ISSUE 12 July 20, 2006
EUROPEAN CORN BORER UPDATE
Flights of the univoltine corn borers are close to completion in most of North Dakota, except for the northern counties. Degree day accumulations near 1177 and 1274 indicate 75% and 90% of the moths emerged, respectively (see map). Field reports indicate low populations of corn borers in corn, so far. For more information on managing European corn borer, consult the July 6th issue of the Crop & Pest Report.
SOYBEAN APHIDS, DROUGHT AND HEAT
Since the arrival of soybean aphid, the occurrence and geography of outbreaks (2001, 2002, 2003, and 2005) have been associated with drought conditions. Drought potentially benefits aphids in several ways. First, slight drought stress may improve the nutritional quality of soybeans for aphids. Second, fewer rain events mean less chance to wash off during thunderstorms. Third, drought conditions (low humidity, less rainfall) reduce the activity of aphid-attacking fungi periods of high humidity. So it is not surprising that soybean aphid problems are emerging in North Dakota.
Will the Forecasted Hot Temperatures Control Soybean Aphids?
With temperatures forecasted to reach the high 90s and even over a 100įF this weekend, it's logical to wonder about the effects of hot temperature on soybean aphids. Temperature affects the longevity and reproduction of soybean aphids, but high temperatures will not control aphids Research at constant temperatures by McCornack, Venette and Ragsdale (see Fig. 1) indicates soybean aphids are temperate insects that do better at cooler temperatures. Longevity declines with temperature.
The cooler the temperature, the longer aphids survive. Reproduction is also better at cooler temperatures. At 90įF constant temperature, soybean aphids longevity drops dramatically and reproductive success is almost nil.
Research at fluctuating temperatures does not find these dire effects of high temperatures. Why? Daily temperatures fluctuate with nightly lows 20 to 25 degrees less than the high temperature for the day. Transitory high temperatures have little effect as long as nightly lows drop below into the 70s. Brief periods of hot temperatures don't affect the rate at which females produce nymphs, only the survival of the nymphs. Nymphs born in the heat of the day donít survive well when temps exceed the 90s. The net effect: aphid populations may not increase as quickly under hot temperatures. Hot temperatures will not stop or control aphid populations. Keep in mind that the 2001 outbreak developed under fairly hot temperatures (highs frequently above 90F) while the 2003 outbreak developed during a cool drought.
Development of the soybean canopy also moderates the conditions that aphids experience. Canopy cover is approaching 100% in earlier planted fields with adequate moisture. As the canopy closes, the highest temperatures and the greatest temperature fluctuation occurs near the top of the canopy. It's not surprising that the highest aphid concentrations are on new vegetative growth, which is well below upper canopy leaves, and on mid-canopy leaves.
Does Hot Weather Affect Insecticide Performance?
Higher temperatures and low humidities can affect insecticide performance in several ways.
Should thresholds be adjusted for drought conditions?
The current economic threshold is an average of 250 aphids/plant on 80% or more of the plants. Economic threshold (ET) and economic injury level (EIL) of 4000 cumulative aphid-days were developed from data gathered under drought conditions. Aphids aggravate drought stress on soybean so there's less room for error. Monitor soybeans closely and treat as soon as possible afer populations exceed threshold. Removing the damaging aphid populations leaves the soybean crop in a better position to take advantage of later rains.
Reprinted from University of Minnesota Crop eNews, K. Ostlie and modified by J. Knodel for North Dakota.
Very high aphid populations are being seen on sunflower. Aphids are small, soft-bodied insects found in clusters. In sunflower, they are usually on the lower sides of the leaves and can be numerous enough to completely cover the underside of the leaves. They are more likely to be on senescing leaves low on the plant. The upper sides of leaves may be sticky from the honeydew excreted by the aphids. Although a number of species of aphids can be found on sunflower, two are common.
The main symptom we see from aphids is wilting caused by their pulling water from the plant as they feed on plant sap. Under our present hot, low-moisture conditions, it is normal for sunflower to appear wilted in the afternoon. However, if the plants are wilted early in the morning and have large numbers of aphids, they may be causing additional stress and damaging the plants. I am not aware of any insecticides registered for aphids on sunflower.
