Sugarbeets XX-16-20

Sugarbeet Root Maggot

Gary L. Hein

Sugarbeet root maggot adults

The sugarbeet root maggot, Tetanops myopaeformis, is the most severe insect pest of sugarbeet in many parts of the High Plains region. Infestations begin in late spring and can reduce plant vigor and stand, resulting in lower yields. Effective management of this insect requires knowledge of the insect’s life cycle and information about the current population level.

Identification (and life cycle and seasonal history)

Sugarbeet root maggot adult flies are similar in size and appearance to the house fly (about 1/4 inch). Unlike the house fly, the body is shiny black with few hairs. The wings of this fly are transparent with a smoky-brown patch located on the front of the wing about one-third the distance from the wing base. Also, the legs have yellowish-white bands on the next to last segment (“ankles”), with the rest of the leg being black. The females have pointed abdomens and the males have rounded abdomens.

The eggs are elongate, slightly curved and white. The larvae are white, legless maggots that grow to about 1/3 to 1/2 inch in length. The head end is tapered to a point and the rear end is blunt. The pupae are tan to brown, elongate capsules about 5/16 inch long.

Sugarbeet root maggots overwinter as full-grown larvae about 10 to 14 inches deep in the soil. As temperatures begin to warm in the spring, the larvae move up close to the soil surface and pupate. In western Nebraska, sugarbeet root maggots pupate in April, and flies begin to emerge in early May. The flies move from last year’s sugarbeet fields to the current fields soon after emergence. The flies are not strong fliers, and movement is generally limited to localized flights to adjacent fields. Fly activity in sugarbeet fields increases under warm and calm conditions. During cool or windy periods the flies remain in sheltered areas along field margins (e.g. weedy, grassy areas or tree rows). Peak emergence and fly activity occur in late May or early June. The females lay eggs in the upper 1/4 to 1/2 inch of soil at the base of the sugarbeet plants or in the crown area of the beet. Eggs are laid in batches of a few to as many as 40, and a female will lay over 100 during her life. Survival of eggs and early larval stages is greatly reduced in dry soils. The larvae begin to feed on the sugarbeet roots and continue to feed for three to four weeks. By late June to early July, feeding ceases, but the larvae remain in the soil around the sugarbeet roots.

Plant Response and Damage

Root maggots feed on the surface of the sugarbeet root causing surface scarring. Deeper scarring and malformed roots may result from heavier feeding. Heavy infestations of the sugarbeet root maggot can cause severe stand loss, particularly with small plants, because the maggots feed on and sever the tap root. Severe damage is obvious because plants become severely wilted or die. If stands are not reduced, losses may still result from reduced plant vigor. Other stresses, such as hail, can more severely impact sugarbeet damaged by the sugarbeet root maggot because vigorous plants are necessary for recovery.

Management Approaches

Cultural practices will not eliminate sugarbeet root maggot problems but can reduce the severity of damage. Areas where close rotations of sugarbeet are used will likely have more serious problems because the flies move from the previous year’s sugarbeet fields to the current fields. If sugarbeet fields are concentrated in an area, more flies will be emerging, and damage potential will be increased. Conversely, in areas where there were

Establishing a vigorous sugarbeet plant as early as possible will also aid in reducing sugarbeet root maggot damage. The larger, more vigorous plants can withstand more damage, and stand reduction will be less likely.

Typically granular insecticides applied at planting have been used to control root maggots. Options have included Counter 20CR and 15G, Lorsban 15G, and Temik 15G. Organophosphate insecticides (Counter, Lorsban) sometimes have caused phytotoxicity problems when applied at planting. Counter has been shown to be the least phytotoxic of the organophosphates, and placement of the granules behind the planter press-wheel can reduce, but not eliminate the damage. All these products are influenced by environmental conditions. For example, control with planting time applications of Temik 15G can be severely reduced during wet springs because of its water solubility, and the chemical may be leached below the zone where control is needed.

Use of Lorsban 4E as a lay-by control of sugarbeet root maggots provides flexibility in managing several problems associated with the granular materials; however, proper timing is critical, and applications must be based on fly population information obtained from sticky-trap sampling. Phytotoxicity (leaf curling) can result from Lorsban 4E applications. Injury will be minimal unless the plant is stressed by other factors (e.g. hot and sunny conditions, wind damage, herbicide injury). Because of its phytotoxicity potential, Lorsban 4E should not be applied with Betamix or Progress, at either regular rates or micro-rates. To minimize damage potential, Lorsban 4E should not be applied within two days before or within one day after a Betamix or Progress application.

