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ISSUE 12   July 29, 2010

SOYBEAN APHID UPDATE

Soybean aphids continue to be very low in most soybean fields in North Dakota (Fig. 1 & 2). However, there are a few hotspots where alate (winged) soybean aphids are starting to show up in fields. Most of these reports are near the Red River Vallley. Alate soybean aphids will become more common as soybean approaches R4-R5, moving within the field or to neighboring fields or even migrating long distances on the winds. Soybean aphids will also start to move down in the canopy as soybeans mature.

       
Figure 1 & 2.
Soybean Aphid IPM Survey map (J. Walker, NDSU)

The weather forecast is good for aphid development, with temperatures in the low 80s. Remember the temperature in the soybean canopy is actually cooler than ambient air temperatures, which would favor aphid reproduction. However, severe thunderstorm can be detrimental to soybean aphids.

The bottom line is to continue scouting soybean fields for soybean aphids until the R6 (full seed) growth stage. Spray fields that reach the economic threshold of 250 aphids per plants on 80% of the plants in field. Remember, there is NO yield loss at the 250 aphids per plant level (economic threshold).

 

SPIDER MITES SHOWING UP IN SOYBEANS

There have been a few fields near Mayville that have needed to be sprayed for spider mites, in spite of our unfavorable rainy weather. When you’re out scouting for soybean aphid, look for spider mites. I’m re-running my article from 2009 on spider mite IPM.

An excellent publication on spider mites has been published by the University of Minnesota, which summarizes everything that you want to know about spider mites on soybeans and corn:  http://swroc.cfans.umn.edu/SWMNPEST/09publications/spidermite.pdf

Mites can be found on the undersides of leaves of the lower half of plant. The life cycle of spider mites can be completed in only 5-19 days with fastest development rates occurring at >90 F. Each female lives for 30 days and she produces about 100 eggs during her lifetime. In hot, dry weather, natural fungal diseases of mites are slowed and populations can increase from a few individuals to millions within a few generations. Remember, spider mites thrive on stressed plants that are nutrient rich!

Leaf injury symptoms appear first as stippling, and then progress to yellowing, browning or bronzing as feeding injury increases. Leaf drop eventually occurs. Feeding injury causes water loss from the plant and reduces the photosynthetic ability of the plant. In severe cases, premature leaf senescence, pod shattering will occur and even plant death will occur. When severe mite infestations occur during late vegetative and early reproductive growth, a 40 to 60% yield loss between treated and untreated soybean has been demonstrated in other north central states. Spider mites can cause yield reduction as long as green pods are present.

When scouting for spider mites, look on the underside of leaves and lower foliage for a tiny mite and fine spider-like webbing. Adult spider mites are small (< 0.1 inch), greenish-white to orange-red in color, and have 2 dorsal spots and 4 pairs of legs (Fig. 3). Nymphs are smaller than adults and have 3-4 pairs of legs. By shaking a leaf or plant over a white sheet of paper, you can see these tiny mites, which crawl slowly over the paper. When spider mites need to move due to diminishing food supply, they climb to the tops of plants, spin a silk ‘balloon’ and are dispersed by the wind, so they can spread quickly within a field or to adjacent fields. It is critical to scout early and check field edges where spider mites infest first. Scouting during during full pod (R4) through beginning seed (R5) stages is critical since these crop stages are the most important contributors to soybean yield. If hot dry conditions exist, mite populations can quickly increase and treatment should not be delayed.


Figure 3. Spider mites (Frank Peairs, CSU, Bugwood.org)

There is no specific threshold that has been developed for two-spotted spider mite in soybean. As a result, several guidelines are available from different states to determine economic infestation of spider mites in soybeans. Minnesota is using this scale to assess mite damage:

0 = No spider mites or injury observed

1 = Minor stipling on lower leaves, no premature yellowing observed

2 = Stipling common on lower leaves, small areas or scattered plants with yellowing

3 = Heavy stipling on lower leaves with some stipling progressing into middle canopy. Mite present in middle canopy with scattered colonies in upper canopy. Lower leaf yellowing common. Small areas with lower leaf loss. Spray Threshold.