WHY THE SUDDEN APPEARANCE OF APHTHONA FLAVA? A LEAFY SPURGE FLEA BEETLE
Aphthona flava was approved for release into the United States in 1988 at several locations in North Dakota: Cass, Barnes, Richland, LaMoure, Steel, Eddy, Morton, and Burleigh counties, as well as in Clay County, Minnesota. In the period from 1987 to 1995, the beetle was reported from only two sites in Barnes County. During this time period, beetle numbers as high as 50 to 80 adults per m2 were recorded. When these two sites were sampled for leafy spurge flea beetle species during 1997 to 1998 and 2001 to 2005, A. flava was not present. Aphthona flava dropped below our radar of detection until this year when USDA scientists documented it in a leafy spurge research plot on the NDSU campus. We immediately canvassed leafy spurge infestations near campus to determine the source population. After sampling only two leafy spurge infestations in Fargo, We detected A. flava along the railroad tracks on the west edge of campus. Sweep samples netted more than 100 beetles per ten sweeps. Aphthona flava has also been detected at very low numbers on the Sheyenne National Grassland (Richland County). Why this sudden showing of A. flava after its apparent disappearance? In Canada, scientist have noted that A. flava requires more heat units for development compared to the other Aphthona species released in North America. The higher than normal temperatures this year may be favorable to A. flava development, thus allowing populations to increase in areas where they have not been detected in the past. Also A. flava does not appear to be as winter hardy as A. lacertosa and A. nigriscutis, the two species credited for reducing leafy spurge in many areas across North Dakota. The past couple of winters have been relatively mild, and larval survival may have been higher during these winters. Whether A. flava continues to increase in numbers and locations remains to be seen. Look for A. flava as you are monitoring leafy spurge infestations for Aphthona flea beetle feeding activity. This orange or orange-brown beetle (Figure 1) with a pale underside looks noticeably different from the other Aphthona flea beetles species, the black flea beetle, A. lacertosa (Figure 2), and the brown flea beetle, A. nigriscutis (Figure 3). Please contact me at 701.231.6292 or Denise.Olson@ndsu.edu should you detect A. flava in leafy spurge infestations.
Figure 1. Apthona flava
Figure 2. Apthona lacterosa
Figure 3. Apthona nigriscutis
TIME TO SCOUT SUGARBEET FIELDS FOR LYGUS BUGS
Lygus bugs in the Red River Valley typically pass the winter as adults in the leaf litter and thatch in shelter belts. Last winterís snow cover should have been conducive to good overwintering survival by local Lygus bug populations. Additionally, the warm temperatures that got our growing season off to a good start and predominated much of the growing season have likely accelerated the development of Lygus bug populations in the area. This is a significant concern because an early start and accelerated development of Lygus bug populations could lead to the development of a large third generation of the insect this year. Also, the hot and dry weather of the past several days is expected to hasten the drying down of other area crops and small-seeded broadleaf weeds that often harbor Lygus bugs. Third-generation Lygus bugs have caused significant feeding injury to sugarbeet in late July through mid-August in past years. Therefore, fields should be monitored during the next few weeks to determine if an insecticide application will be needed.
Description. Lygus bug refers to a complex of several species. The species that commonly infests and causes feeding injury to RRV sugarbeet is the Tarnished Plant Bug, Lygus lineolaris. Adults are about 1/4 inch long and 1/8 inch wide, and their color can range from dark greenish yellow to a dingy, mottled brown. Most have a pale yellow V-shaped mark near the middle of their back and two faint light yellow patches near their hind end (Fig. 1).
Figure 1. Lygus lineolaris adult
Immature Lygus bugs are called nymphs. They pass through five nymphal molts before reaching adulthood. First-instar nymphs are very small (1/25 inch long), wingless, and look like a large, bright green aphid. Also, they have a faint black spot (actually a scent gland) on the center of their back. Later-instar nymphs have 4 more true spots (Fig. 2).
Figure 2. Lygus lineolaris nymphs
Damage. Lygus adults and nymphs use piercing and sucking mouthparts to feed on the plant. They pierce the plant and inject a salivary toxin which liquifies and kills plant tissue. The bugs then suck up the resulting liquid. Feeding injury in sugarbeet is usually concentrated in new leaves and petioles. Symptoms of Lygus feeding injury include leaf curling and wilting, seepage of a black oil-like exudate (Fig. 3a.), swollen feeding scars on petioles (Fig. 3b.), and often a gray to black sooty appearance on new growth near the plant crown.
Figure 3a. Leaf curling and black exudate
seeping from Lygus feeding sites
Figure 3b. Healed scar from Lygus feeding injury
Lygus injury is believed to cause plants to respond by using carbohydrate reserves to produce new leaves and stems. This can lead to major sugar yield losses if it occurs later in the season when the reserves should be building up in the root.
Sampling. Sampling should be done with care because adults can fly away and nymphs usually hide or drop from the plant when the canopy is disturbed. Also, young nymphs blend in well with the beet canopy due to their green color, and can be difficult to detect.
Threshold: an insecticide application is probably needed if harvest is at least 3 weeks away and if the infestation exceeds one Lygus bug per plant (adults and nymphs combined). An insecticide application is not likely to be economically justified if harvest will occur in 3 weeks or less. For more information on control strategies, consult the Insects section of the 2006 Sugarbeet Production Guide. The online version is at: