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ISSUE 1   May 14, 2009


With the flooding this past spring in many areas, there are plenty of pools of standing water for mosquitoes, and running streams for black flies to breed in. It is not a surprise that will we be dealing with these biting insects that interfere with fun outdoor activities.

Adult mosquito
Adult mosquito
, Aedes triseriatus
(photo by Susan Ellis, Bugwood.org)

Mosquitoes: Mosquitoes need water to complete their life cycle and the first three life stages (egg, larva, and pupa) require water for development. In the summer, mosquitoes can complete their life cycle in 7-30 days depending on species, temperature and environment. Residential areas are a major source of breeding sites. Man-made objects such as bird baths, uncovered boats, discarded tires and plant pots allow mosquitoes to breed in our own backyards. Ditches along roadways often become filled with water and are also ideal breeding sites for floodwater mosquito species. To minimize breeding site for mosquitoes around the home:

  1. Remove standing water that has accumulated in man-made objects tires, bird baths, uncovered boats and plant pots, anything that will hold water.
  2. Flush water out of bird baths and pet dishes at least twice a week
  3. Clean debris out of roof gutters.
  4. Correct any drainage problems in yards so rainwater will not pool in low areas.
  5. Fill tree holes with cement or sand.
  6. Swimming pools or spas should be chlorinated. If not in use, cover.
  7. Repair leaky faucets or air conditioning units.
  8. Keep grass and shrubs around your house well-trimmed.

For protection against mosquito bites and diseases that they vector (such as West Nile Virus) follow these guidelines:

  1. Wear long pants and long-sleeved shirts and earth-tone color clothing (brown, olive, khaki).
  2. Use mosquito repellent containing DEET, picaridin, or oil of lemon eucalyptus (biopesticide).
  3. Fix holes in screen windows and doors to prevent mosquitoes from getting indoors.
  4. Stay indoors at dawn and dusk when mosquitoes are most active.
  5. If you are planning an outdoor picnic or activity, spray trees and shrubs with ready-to-use insecticide sprays, aerosols or foggers the day before your activity. Always read and follow the pesticide label.

Additional mosquito information is available at the website for the Centers for Disease Control and Prevention.


Black Flies: Black files are pests of humans and animals and most complaints are in May to June in North Dakota. They are commonly called buffalo or turkey gnats and belong to the insect family Simuliidae. The immature stages (eggs, larvae, and pupae) develop in moving water and will die within 10-75 hrs without moving water to provide oxygen and food. Only the females bite and males feed on plants. Bites can be painful and cause dermatitis and itching for several days. Black flies are attracted to dark colors so wear lighter colored clothing. Unlike mosquitoes, black flies only bite during the day and prefer low winds. Losses to livestock and poultry are difficult to estimate, but decreased milk, beef and egg production are common when high populations of black flies are present. Black flies are also known to vector diseases. Flies live for about three weeks.

Adult black fly
Adult black fly
(photo courtesy Pest
Control RX.com)

There is little that the homeowner can do to control black flies, except to avoid outdoor activities during peak black fly season. Using protective repellents have only some relief depending on species of fly, temperature, humidity and time of day. Fine mesh screening can be used over hats to protect the face. The best method of control is to reduce the number of breeding areas. Removal of vegetation and objects in streams will help minimize the number of larvae, since larvae attach to submerged objects during development. Temporary damming of water can also lower populations as immature stages require running water to survive. Any insecticide used as a fogger provides only temporary relief as black flies can fly 7-10 miles or can be blown by the wind even further. Treatment of streams is only effective if timed correctly and on a large scale community effort by local government programs. A bacterial biopesticide, Bacillus thuringiensis var. israelensis, is a larvicide that is nontoxic to nontarget organisms such as fish, birds, etc. and can be applied to streams by aircraft.



White grubs are larvae of scarab beetles, also known as June beetles. Larvae are white with a brown head capsule, C-shaped and about 1 inches long at maturity. They feed on the roots of many vegetable crops grown in the home garden, including potatoes, corn, beans, beets, turnips and other root crops. White grubs cause injury by pruning roots and chewing large, shallow circular holes in roots. Sometimes the plants wilt or turn yellow; however, often the damage is not noticed until harvest.

White grubs
White grubs
(photo courtesy University of
Georgia Entomology Archive, Bugwood.org)

In North Dakota, the life cycle of white grubs normally takes three years to complete its life cycle.

