ISSUE 7 June 15, 2006
TIME TO CUT ALFALFA EARLY TO AVOID ALFALFA WEEVIL INJURY. REMEMBER TO SCOUT RE-GROWTH!
Sporadic alfalfa fields have been reported having economic populations of alfalfa weevil with more than 30 to 40% of the stem showing tip feeding or larvae present. A sweep net is also used to determine if treatment is necessary. When there is 20 or more larvae per sweep and early harvest is more than a week away, a treatment is justified. Fortunately, most of these fields are ready to be cut eliminating the need for any insecticides. Most larvae, especially the smaller ones, will be killed by exposure to direct sunlight and high soil temperatures after cutting. Others will starve since the cut alfalfa will not be sufficient to support weevil larvae. It is recommended to remove hay windrows or bales as soon as possible to increase mortality of susceptible larvae stages. It is good insurance to scout regrowth for signs of feeding injury to the new alfalfa growth. Late 4th instar larvae ( inch long) consume 80% of the total foliage eaten in all the larval stages. According to the degree day model for alfalfa weevil, southeastern, central and northeastern North Dakota is in the mature 4th instar stage (595 DD), where as the southwestern and northern tier is in the 3rd instar stage (504 DD) -see map. Under high populations, fields may fail to green-up because late instar larvae and maybe newly emerged adults are consuming the new crown buds as fast as they are formed. Other factors like dry weather and high temperatures can affect growth of crop and development of alfalfa weevil. Alfalfa weevil populations can also vary significantly between fields. So, close scouting in each field is important. Treat regrowth fields when 50% of the stems have feeding injury. Remember, it’s good insurance to scout the 2nd crop regrowth for late instar larvae feeding injury, even if the 1st crop scouting assessment did not represent a treatment threshold situation.
WHEAT MIDGE EMERGENCE UNDERWAY IN SOUTHEAST REGION OF NORTH DAKOTA
Wheat midge degree days are moving along fast with the above average heat units this summer! Most of the southeastern region of North Dakota is near 1300 degree days when the female wheat midge emerges (see map).
Female emerges at 1300 degree days and emergence is 90% complete at 1600 degree days (degree day base = 40 F). Conditions that are conducive to wheat midge emergence is moist soils. Although the risk for wheat midge is low for North Dakota this year, it is always a good idea to scout any wheat/durum fields in the susceptible crop stage – heading to early flowering during female wheat midge emergence. The midge emergence and wheat growth degree day models that is available on NDAWN (see website below)) allow estimates to be made based on specific planting dates. The model identifies the expected growth stage of wheat and will identify emergence time and associated risk to egg laying and infestation by wheat midge.
Scouting should be conducted at night (after 9:00 PM), warm night temperatures >60 degrees F, and light winds <6 mph. Economic thresholds are one wheat midge per 4-5 head for wheat and one wheat midge per 7-8 heads for durum. 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 environmental conditions. Observations over the years in North Dakota indicate that by about 1800 DD, adult numbers decline to the point where field activity is below economic levels. However, in areas where reduced or minimum tillage is common, significant adult activity has been reported and observed up to about 1900 DD. Remember not to confuse the Lauxanid fly for the wheat midge fly. The Lauxanid fly is larger (2.5-4 mm), more robust body, and yellowish-brown in color; in contrast, the wheat midge is smaller (2-3 mm), more delicate fly, and orange in color . No field reports are available yet. Stay tune for updates on wheat midge.
IPM INSECT SCOUTING UPDATES
- No soybean aphids have been detected in soybean fields, V1-V4 crop stage, in Traill, Steele, Griggs, Barnes, Pembina, and Grand Forks Counties.
- Low numbers of small grain aphids have been detected in the northeast and southwest regions of North Dakota.
- The first Barley thrips were observed in barley in Griggs County.
- Low numbers of grasshoppers (<10 nymphs per square yard) have been reported throughout North Dakota.