4 = Lower leaf yellowing readily apparent. Leaf drop common. Stipling , webbing and mites common in middle canopy. Mite and minor stipling present in upper canopy. Economic Loss.

5 = Lower leaf loss common, yellowing or browning moving up plant into middle canopy, stipling and distortion of upper leaves common. Mites present in high levels in middle and lower canopy.

Remember to use an organophosphate insecticide (e.g. Lorsban, Dimethoate) instead of a pyrethroid insecticide to avoid flaring mite populations. Reasons for the increase in mite populations include:  disruption of the natural enemies that control spider mites (predatory mites); increased movement of mites out of fields, and increased reproductive rates of female mites. Early detection facilitates timely and effective rescue treatments. Current insecticides for soybeans provide short-term protection, maybe 7-10 days, from the pest. Fields will need to be monitored continually for resurging populations. The efficacy of an insecticide can be improved significantly with sufficient coverage (>10 GPA of water) and application at high pressure to penetrate foliage.

 

VIDEO ON SCOUTING FOR BANDED SUNFLOWER MOTH EGGS

Pheromone trap catches indicate that adult banded sunflower moth numbers are high (Fig. 4). Sunflowers in R2-R3 crop stage are preferred for egg laying stage. A video was produced to explaining how to scout for eggs and how to calculate the Economic Injury Level.   Follow this link to watch the video:  www.ag.ndsu.nodak.edu/aginfo/entomology/entupdates


Figure 4.
Banded sunflower moth pheromone trapping map (T. Mittelsteadt, NSA)

Head mounted optivisors can be purchased from Great Lakes IPM (http://www.greatlakesipm.com/) or Gemplers (http://www.gemplers.com). (Note mention of a product is not an endorsement by NDSU.)

  

WATCH FOR SUNFLOWER MOTH WHEN SCOUTING FOR OTHER SEED-INFESTING INSECT PESTS

Sunflower moth has also been detected in pheromone traps in North Dakota at low numbers (Fig. 5).


Figure 5.
Sunflower moth pheromone trapping map (T. Mittelsteadt, NSA)

However, this moth is blown in on southerly winds and can quickly increase in numbers. Sunflower moths are grayish-tan (Fig. 6) and will be attracted to the early blooming sunflowers. It deposits its eggs on the face of the flower.


Figure 6.
Sunflower moth adult (L. Charlet, USDA-ARS)

Eggs hatch within 48 to 72 hours and newly emerged larvae feed on pollen and florets. Larvae begin tunneling into seeds upon reaching the third instar growth stage. Tunneling continues throughout the remainder of larval development. Larval development from emergence to full maturity takes about 15 to 19 days. Larvae spin webs over the face of the sunflower head, which accumulates disk florets and frass. This gives a trashy appearance to the head. Mature larvae move to the ground where they spin cocoons in which they overwinter.

Damage is similar to that caused by the banded sunflower moth with larvae feeding on the florets and tunneling through seeds and the receptacle area (back) of sunflower heads. Larvae have alternating dark and light-colored longitudinal stripes on a light brown body (Fig. 7). The larva is about 0.75 inch (19 mm) long at maturity.


Figure 7.
Sunflower moth larva (P. Beauzay, NDSU)

Since female moths lay eggs on the face of sunflower heads, insecticide should be applied in early flowering (R5.1 - R5.3). Remember to protect our pollinators when spraying and treat in the late evening or early morning when bees are back in hives. Pollinators are important to sunflower yields through increased pollination.

The Economic Threshold is when 1 to 2 moths are found for every 5 plants inspected.

Janet Knodel
Extension Entomologist
janet.knodel@ndsu.edu


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