NDSU Crop and Pest Report

Entomology


ISSUE 10  July 3, 2003

CEREAL APHIDS STILL A MAJOR CONCERN

Though nothing was mentioned in last week’s newsletter regarding aphids, they are still a major concern in our small grains. The areas of concern will move northward to the younger wheat. The Bird Cherry-oat aphids that are colonizing our fields have begun migration from fields in the southern counties and are moving northward to younger fields. In many cases they are already in the northern fields and colonies are increasing in size.

IPM survey reports from the north central counties were finding up most fields with the 25% infested stem levels. These numbers will change quickly this week. Most of the surveyed wheat in those counties was in the tillering to jointing stages. Controlling aphids at the flag leaf stage generally produces the greatest yield response when compared to untreated checks.

IPM survey map

Barley Yellow Dwarf Virus

Of great concern in the small grains is the appearance of Barley Yellow Dwarf Virus (BYD), which is transmitted to our wheat, barley and oats by the aphids. The Bird cherry-oat aphid is an efficient vector of BYD, and vectors the most virulent strain, PAV. There are lab confirmed reports of BYD infections in fields in our southern counties. Growers in northern counties with younger wheat are encouraged to scout for and respond to aphid infestations quickly.

Trying to protect wheat from BYD is difficult. In the past, I have suggested a more aggressive approach for making treatments to try and minimize infection risk. Treatments should be considered if 50 to 75% of the stems have at least one aphid present (this is less than the 85% infested stems normally used as a guideline). It may be difficult to assess populations of aphids at these low levels. Keep in mind, also, that infections in younger plants have greater impact on yield than infections in older wheat. Infections during vegetative stages may reduce yields 30% or more; infections in reproductive stages may only impact yield 5 - 10% or less.

 

SUNFLOWER INSECTS PICK UP MOMENTUM

Sunflower Beetle Larvae Hatching

As larvae hatch, scout fields to determine if the larval threshold has been reached. The treatment threshold is 10 to 15 larvae per plant. Leaf damage will likely be light from early larval feeding. However, defoliation will reach the damaging level of 25 to 30% if populations are present at the treatment threshold and are allowed to develop. As the larvae grow in size, their leaf consumption will increase both in quantity and speed. Delaying treatment, though the larval numbers suggest the need for control, only results in a ragged, stressed plants during flowering.

Insecticides labeled for sunflower beetle include: Asana, Baythroid, carbaryl, Furadan, Lorsban, Scout, and Warrior.

Spotted Stem Weevil Emerging

There is only one generation per year. Adults emerge in mid to late June feeding on the epidermal tissue of the sunflower foliage and stem. This feeding does not affect plant vigor. Mating occurs soon after emergence of adults. Just prior to egg laying, females descend to the lower portion of the plant to deposit eggs individually in the stem tissue. Approximately 50 percent of oviposition occurs by mid July.

Field monitoring for sunflower stem weevils to estimate population is important. However, adults are difficult to see on the plants due to their small size, cryptic color, and "play dead" behavior. They are inactive on the plant or fall to the ground when disturbed and remain motionless. Adults can be found on both surfaces of the leaves, the lower portions of the stem, in leaf axils, within the dried cotyledons, or in soil cracks at the base of the sunflower plant.

Field scouting for adults should begin when plants are in the eight to14 leaf stage, developmental stage V-8 to R1, or late June to early July, and continue until mid-July.

Sunflower Midge Emergence Begins

SF midge emergence began during the July 4th week at Mapleton, ND according to Dr. Gary Brewer and the sunflower insect research project group. Though there still is no treatment recommendation for this insect, field scouting may reveal adult midge, eggs and soon larvae at the base of the leaves or perhaps bracts on older plants. The female midge prefer to lay eggs on sunflower buds with a diameter greater than 1 inch. Larvae initially feed on margins of the head between the bracts surrounding the heads.

 

UNIVOLTINE CORN BORER EMERGENCE UNDERWAY

One more insect to begin looking for in our field crops. A degree day model was developed to predict occurrence of the univoltine flight of corn borer moths. As with other degree day models, it should help us identify priority times for field scouting. The models pinpoint the occurrence of key biological events. In this case the model is indicating the proportion of moths that have emerged based on temperatures.

Degree Day (Modified Base 50EF) Model for Moth emergence of Univoltine-type European Corn Borer

Accumulated Degree Days

Proportion of Emerged Moths

911

10 %

986

25 %

1078

50 %

1177

75 %

1274

90 %

Current degree day accumulations around the major corn production areas would indicate that we are around the 10 to 20% emergence level of moths. By next week, when the 50% level is reached, we should be able to start picking up egg masses in fields.

Degree Day accumulations map

 

LEAFY SPURGE BEETLES EMERGING

The adult leafy spurge beetles used for biological control of this noxious weed are beginning to emerge around the region. If you are interested in obtaining beetles for release in your spurge trouble areas, contact your local weed control officer to see if there are any collection dates scheduled for your area.

Phillip Glogoza
Extension Entomologist
pglogoza@ndsuext.nodak.edu

 

APHID ALERT: Potato insect update for the Northern Great Plains, week ending June 27

http://ipmworld.umn.edu/alert.htm  

Last week's prolonged (45 h) low level jet (wind event) and accompanying thunderstorms provided perfect conditions for the long distance transport of aphids and leafhoppers from the south and their deposition across the Northern Great Plains.

