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ISSUE 8   July 1, 2010

INSECTICIDES DO NOT CONTROL WHEAT STEM SAWFLY

Wheat stem sawfly populations are reported to be high again in wheat fields in southwest North Dakota. Some producers are justifiably nervous from the yield losses that they suffered from last year. Thus, producers are interested in spraying insecticides for control of adult sawflies. However, before you waste your time and money, read this article!

In 2009, NDSU Extension Entomology conducted a large plot (25-acre) insecticide trial near Mott where we sprayed three times with a pyrethroid for sawfly control. We found that three applications of a pyrethroid insecticide timed for the beginning, peak and end of sawfly flight reduced infested stems by half compared with the untreated check. However, the yield gain was not significant and only a 3.3 bushels per acre in the insecticide-treated plot versus the untreated check. The estimated cost of the insecticides was $30 per acre (or $10 per acre per insecticide application). If wheat is valued at $5 per bushel, the gross revenue increase due to yield is $16.50 per acre. This results in a net loss of $13.50 per acre ($16.50 minus $30), in spite of the three applications of insecticides!

NDSU Extension Entomology also studied insecticides and spray timings for control of wheat stem sawfly at Hettinger and Makoti in 2009. The following treatments were evaluated: 1) untreated check, 2) foliar insecticide at the 4-6 leaf stage, 3) foliar insecticide at flag leaf stage, 4) low rate of insecticide seed treatment, 5) high rate of insecticide seed treatment, and 6) low rate of insecticide seed treatment + foliar spray at the 4-6 leaf stage. Cruiser 5FS was used as the insecticide seed treatment and Warrior II (pyrethroid) was used for all foliar insecticide applications. All seed treatments were applied commercially. Results from Makoti are shown (Fig. 1). For percent infested stems, all of the insecticide treatments were comparable except for the low and high rate of Cruiser had a significantly higher percent infested stems than the low rate of Cruiser + Warrior II at the 4-6 leaf stage. Infested stems of the insecticide treatments were not different from the untreated check. As expected, there were no significant yield differences among the different insecticide treatments or between insecticide treatments and the untreated check. The untreated check had the lowest yield, which was probably due to root rot diseases because no fungicide seed treatment was used (bare seed).


Figure 1.
2009 Wheat stem sawfly insecticide timing trial at Makoti.

Insecticides generally have not been effective against the wheat stem sawfly. The egg, larval and pupal stages are well-protected inside the plant stem. Spraying for adults has not been successful because newly emerged adults can migrate into a field that was sprayed, the sawfly emergence window is so long and adults that emerge after spraying have reduced exposure to insecticide. The adult has no mouthparts and does not feed or drink water, which minimizes exposure to insecticides. Overall, insecticides are relatively costly for a low-value, large-acreage crop such as wheat; ineffective in controlling wheat stem sawfly; and damaging to beneficial parasitoid populations.

 

SCOUTING FOR WHEAT MIDGE CRITICAL IN NORTHERN TIER OF NORTH DAKOTA

With the localized hot spots for the wheat midge this year, field scouting will be important. Please see the past issue no. 7 of the Crop & Pest Report for 2009 Wheat Midge Larval Soil Survey map and scouting protocol. The current DD map (Fig. 2) indicates that wheat midge emergence is starting in the northern tier.


Figure 2.
Wheat midge degree day map (NDAWN)

Wheat is in the susceptible growth stage (heading to early-flowering) when the wheat midge were at peak emergence in many areas. An insecticide should be applied during heading and when the adult midge density reaches one midge per four to five wheat heads for hard red spring wheat or one midge per seven to eight heads for durum. A late insecticide application should be avoided to minimize negative impacts on the parasitoid. Wheat midge larvae feed on the kernel and negatively affect yield, grade and quality.