- Low numbers of sunflower beetles (non-economic levels) have been found in north central region of ND.
LEAFY SPURGE FLEA BEETLES ARE READY FOR REDISTRIBUTION
It is time to start checking those patches of leafy spurge where Aphthona flea beetles have been released during previous summers. Adult beetles have been observed in southeast North Dakota the first of June and I swept up to 30 beetles/m2 in research plots near Warwick on June 7. Mid-June to early July is the best period to collect adult flea beetles for redistribution. It is best to collect beetles after 10:00 a.m. when the air temperature has warmed to at least 70 F and the spurge is dry. Beetles are easiest to collect when there is little or no wind. Look for signs of adult feeding on the leafy spurge vegetation (Fig. 1) in areas where they have been released.
You need to redistribute flea beetles when an established population yields 500 to 1,000 adults per five minute sweeping period. When numerous beetles are collected, remove the vegetative trash and other insects from the sweep sample and pour the flea beetles into a graduated container. Every 10 ml equals approximately 1,000 flea beetles.
For redistribution, the beetles need be placed in the paper bag or container with some leafy spurge. Transport the beetles in a cooler with blue ice. Remember the beetles are living organisms and the containers of beetles should not be left in the sun or a hot vehicle. Beetles need to be released at the new release site preferably on the same day of collection. If the beetles cannot be released as soon as possible, they can be stored for a maximum of one week at 40 F.
Aphthona flea beetles usually establish and control leafy spurge sooner when large numbers are released into habitats that are similar to the habitat from which they were collected. A minimum of 1,000 adults should be released at a single point, marked with a stake, along the margins of the leafy spurge when the infestation is dense. Release the beetles at several points in large spurge infestations. New flea beetle releases should yield at least 50 beetles per five sweeps the following summer. When the population is less than 50 beetles, make additional releases.
Flea beetles do have limitations and do not have an equal impact across all habitats. Flea beetles will establish sooner when released in moderate spurge densities of 60-90 stems per square yard with little grass cover and thatch. Aphthona flea beetles establish faster on the south facing slops followed by the western and eastern slopes and usually noticeably slower on north facing slopes. The black flea beetle, A. lacertosa, establishes at sites ranging from high and dry to cooler and moister habitats with shade and denser stands of spurge, and where spurge is growing in soils of silt loam, silt clay loam, clay loam, loam, or loam/fine sand loam. The brown flea beetle, A. nigriscutis, is more successful in areas that are higher and drier with well-drained loam soils. When releasing beetles into a spurge infestation for the first time, release a mixture of both species to determine which species is best suited for that spurge habitat.
Aphthona flea beetles are not a "quick fix" for controlling leafy spurge. The beetles may take several years to reduce a leafy spurge infestation. Every spurge infestation is different and the flea beetle population development will vary across habitats. Beetles may need to be integrated with other management tactics to achieve desirable results. Herbicides plus flea beetles have shown to give better results than either tactic used alone. Tordon (picloram) plus 2,4-D at 1 quart plus 1 quart per acre (0.5 + 1 pound per acre) can be applied during early September to mid-October to leafy spurge with an established flea beetle population. The herbicide treatment will help to open up the canopy, and allow the flea beetles to deposit their eggs at the base of the spurge plants. Grazing by sheep or goats, or fire can also be used to open up the spurge canopy and remove excess plant trash from the soil surface. When integrating these management tactics with an established flea beetle population, they should only be used after mid-August, when egg laying by the beetles is completed.
Some county Weed Control Officers or county Extension Agents host field days during June to early July for collecting and redistributing the flea beetles. Contact your county Weed Control Officer or Extension Agent for the date, time, and location of a field day in your area. For more information on using the Aphthona flea beetles for leafy spurge control, refer to Leafy Spurge Control Using Flea Beetles (W-1183) at
Figure 1. Aphthona flea beetles feeding on
leafy spurge vegetation (source: USDA-APHIS)