ISSUE 3 MAY 21, 1998
Grasshopper emergence continues around the state. Higher numbers of nymphs are being reported in the north central and southwest counties. Look for and monitor hatching sites closely during the next couple of weeks. In the southeastern counties, where some border treatments have been applied during the past two weeks, scout hatching sites before scheduling any new treatments. The rains should cause some mortality of the young nymphs, perhaps making treatments unnecessary.
Flea beetle activity is increasing in our canola production areas. Good moisture in many areas will favor plant development and make seedlings more tolerant to flea beetle injury. However, the return of warmer weather is stimulating lots of activity by beetles. Canola emerging during the next three weeks will be very vulnerable to infestations. The north central counties have the largest populations, while numbers decline towards the northeast counties. Canola is at risk to flea beetle injury up until the time fully expanded true-leaves are present. Canola treated with Gaucho should have protection from beetle feeding during the first two weeks of emergence. Lindane treated seed only has protection for 5 to 6 days after emergence.
Army cutworms and Dingy cutworms should be cycling through, finishing their feeding and beginning to pupate. Continue to watch for spring hatching cutworms such as Red-backed, Darksided, and Variegated. These cutworms will be small at this time, but could pose significant threat to field crops being planted now.
PREDICTING SUGAR BEET ROOT MAGGOT ACTIVITY IS VERY IMPORTANT !
There are positives and negatives to all that has happened in sugar beets thus far. Early planting was a positive except for being a little on the frost risk side. The nice warm weather has been a positive for good sugar beet stand establishment. Then came the rains ! In areas that have had a lot of rain, in the 3 - 5' or more range, soil insecticides applied at-planting are going to be washed off of the carrier and through the soil profile. This will greatly reduce SBRM control. Again, this is for areas that have received the heavy rains. In areas that were below 3' of precipitation and where granular insecticides were used at-planting, the active ingredient should be available to provide control. Soil type, PH and many other things come in to play as far as the durability of granular insecticides in the soil. Well drained soils (lighter textured) obviously will moved insecticide through the profile faster than a heavy soil. In 1996, a three inch rain after planting in a well drained soil was enough to neutralized granular insecticides. The unnecessary use of post treatments should be avoided in areas where the precipitation was <3'.
Scouting for SBRM pupae can now to determine when adult emergence begins, or the progression of emergence. Sift through the top 4 inches of soil of last years beet fields to find the pupal stage of the SBRM. The pupal stage looks like a gelatin capsule, but is about 1/3 the size and will vary from cream to yellow-brown, to dark brown in color. Coloration of the pupae will give a rough estimate of the time to emergence of adults from the soil. Finding cream colored pupae indicates that pupation has just begun and depending on temperatures, emergence is a week to 10 days away. The pupae darken as they mature. When greater numbers of the dark brown pupae are found by sifting through the soil, adult emergence can be expected in 1-3 days. Experience with this type of sampling can result in a fairly accurate emergence forecast for fields in a given area.
Forecasting Adult SBRM Emergence and Activity
Hopefully beet growers have sticky stakes out around fields in areas where the SBRM has been a problem. Growers may utilize the stakes, or field personnel from the sugar companies may be using them to monitor adult densities. The fly catches should be used in combination with degree-day (DD) accumulations for post treatment decisions. Two different Degree_Day (DD) temperature accumulations have been used to forecast adult root maggot activity with success. One relies on soil temperature to estimate SBRM emergence from old beet fields while the other uses ambient (air) temperature DD accumulations for predicting SBRM fly activity in to new beet fields.
Using soil DD accumulations
Improved access to soil temperatures has prompted examination of soil_DD to fly emergence as a tool which would give warning of pending fly emergence from old beet fields. Soil DD accumulations of 450 ( threshold of 47EF, accumulations start on 1 April) correlate with 50% emergence of adults. Soil DD are used to estimate when SBRM are emerging from old beet fields and should not be used for post treatment decisions. There can be circumstances that prevent SBRM from moving out of old beet fields and in to new beet fields. Use ambient (air) DD for post treatment decisions.
