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ISSUE 13   July 27, 2006

PEST ALERT: TWO-SPOTTED SPIDER MITES IN SOYBEAN IN ND

Two-spotted spider mites have been observed in about 30% of soybean fields surveyed in ND. Some locations include: Oakes, Glenfield, Devils Lake, Galesburg, Arthur, Casselton, and Wyndmere. Overall, mite populations are low and found on the underside of leaves of the lower half of plant. Unfortunately, the weather forecast for dry and hot weather will aggravate and potentially increase mite populations. The life cycle of spider mites can be completed in only 5-14 days with fastest development rates occurring above 91F. Each female lives for 30 days and she produces about 300 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. Mites thrive on the stressed plants that are nutrient rich!

Damage: Leaf injury symptoms appear as stippling first and then progresses to yellowing, browning or bronzing as feeding injury increases, and eventually leaf drop (see photo). Feeding injury causes water loss from the plant and reduces the photosynthetic ability of the plant. In severe cases, premature leaf senescence and pod shattering will occur and even plant death. When a 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.

Leaf injury
Leaf injury

Scouting: When scouting for spider mites, look on the underside of leaves and lower foliage for a tiny mite and fine spider-like webbing (see photo). Adult spider mites are small (< 0.2 inch), greenish-white to orange-red in color, and have 2 dorsal spots and 4 pairs of legs (see photo). 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 (yellow dots), which crawl slowly over paper. Predatory mites move faster than the two-spotted spider mite. When spider mites need to move due to diminishing food supply, they climb to the tops of plants and are dispersed by the "ballooning" on the wind. This allows them to spread quickly within a field or to adjacent fields.

Spider mite webbing
Spider mite webbing (D. Cappert, www.insectimages.org)

Adult spider mite
Adult spider mite (D. Cappert, www.insectimages.org)

Treatment Thresholds: Before pod set, the threshold is when 20-50% of soybean leaves are discolored. After pod set, the treatment threshold is 10 to 15% of the leaves discolored (Gray 2005, http://www.ipm.uiuc.edu/bulletin/print.php?id=354).

If spider mites are above threshold and significant pod or seed fill remains, an organophosphate insecticide (e.g. Lorsban, Dimethoate) is recommended over a pyrethroid insecticide. Pyrethroids (e.g. Asana, Baythroid, Decis, Mustang Max, Proaxis, Warrior) can flare (increase) mite populations within 7-10 days after application due to: disruption of the natural enemies that control spider mites (predatory mites); increased movement of mites, and increased reproductive rates of female mites.

IPM: When scouting soybean fields, consider which insect pests (soybean aphid and spider mites) are present and their population levels. If the heat and drought stress continues, we may see an increased risk for spider mites and reduced risk for soybean aphids (increased mortality and decrease reproductive rate due to hot temperatures >90F). If heavy rains occur, mite and aphid populations can collapse. Early mite infestations are often concentrated on field edges, and spot treatments can be effective. However, under dry conditions, mites usually will occur throughout the field and spot treatments are unlikely to prevent the infestation from spreading. Early detection facilitates timely and effective rescue treatments, but current insecticides do not provide long-term protection from the pest. Fields will need to be re-monitored continually for resurging populations. Insecticide efficacy can be improved significantly by using >10 GPA of water and applying at high pressure to penetrate foliage. Insecticides labeled for mite control have 21 (Dimethoate) to 28 day (Lorsban) harvest intervals. Consequently, if infested fields still have green seeds but seeds are filling, it may be better to accept some yield loss from mites and not treat, than treat and be unable to harvest. Remember that spider mites are not normally a significant pest problem in ND. However, hot dry conditions are conducive for spider mite development and field scouting is necessary to detect outbreaks early and facilitate timely effective treatment actions. See Red River IPM webpage for more photos of mite feeding injury: http://www.nwes.umn.edu/ent/redent.html

 

INSECT IDENTIFICATION QUIZ

Question: What is this arthropod? It has been observed on the underside of leaves in soybean, corn, alfalfa, and other crops in North Dakota this year.

Phytoseiulus mite
(courtesy Bugalogical Control Systems, M. Verkooy)

Answer: This is a predatory mite called Phytoseiulus species (Acarina: Phytoseiidae). Mites are not insects, they are more closely related to spiders. Predatory mites are one of the most successful commercially available biological control agents. Predatory mites have been particularly successful in greenhouses and vegetable production (strawberries). Adult Phytoseiulus eat from 5-20 prey (eggs or mites) per day and they reproduce more quickly than the spider mites at temperature above 82F. Predatory mites will starve to death if they cannot locate new colonies of spider mites. Scouting is therefore important to ensure proper timing and placement when releasing predators. Unfortunately, little is know about the role of predaceous mites in soybeans.

