 that of the same week of 1998 and 1999, green peach aphid captures are below that of 1999 but more than we were capturing on the same date in 1998.
We continue to find green peach aphids on wild mustard. Wild mustard is growing profusely in many drowned out row crops. Matt Carroll (entomology Ph.D. student) and his crew found over half the mustard plants in two separate fields had green peach aphids. Wild mustard is not a host of PLRV or PVY but the aphids that we have seen on this plant (green peach and turnip aphid) are good vectors of leafroll (PLRV) and PVY. If these wild mustard infested fields persist, we would expect to capture green peach aphids throughout the month of August, which is not good news for seed potato growers.
If the green peach aphid flight activity patterns observed in 1998 and 1999 are repeated in 2000 we will see a marked increase in green peach aphid flight activity within the next two weeks. There do not appear to be large populations of green peach aphid on canola perhaps because much of the canola is more advanced than at the same time last year, but the large populations supported on wild mustard are of concern.
PVY spread cannot be stopped with insecticides. Reducing inoculum in seed fields by roguing and isolating seed fields (about 150 yards) from a known source of virus and aphid vectors will help. It is too early in most areas to consider top killing as tubers are just beginning to form. However, seed growers should consider killing the earliest generation fields as soon as possible to keep these basic seed lots as free of virus as possible.
You should immediately begin scouting potato fields for green peach aphids. These aphids will colonize at field margins so concentrate your scouting efforts on the outer 20 rows. Pick 20 leaves from 5 locations, selecting only lower leaves as green peach aphids can only be found in the lower canopy. An economic threshold for green peach aphid in seed potato fields is essentially "at detection" or 3-10 wingless green peach aphids per 100 leaves. The same threshold should be used for processing potatoes known to be susceptible to net necrosis, e.g, Russet Burbank. All other processing and chip varieties can use a threshold of 30-50 green peach aphids per 100 leaves.
There are only three insecticides that provide reliable green peach aphid control: Monitor® (methamidophos); Provado® (imidacloprid); and Fulfill® (pymetrozine). In our research plots these three compounds have consistently provided 90-99% control. Because green peach aphids are found on the lower canopy, high pressure, high gallonage sprays provide the best control. Fulfill has the added advantage of being transported across the leaf membrane and is mobile in the vascular tissues. Fulfill kills more slowly than Provado or Monitor and aphids may be found in the crop 3-4 days after application. However, populations invariably drop by 1-week after application. Fulfill must also be used with a non-ionic surfactant so read the label before using this product.
Edward (Ted) Radcliffe
Department of Entomology
University of Minnesota