ISSUE 9 July 9, 2009
SCOUT FOR LEAFY SPURGE FLEA BEETLES
Leafy spurge flea beetles (Aphthona species) are an effective
means of controlling the noxious weed leafy spurge in North Dakota (Fig. 1).
This group of flea beetles is host-specific to the leafy spurge plant, which
makes them an ideal biological control choice.
Figure 1. Black leafy spurge flea beetle
(photo by USDA APHIS PPQ Archive, Bugwood.org)
Most of North Dakota has accumulated over 1,000 growing degree day (GDD, Fig. 2) and scouting should begin for adult leafy spurge flea beetles. Flea beetles should be collected between 1,200 and the 1,600 (see map below) using the sunflower GDD on NDAWN. Adult flea beetles can be collected with sweep nets. After late July, flea beetles begin to lay eggs and should not be moved. Leafy spurge flea beetle typically take three to five years to establish and impact leafy spurge infestations.
Figure 2. (Source: NDAWN)
WHEAT MIDGE EMERGENCE AND SCOUTING
The 2009 risk forecast for wheat midge infestation was low
throughout most of the state based on the previous season’s cocoon counts in the
soil (see previous issue of Crop & Pest Report #3, May 28, 2009). Although the
risk is low, isolated pockets of economic populations of wheat midge can still
occur that wheat midge is emerging at the same time as wheat is in the
susceptible stage (heading to early flowering), and environmental conditions
(moist soils, warm temperatures, calm winds) are favorable.
A degree day (DD) model using daily temperatures to calculate DD accumulation can be used to predict the emergence of adult wheat midge. Events for the midge population (degree day base = 40 F) are listed in the table below.
|450||Midge breaks the larval cocoon and moves close to soil surface to form the pupal cocoon.|
|1300||10% of the females will have emerged.|
|1475||About 50% of the females will have emerged.|
|1600||About 90% of the females will have emerged.|
Typically, wheat midge is emerging during the first week of July in the northern counties of North Dakota. However, this year the DDs are behind normal and just at <1,300 DD in the northern tier (Fig. 3). Wheat midge DD maps for North Dakota are available on the NDAWN website, use March 1 as the start date (enter in ‘planting date’ box) and select ‘Midge degree days.’
Figure 3. Wheat midge degree day map (Source: NDAWN)
Scout for adult wheat midge at night (after 9:00 PM) during warm temperatures >60 degrees F, and light winds <6 mph. The adult wheat midge is an orange color, fragile, very small insect approximately half the size of a mosquito (Fig. 4). It is about 0.08-0.12 inch (2-3 mm) long with three pairs of long legs. There is a pair of wings, which are oval in shape, transparent and fringed with fine hairs. Two eyes are conspicuous and black in color. Typically, the most significant flight period for the entire wheat midge population extends over a 14 to 18 day window of time within a region. Individual adult midge may survive from 3 to 7 days, depending on favorable conditions (warm, calm, humid weather).
Figure 4. Adult wheat midge
Economic thresholds are:
- Hard Red Spring Wheat = one wheat midge per 4-5 heads
- Durum = one wheat midge per 7-8 heads.
Hard red spring wheat at high risk for wheat midge infestation can be identified based on degree days. Degree days are then used to help identify the high risk planting window for hard red spring wheat. Wheat fields planted between 200-600 DD will be heading (susceptible crop stage) at the time wheat midge is emerging and are at greatest risk to infestation. In North Dakota, planting dates at high risk in southern tier are May 7 to June 1, in central tier - May 12 to June 12, and in northern tier - May 18 to June 16 (Fig. 5). The window of high risk planting dates is wide due to the cool temperatures (low DD units per day) this spring. As a result, the majority wheat fields will be at risk for wheat midge infestation; fortunately, the population level is low. So, the bottom line is it’s always ‘good’ insurance to scout fields for wheat midge infestations and avoid any unexpected yield losses with the good wheat prices! A NDSU Extension Entomology bulletin on wheat midge is available at the following weblink: http://www.ag.ndsu.edu/pubs/plantsci/pests/e1330.htm
SOYBEAN APHID: FIRST NORTH DAKOTA
Soybean aphids (Fig. 6) have been found in two southeastern counties (Cass and Barnes Counties) of North Dakota this past week.