 

TIME TO SCOUT FOR SUNFLOWER HEAD INSECT PESTS SEED WEEVIL, BANDED SUNFLOWER MOTH, AND LYGUS BUG

Red Sunflower Seed Weevil:

Field reports of adult seed weevils on early blooming sunflower fields indicate the beginning of weevil emergence and high populations in south central and central ND. This is a reddish-brown weevil about inch (see photo). Newly emerged adults feed on the bracts, sunflower buds, and pollen. Pollen is necessary for fertile egg development. Field scouting for adults should begin when plants are showing yellow ray petals (R5.0) to 30% of the head shedding pollen (R5.3), and should continue until most of the plants have reached 70% pollen shed (R5.7). The easiest method for scouting for adult seed weevils is using a can of mosquito repellent with DEET. Spray the sunflower heads and wait 15 or more seconds for the adult weevils to move to the face of the head. Then, count the number of adult weevils on numerous sunflower heads at several locations and calculate the average number of adult weevils per head. The economic threshold is only one adult weevil per head for confections and about 6-8 adult weevils per head for oils. The best time to treat is when more than half of the plants in a field are beginning to show yellow ray petals (R5.0) to 30% of the head shedding pollen (R5.3) and the rest of the plants in the field are still in the bud stage. If spraying is done too early, weevils can re-infest a field requiring a second treatment.

Red Sunflower Seed Weevil
Red Sunflower Seed Weevil

Banded Sunflower Moth:

Field reports indicate high populations of banded sunflower moth in the northern tier of ND, especially in the north central and northeast regions. Eggs can be found on the bracts of sunflower heads. Pheromone trap captures indicate peak activity during the week of July 13-17 in Prosper (Source: L. Charlet, USDA ARS). Fields should be scouted when plants are in the mid bud stage (R3) for eggs and in the R3 to early bloom stage (R5.1) for adult moths. Two economic thresholds are available for adult moths in oil or confection sunflowers based on the time of day that you sampling for the adult moths:

EVENING OR EARLY DAWN
One adult moth per 2 plants

DAY (late morning to early afternoon)
During the day, adult moths rest on foliage and flutter from plant to plant when disturbed.

Use the following formula based on treatment costs, plant population and market price to determine the number of adult moths per 100 plants during DAY.

EIL

= (

(Treatment Cost ($) / Market Price)

) x 582.9) - 0.7

(moths per 100 plants)

Plant Population

A sample calculation of the EIL based on moth sampling during the day for the following conditions is given below.

Insecticide treatment cost = $8.00/acre
Market price = $0.09/lb.
Plant Population = 20,000/acre

EIL

= ((

$8.00 / $0.09

) x 582.9) - 0.7 = 1.9 moths per 100 plants

20,000

For this set of variables, an infestation of about 1.9 moths per 100 plants will result in sufficient larvae to destroy seeds in the sunflower head equal to the $8.00 treatment cost per acre in a field of 20,000 plants per acre with a market value of 9 cents per pound. If the adult population has reached or exceeded this level, then the grower should consider the use of a chemical insecticide to prevent larval seed damage.

The egg-sampling method was described in the 11th issue of Crop & Pest Report, July 13, 2006. Or, see website for more information:

http://www.ndsu.nodak.edu/entomology/econ_calc.xls

The best time to apply an insecticide is when the insects are hatching from eggs or young larvae are present (prior to third instar tunneling into seeds). Typically, this is when the majority of the sunflower plants are R5.1 plant stage or when the plant has just begun to shed pollen. However, this year high temperatures are pushing insect development and young larvae are hatching and causing injury to buds at stages R3-R4. Waiting until stage R5.1 could result in bud damage and reduced seed set. However, if an early application is made the field should be checked again in 3-4 days for re-infestation.

Lygus bug:

Lygus bug is mainly a pest problem in confection sunflowers causing kernel brown spot (see photo). Adult Lygus bugs are about inch in length, and pale green, light brown, or dark brown with a distinctive triangular marking on its back (see photo). Immature nymphs hatch from these eggs and look like aphids. Field reports indicate that Lygus bugs are common in sunflower fields. Scout for adults or nymphs on the sunflower head or foliage. One Lygus bug per 9 heads (economic threshold) results in economic loss to the producer through the reduction of seed quality. Spray timings coincide with timings for control of banded sunflower moth and sunflower seed weevils early pollen shed (R5.1). A second application, seven days later, is recommended for optimal control of Lygus bugs in confection sunflowers. Oilseed sunflowers are not believed to be at risk to injury from Lygus feeding.

Kernel brown spot from Lygus bug injury
Kernel brown spot from Lygus bug injury

Lygus bug
Lygus bug
(courtesy M. Boetel, NDSU)

 

BEE ALERT: PROTECT OUR VALUABLE HONEY BEES!

Remember to protect honeybees when spraying any flowering crops and only spray when its absolutely necessary. Pollinators can increase yield of crops and are vital to our ND beekeeping industry. If it is necessary to spray, apply insecticides when there is minimal bee activity, preferably during the late evening or early morning hours. Late evening is better than early morning. Honey bees are primarily active during the day. During most summer evenings, honeybees leave fields by 8 PM and do not return until 8 AM or later the following day. Minimized the potential for pesticide spray drift by not spraying on windy nights (>10 mph). Be sure to know the location of beehives and contact the beekeeper at least 48 hours before insecticide application. The beekeeper may move hives or cover the hives so bees will be contained until the residual risk of insecticide has decreased. A list of the locations of 2006 beehives by county can be obtained from the ND State Department of Agriculture:

http://www.agdepartment.com/Programs/Plant/Apiary.html

Honey bee
Honey bee (courtesy, L. Tobias)

Janet Knodel
Extension Entomologist
Janet.Knodel@ndsu.edu


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