First year - In May and June, beetles emerge and at night fly to trees to feed, such as willow and poplar trees. After mating, females deposit 35 - 60 white eggs in the soil during the day. Eggs hatch in approximately 30 - 50 days, depending on soil temperature. First instar larvae begin feeding on organic matter after hatching, and later feed on plant roots. Most larvae reach the second instar stage before soil temperatures begin to decline in the fall. With cooling soil temperatures, larvae descend into the soil profile where they spend the winter below the frost line.

Second year - Larvae begin their upward migration as soil temperatures increase in the spring. Most larvae do not reach the 0 to 6 inch soil layer until the last week of May. It is during the second year that larvae are expected to cause the greatest level of feeding injury. Larvae molt to the third instar by July and continue feeding through the entire summer, being found in the upper 6 inches of the soil at the base of plants until a killing frost. At that time, third instar larvae descend in the soil profile to overwinter below the frost line.

Third year - Larvae are found in the upper soil layers by early May. The larvae feed on seedling roots, but seldom cause significant injury. By early August, pupae and adults can be found at depths of 6 to 18 inches in the soil. Adults represent the largest percentage of the overwintering population. The next May and June the adults emerge, repeating the three year cycle.

During soil sampling in the late summer and fall, all larval instars, pupae, and adults can be found. However, usually one brood dominates, representing the greatest proportion of the population in all three years. As long as one brood dominates, significant feeding injury is expected only in one year out of three. The year of greatest injury should correspond with the second year of the life cycle, when second instars are the most numerous in the spring. Based on past infestation patterns in southeastern North Dakota, 2009 should be in the second year for the dominant brood, which is when white grubs cause significant injury to plants.

Management of Grubs in Home Gardens:

  1. Expose grubs to predators during the late summer or early fall by plowing and turning the soil.
  2. Avoid planting garden vegetables in areas where grass, corn, potatoes or old strawberry beds were the previous year. Grubs are often a pest problem when areas of lawn were turned over in the spring for garden use.
  3. Keep the garden weed-free and clean. Adult female beetles prefer to deposit eggs in soils with lots of weeds.
  4. There are no insecticides registered for control of white grubs in home gardens because they are impractical and ineffective. Entomopathogenic nematodes are biocontrol agents of white grubs and are a natural component of most soils. These tiny worm-like hunters attack and killing white grub larvae. Nematodes are commercially available from most garden supply stores.



Onion maggots can occasionally be a serious pest in the home garden. Adult onion maggot flies resemble house flies with long legs and are about 1/3 inch long. Larvae of flies are called maggots and are headless and legless. Onion maggots bore into plant stems, causing the plants to turn yellow and wilt. Their tunnels make it easier for diseases to enter the bulbs and cause rot.

Onion maggots have three generations per year. Adult flies begin to emerge from pupae in the spring, usually in mid-May. Peak flights of adults occur about late May or early June. Adult onion maggot flies can survive for 2 - 4 weeks and may lay several hundred eggs. Female flies lay their eggs on the soil at the base of the seedling or in leaf sheaths, beginning about 7 to 10 days after emergence. The eggs hatch into maggots 2-5 days after being laid. The maggots develop through three larval stages in 2 to 4 weeks and feed externally or tunnel into the stem and bulb. Larvae use their hooked mouthparts to feed on the tissue of the onion plants. When mature, the maggot leaves the bulb and enters the soil to pupate at a depth of 1-4 inches. First and second generation pupae remain in the soil for 2-4 weeks before adult emergence occurs. The second flight of adults occurs in early July and the third flight in late August and September. Larvae of the third generation develop into pupae and will overwinter as pupae in the soil.

life stages of onion maggot
Life stages of onion maggot
(photo courtesy
J. Ogrodnick, Cornell Univ., NYSAES)

Management of Onion Maggots in Home Gardens

  1. Plant new plants in a different location from last year.
  2. Avoid over-fertilizing with manure and high organic matter soils. Maggots prefer to lay eggs in rich soil.
  3. Deep tilling or double-digging soil in the fall will help reduce the overwintering population of pupae.
  4. Remove weeds at least two weeks prior to planting.
  5. Cover seedbeds with a protective cloth or cover individual plants to prevent adults from laying eggs near plants. Various synthetic materials like polyethylene or woven plastics are available as physical barriers to prevent pest infestations and protect plants. Individual cone-shaped screen covers, made from standard window screen fixed to a wooden frame, can be useful in the home garden. They are easily constructed and can be used year after year. Covers can be removed when plants grow big enough to tolerate damage. Be sure transplants are maggot-free before planting them. Once maggots are infesting a crop, there is no reliable way to control them.
  6. Remove infested onions from the garden, so that maggots will not be able to complete their development.
  7. Prevention is the best management strategy as there are no insecticides available for use in the home garden.