Using ambient (air) DD accumulations
Calculating ambient DDs is similar to the soil method, the only difference is that 47.5EF is used as the base temperature and an upper threshold of 99EF is used. With this procedure, the first 80E day following accumulation of 600 DD usually coincides with 50% (peak) activity in new beet fields. However, post liquid treatment for SBRM control need to be applied 3 days before peak fly activity, which would be around 520 to 550 DD and when the daily temperature gets up to 80E. High winds and low temperatures can keep SBRM from moving from the old beet fields to the new beet fields. It is very important that 520 to 550 DD have accumulated and the weather has warmed up to 80E for post liquid applications.
Post treatments of granular insecticides may be a very good option for controlling the SBRM since we have adequate to more than adequate soil moisture at the present time. Granular insecticides can be applied up to ten days ahead of peak fly activity. The control achieved by post granular applied insecticides is precipitation related. If no rain occurs, control will be limited. If a nice rain occurs after application, very good control should be expected.
Degree-day information is posted on DTN by Mike Beltz, American Crystal Sugar, Moorehead, MN. Fly catches can be retrieved from field personnel from your growing district. Projections of anticipated peak SBRM fly emergence for most of the Red River Valley to be the end of May to the 2nd of June. Be ready !
Timely prediction of fly activity and populations, along with effective post-emergence treatment options may eventually lead to reduced at-planting treatments.
SHELTERBELT INSECTS . . . WHAT TO WATCH FOR
Throughout the growing season, a succession of insect pests feed on tree foliage in our shelterbelts around the state. In most cases, these insects are present at low levels with little impact to the trees. Outbreaks do occur, but are unpredictable. Impact on trees is determined by when they are defoliated and how frequent. Defoliation of trees in the spring is more harmful than late summer. Defoliation several seasons in a row depletes stored energy reserves leading to die back of branches and roots.
Some comments about the most frequent leaf feeding insects in shelterbelts are summarized below.
Spruce sawflies: There are five species which may attack spruce. The most common is the Yellowheaded spruce sawfly. The caterpillar-like sawfly has a yellowish brown head and is shiny olive green with paired grayish green lines running the length of the body. Larvae hatch from late May to June. They feed on new foliage first, moving to older needles later. There is a tendency for these sawflies to concentrate their attacks on trees previously defoliated.
Forest Tent Caterpillar: They feed on a wide variety of deciduous trees. Outbreaks last for 4 to 5 years. Eggs are laid in the summer, but do not hatch until the next spring. The hairy larvae have bluish bands with distinct white markings on the back. They can be found as soon as foliage appears on trees. They feed for about 5 to 6 weeks. The caterpillars mass together when not feeding. Two years or more of heavy defoliation leads to a decline in the health of trees.
Cankerworms: These larvae hatch in spring. They are green to reddish brown to black with pale stripes. Caterpillars crawl with a looping motion. Severe defoliation of deciduous trees in back to back years leads to branch die back.
Spiny Elm Caterpillar: The butterflies lay eggs in the spring. Larvae are black with numerous white dots. There are red dots on the first seven abdominal segments. The body is covered with numerous branched spines. The caterpillars mass together when feeding causing noticeable defoliation. There is little permanent damage unless infestations persist for several years.
Elm Sawfly: The adults emerge from late May to June; they are black, stout-bodied insects with smoky wings. The adults look like large flies at first glance. The sawfly larvae hatch in mid June. They are found primarily on elm and willow. The older larvae are yellowish green with a distinct blue and black line down the middle of the back.
Willow Sawfly: This sawfly may have two generations in some areas. Adults emerge in the spring. The larvae are black with prominent yellow spots on the side of the body.
Fall Webworm: Moths emerge in early summer to lay eggs. The caterpillars are covered with long silky hairs. The bodies are white with a black stipe on the back. There are orange spots at the base of the hairs on the side of the body. These caterpillars spin large silky webs around foliage. The colony feeds within the web, expanding its size as more food is needed. The unsightly webs provide protection to the caterpillars, making control difficult.
Insecticides for use in shelterbelts to control these insect pests are limited to diazinon, carbaryl (Sevin), and malathion. The caterpillars can also be treated with Bacillus thuringiensis (Bt) products. The Bts will not control the sawfly pests. If treatments are limited to landscape plants, there are other insecticide options available, such as chlorpyrifos (Dursban), cyfluthrin (Tempo), and acephate (Orthene).