Figure 6. Soybean aphids (P. Beauzay)
Numbers are low at present - <10 aphids per plant and <15% of the plants infested on early V-2 to V-3 stage beans. The economic threshold is 250 aphids per plant on 80% of the plants in field from late vegetative to R5 (beginning seed). In North Dakota, we typically don’t treat for high populations of soybean aphids until late July into August. Nevertheless, it’s a good time to get started monitoring for soybean aphids. Populations can build quickly with the moderate temperatures, which are favorable for aphid development. We are being to monitor our soybean sentinel plots for the IPM PIPE (http://sbr.ipmpipe.org/cgi-bin/sbr/public.cgi) this week. We will post frequent updates from scouting reports in the Crop & Pest Report. So, stay tuned!
CONTINUE TO SCOUT FOR CEREAL APHIDS
Scout fields for cereal aphids through early flowering. If weather continues dry with moderate temperatures (mid-70s F to 80s F), aphid populations can explode quickly and reach economic threshold. To protect small grains from yield loss due to aphid feeding, use either economic threshold:
- 85% stems with at least one aphid present, prior to complete heading.
- 12-15 aphids per stem prior to complete heading
After heading, the yield loss due to aphid feeding is minimal.
DIAMONDBACK MOTH ALERT IN FLOWERING CANOLA
High numbers of adult moths (>100 moths per trap week) are being
captured in pheromone traps (Fig. 7) in north central and northeastern North
Dakota and northwestern Minnesota. Field should be monitored from bloom to early
pod development for larvae of diamondback moth by beating plants to dislodging
the larvae from plants. After beating plants, count larvae on ground or dangling
from plants on a silk thread. Check several locations per field.
Figure 7. Diamondback moth pheromone trap ( J. Knodel)
Larvae are green and about ½ inch long when mature (Fig. 8). Larvae feed on the leaves, buds, flowers, seed pods, the green outer layer of the stems, and occasionally, the developing seeds. The amount of damage will depend on the crop stage and the larva densities and size. Extensive feeding on the flowers will delay plant maturity, cause the crop to develop unevenly, and significantly reduce seed yield (Fig. 9). As leaves wilt and drop in late July to early August, the larvae will feed on the stem, pods, and developing seeds. Damaged seeds will not fill completely and may shatter. Severely damaged pods appear whitish in contrast to the normal yellowing and browning of ripening pods.
Figure 8. Larva of diamondback moth (J. Knodel)
Figure 9. Aborted flower bud damage caused by
diamondback moth (J. Knodel)
The action threshold for diamondback moth in canola at the pod stage is about 20 larvae per square foot (two to three larvae per plant). No threshold has been established for the early flowering stage, however, insecticide applications are likely required at larval densities of 10-15 larvae per square foot (one to two larvae per plant). Early monitoring of adults and larvae, and judicious use of insecticides only when fields are above thresholds are the best pest management practices for preventing losses from diamondback moth on canola.
A number of natural factors can also negatively affect diamondback moth populations. For example, heavy rainfalls can drown many larvae. Humid conditions associated with rainfall can also favor the development of fatal fungal diseases like Entomophthorales. In addition, there are several parasitic wasps and predators (flies, lacewings, minute pirate bugs, spiders and birds) that prey on the larvae of diamondback moth.
CORRECTION - BARLEY THRIPS
In the barley thrips article in the Crop & Pest Report #8 for
July 2, 2009, the insecticide rate for methyl parathion for control of barley
thrips was incorrectly listed. The correct rate is 8-12 fl oz per acre. Always
read and follow the use directions from the pesticide label. The label is the