Many area soybean producers are wondering if the adoption of an insecticide seed treatment will offer control of soybean aphid and whether this associated input cost is economically justifiable when a foliar insecticide application may also be needed. Extension Entomologists have been evaluating this issue. In 2008, NDSU Extension Entomology evaluated the following treatments at Johnson, MN for control of soybean aphid in a large strip trial:

  1. Untreated check
  2. Insecticide seed treatment alone (Cruiser MAXX at 3 fl oz/cwt)
  3. Foliar insecticide alone (Warrior II at 1.28 fl oz/acre) applied at the economic threshold (ET) of 250 aphids/plant in 80% of the field
  4. Combination of seed treatment plus foliar application at ET.

Soybean aphids reached ET during the first week of August, and Warrior II insecticide was applied on 7 Aug by a commercial applicator. Soybean was at R3 to R4 growth stage.

In 2008, aphid invasion occurred in late July and the population built up very rapidly. For pre-spray aphid counts on August 7, there were no significant differences in aphid densities among treatments (Fig. 1).

Treatment means indicated that aphid populations were at or just above ET. For aphid densities at seven days after application (14 Aug) and at 14 days after application (20 Aug), foliar sprayed treatments had significantly lower densities than the seed treatment alone or the untreated check. Treatment means on these dates indicated that aphids were well above ET for the untreated check and for the seed treatment alone, while mean aphid counts on the foliar treatment and seed treatment plus foliar were considerably below ET. For yield, the foliar treatment and the seed treatment plus foliar had significantly higher yields than the untreated check and the seed treatment alone. There was no significant difference in yield between the untreated check and seed treatment alone, or between the foliar spray treatment and the seed treatment plus foliar spray (Figure 2). These data indicate that there was no advantage in using an insecticide seed treatment to control soybean aphids when aphids invade the soybean field late in the season.

In 2008, Dr. Ian MacRae of the University of Minnesota evaluated foliar insecticides, foliar insecticides + fungicides, foliar fungicides, insecticide seed treatments, and insecticide seed treatments + foliar insecticide + foliar fungicide combinations for control of soybean aphid in Lamberton, Minnesota. Treatments included:

  1. Untreated check
  2. Cruiser MAXX seed treatment
  3. Warrior II applied at V3
  4. Headline applied at R3
  5. Warrior II applied at R3
  6. Warrior II & Headline applied at R3
  7. Cruiser MAXX & Headline applied at AP (at plant) & R3
  8. Cruiser MAXX, Warrior II & Headline applied at AP & R3
  9. Cruiser MAXX & Warrior II applied at AP & R3
  10. IPM (ET) Threshold

The R3 treatments were applied 29 July and the ET treatment was applied on 2 Aug. Warrior II was applied at the rate of 1.6 fl oz/acre and Headline was applied at the rate of 6 fl oz/acre. Treatments that kept soybean aphid population below ET and provided the highest yield included all of the foliar-applied insecticides at R3 (Treatments 5, 6, 8 and 9) and at ET (Treatment 10) (Figs. 3 & 4). Dr. MacRae looked at the economics of an intensive management approach costing $30-35/acre (insecticide seed treatment at $10-12/acre, foliar fungicide at $14/acre, and foliar insecticide at $10/acre) versus an IPM approach (treating at ET) costing $16/acre (scouting costs at $6/acre, foliar insecticide at $10/acre). The difference between the intensive and IPM treatments was at least $15/acre. If the average soybean producer in northwestern MN produces 550 acres of soybean, each producer would save approximately $8,250 if using the IPM approach over the intensive management approach!


These data indicate that a well-timed foliar insecticide applied at ET was the best pest management strategy to optimize control of soybean aphid and prevent unnecessary repeated insecticide applications. The foliar insecticide applied at ET generally had higher yield and was the most economical treatment. Seed treatments may have some early-season control of aphid infestation only when infestation occurs early in the season (June). However, peak soybean aphid infestations typically occur later in the year (late July or early August) in North Dakota and northwestern Minnesota. Remember, insecticide seed treatments are effective in controlling other early season pests of soybeans, such as wireworms, seed corn maggots and overwintering bean leaf beetles.

Janet Knodel
Extension Entomologist

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