 

START SCOUTING FOR SOYBEAN APHIDS

The cool weather has not been optimal for soybean aphid development and this has slowed any field infestations. The current weather forecast is for warming temperatures, and temperatures in the high 70s to low 80s will become more favorable for soybean aphid development. Sixteen soybean fields were surveyed by the IPM Survey scouts last week. Soybean aphids were found in only one field in northern Richland County (Fig. 3). Low population levels (about <10 aphids per plant) were also reported in southwestern Minnesota, northern Iowa and no soybean aphids in South Dakota. So, now is a good times to start scouting soybean fields for soybean aphids.


Figure 3.
Soybean aphid IPM map (J. Walker, NDSU)

Regular scouting (once a week) is important to determine which fields have economic population levels of soybean aphids. Fields near buckthorn, the overwintering host, may be colonized at emergence and require earlier scouting. In areas without buckthorn, winged aphids migrate from other areas in mid-season. Late July and early August infestations in North Dakota have been strongly influenced by migrating aphids from soybeans south and east of the region. Check 30 to 40 plants per field. Examine the entire plant, particularly new growth. Use an action threshold of 250 aphids per plant if populations are actively increasing on 80% of the field for plants in late vegetative to R1 (beginning bloom)-R5 (beginning seed) soybeans.

Avoid early-season application of insecticides for control of sub-economic populations of soybean aphids. Why? Hereís four good reasons to avoid spraying early: 1) kills beneficial insects like lady beetles, lacewings, nabids that keep aphid populations low and below economic threshold levels; 2) increases secondary insect pest outbreaks, like spider mites. Early spraying of pyrethroids for aphids causes mite populations to flare; 3) requires second insecticide application to control aphids and/or mites increasing input costs; and 4) increases the risk of aphids developing insecticide resistance due to unnecessary applications of insecticides.

 

BANDED SUNFLOWER MOTH EMERGENCE STARTING

Emergence of the banded sunflower moth is underway as the first moth was trapped in a pheromone trap in Mapleton, Cass County, ND.

 

UPDATE ON DEGREE DAYS FOR COLLECTING LEAFY SPURGE FLEA BEETLES

The accumulated growing degree days (AGDD) for sunflower (base of 44 F) can be used as a guide to determine when to begin scouting for adult flea beetles. Begin scouting for adult flea beetles when the AGDD approaches 1,000. Flea beetles should be collected between 1,200 and the 1,600 using the sunflower GDD from NDAWN. Most of North Dakota has accumulated between 1,200 and 1,600 AGDD, except the northwest corner (Fig. 4).


Figure 4.
Leafy spurge flea beetle degree day map (NDAWN)

 

FUN INSECT QUESTION

What is this bright yellow fly that is common in sunflower and other crops near sunflower fields? Is it a major insect pest?


Figure 5.
Mystery insect (NDSU Extension Entomology)

Iíve been getting many questions on what this common, showy yellow fly is and is it a pest. This is one of the Sunflower Maggots, Strauzia longipennis (Wiedemann) (Diptera: Tephritidae). This sunflower maggot is the only tephritid species found in the stalks of cultivated sunflower. It is a widespread species, occurring in most areas of the United States and many Canadian provinces. The yellow adult (Fig. 5) has a wing span of about 0.5 inch (13 mm) and a body length of 0.25 inch (6 mm). The wings bear broad dark bands that form a fairly distinct F-pattern near the wing tip. The adult fly is very active during the day and is present in fields until late July.

Damage: Economic loss due to larval feeding has not been documented for this species, even though larvae are commonly found in up to 100% of sunflower stalks. Feeding is confined to the pith, which acts as a supporting structure, and is not critical to plant nutrition. Secondary fungal infections are also associated with tunneling by the larvae within the stalk. Stalks are not weakened and seed yield is not reduced even with severe pith destruction. Insecticide use has not been warranted for control of sunflower maggot and no scouting methods or economic thresholds have been established.

Janet Knodel
Extension Entomologist
janet.knodel@ndsu.edu


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