Adult potato leafhoppers are abundant in the region and early stage nymphs (mostly first instar) can now be found feeding on the under side of potato leaves. Potato leafhoppers have 5 nymphal instars. Insecticidal treatments should be applied before the nymphs reach fourth and fifth instars. The recommended threshold for treatment to control potato leafhopper is very low: 10-15 late instar nymphs per 100 true leaves (as opposed to leaflets) will cause economic damage. Potato leafhoppers are easily killed by almost any persistent insecticide. Dimethoate, Asana, and Baythroid (Leverage) are examples of products that work very well. Application of these products at rates as low as 1/4 label rate provide good control of potato leafhopper nymphs and tend not to flare aphid outbreaks.

Potato aphids are very abundant in some fields, e.g., densities approaching 1 per leaf were observed near Climax, MN. The most abundant winged aphid captured in suction traps in Manitoba last week was buckthorn aphid. Green peach aphid numbers are still very low, but potato colonization has been detected. Typically, we capture few winged green peach aphids in our traps before the end of July. The initial colonizers probably are the progeny of long distance migrants that presumably arrived virus_free. Of course, these aphids can acquire and move virus if they encounter infected potatoes.

Colorado potato beetle eggs are now hatching in northern Minnesota and eastern North Dakota. In warm weather these insects can complete a larval instar in as little as 2 days. They have only 4 instars and the last two are voracious feeders. Growers should monitor their fields closely and be prepared to spray next week. Insecticides applied when Colorado potato beetle are just hatching or in their early instars are much more effective than insecticides applied when the beetle larvae are in their third or fourth instars. Insecticide resistance can limit product choice. If a nicotinyl insecticide, e.g., Admire or Platinum, was applied at planting, it shouldn't be necessary to spray for 1st generation Colorado potato beetle control. However, if foliar sprays are subsequently used on the crop, you should chose a different class of insecticide.

Edward B. Radcliffe
Department of Entomology
University of Minnesota
radcl001@umn.edu

 

LEAFMINERS IN AREA SUGARBEETS

Leafminers are showing up again in Red River Valley sugarbeet fields this season. Infestations have approached the economic threshold (50% of plants infested) in a few fields; however, no major losses have been reported. Most reports have come from the Grand Forks area. Description & damage: adults are small, clear-winged flies with brown or grayish to brown or greenish bodies that look like small hunch-backed house flies. They hold their wings above their backs when resting and have numerous hair-like spines on their backs (Fig. 1)

Leafminer image

Figure 2.  Full-grown leafminer larva inside "mine."

Females lay groups of 3 to 10 white, oval eggs on the undersides of beet leaves. Eggs hatch within 3 to 10 ten days, depending on air temperatures. After hatching, the larvae (pale-green to whitish colored worms; tapered from front to back [Fig. 2]) tunnel into and feed between the upper and lower surface of the leaf, creating "mines".

Necrotic blotch image

Figure 3.  Necrotic blotch on sugarbeet leaf from leafminer injury.

The mines expand and run together as feeding progresses, and the injury leads to necrotic leaf blotches (Fig. 3).

Leafminer larvae are easily detected by holding an infested leaf up toward a source of light such as the sun. Leafminers can have up to 3 generations per year, although the first generation usually causes the most damage in sugarbeets. The insects are probably ending their first generation and dropping to the ground at this time. Although major problems are not expected to develop later in the season, fields should be monitored for the next couple of weeks to determine if treatment will be needed.

Scouting: Field scouting and early detection are crucial for effective control. Scouting is important because leafminers may only be a major problem isolated fields. Early detection will allow for more effective use of insecticides because they work best when applied just before or at egg hatch; however, very good control has been achieved after larvae have already tunneled into leaves. To scout a field, sample several sets of 10 random plants in several representative areas within a field. The more samples taken, the more reliable the estimate will be. The current recommendation is for fields to be treated if 50 percent or more of the plants have egg masses visible mines.

Control: Refer to Table 1 for insecticide options.

Please note that products without leafminers listed specifically on the label can be used as long as they are registered for use as a foliar application on sugarbeets; however, the manufacturers are not liable for losses or unsatisfactory performance associated with control failures if their label does not explicitly offer leafminer control. Also, if planning to tank mix with other materials such as microrate herbicide treatments when spraying for leafminer control, remember that liquid formulations of oil-based insecticides add oil to the mix and may increase chance of crop injury from the herbicides. To minimize the chance of crop injury, reduce the amount of crop oil by 50% or by the amount of insecticide volume used in the mixture.

Table 1. Treatment options for controlling leafminers in sugarbeet.

Product

Use Rate / Acre
(formulated product)

Re-entry Interval
(hours)

Asana XL 0.66 EC*

9.6

12

D Z N diazinon 50W RUP

0.75 – 1.0 lb

24

D Z N diazinon AG500 RUP

0.75-1.0 pt

24

D Z N diazinon AG500 WBC (water-based concentrate) RUP

10.0 - 13.5 fl.oz

24

D Z N diazinon 500AG RUP

0.75-1.0 pt

24

Lannate SP* RUP

0.25 - 1.0 lb

48

Lannate 2.4LV* RUP

0.75 - 3.0 pt

48

Lorsban 4E

1 pt (2/3 pt banded)

24

Mustang Max 0.8EC

2.24 – 4.0 fl oz

12

RUP – Restricted Use Pesticide

*Labeled and legal for use in sugarbeet, but manufacturer does not include leafminer control on label and satisfactory control is therefore not warranted.

Always read, understand, and follow all label instructions and precautions!

Mark Boetel
Research & Extension Entomologist
mboetel@ndsuext.nodak.edu


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