In areas of very serious maggot damage potential, layby treatments have been used in addition to planting time applications. In years when rainfall between planting and peak fly activity has been sufficient, planting time organophosphate treatments should provide good control and supplemental lay-by treatments should not be needed. However, if very little rain has fallen between planting and peak fly activity, a supplemental lay-by treatment may be needed to provide additional control. Lay-by treatments also may be beneficial in years when peak fly activity occurs later than normal in the season (e.g. mid June in western Nebraska) because planting time treatment would no longer be effective.

Once maggot damage begins to appear in the field, effective options to correct the situation are limited. Irrigation can help reduce damage once the maggots are feeding on the sugarbeet. Moist soil conditions will cause the maggots to move higher on the roots and be less likely to sever the tap root. Irrigation also will reduce water stress and the potential for stand loss. A lay-by nitrogen application may stimulate beet growth to help plants recover from damage. The value of this practice may be questionable if adequate fertility has already been applied. After damage has been observed, Temik 15G, because of its high water solubility, can be knifed in on the water side of the row (furrow irrigation) or banded over the top of the row (sprinkler irrigation) and watered into the soil. Very little control will be obtained if watering (or rainfall) does not occur after chemical application or if the insecticide is applied too late. Other insecticides are not water soluble enough to provide control of established maggots even with watering.

Sampling Adult Populations: Sugarbeet growers in areas where the sugarbeet root maggot is a problem can improve their management by using the orange sticky-stake trapping method originally developed in Idaho (Blickenstaff trap). This method can be used to monitor the development of fly populations in and around sugarbeet fields in May and June. In many areas of the region root maggot populations fluctuate. Without population information it is impossible to make an informed decision on the need to treat or how to treat for sugarbeet root maggot. Growers in these areas may be caught off guard when a problem eventually develops or they may waste dollars on treatments that aren’t needed. In areas where the root maggot is continuously a serious problem, growers have had serious control problems even with the use of planting-time insecticides. The sticky-stake method can be used to determine both the need and the proper timing for a supplemental lay-by treatment that will improve control in these serious situations.

The orange sticky-stake trapping method should be deployed early — the first week of May in western Nebraska — to catch the first fly activity of the season. As the season progresses, the size and duration of the fly population can be determined. Information gained from the use of the sticky-stake fly traps can be used to:

1. Determine the current population level in the field and assess the need for insecticide treatments in subsequent years in adjacent fields. Anyone just learning to use the trapping system should use this option. This allows one to get used to the trapping method and gain insight into the fly population level in your area. The presence or lack of dying beets in the field is not an accurate way to determine if flies are a problem. Monitoring the flies can give a reasonable idea as to the damage potential of the maggots in the area.

2. Determine the damage potential for the current root maggot fly populations. Decisions can then be made on the need for lay-by insecticide treatments and the proper timing of these treatments.

Using Trap Data in Decision-making

1. Record the number of sugarbeet root maggot flies caught on each trap at each observation.

2. Keep an accumulated total for the traps and determine the field average. The accumulated total is determined by adding the number of flies in a trap since the beginning of the season (number of flies per trap).

3. Decisions can be made concerning the use of an insecticide the next year based on the average accumulated fly trap catch for the field.

a. If fly populations are very low with a total accumulated catch per trap of less than 20 flies for the season, a planting time treatment would likely not be needed; however, the fly population will need to be monitored the next year to determine if it’s building and may pose a threat.

b. If fly populations are moderate with a total accumulated catch per trap of 20-80 flies for the season, the damage potential is moderate and one of several treatment options can be used.

• Apply a planting time soil insecticide to control the root maggot problem. This can be effective, however many factors influence the insecticide in the weeks between planting and when it is needed. Also, because of the phytotoxicity risk from some products, this option should be used only when there is demonstrated risk from root maggots (i.e. previous damage or high fly populations).

• Use an early lay-by application of a granular soil insecticide for root maggot control. This option reduces the risk from phytotoxicity, but lack of water (precipitation) to move the chemical into the soil may reduce control. This would be the best option if overhead sprinkler irrigation is possible.

• Forego an at-plant insecticide and rely on a liquid lay-by application based on the trapping threshold to provide control of the maggot population. This option works well, but fly monitoring and proper timing are critical. (See No. 4 below.)

c. If fly populations are very high (more than 80 per trap), a planting time soil insecticide may be the best option to begin control of root maggots. If the fly populations in a field treated at planting are very high during the season, a lay-by application of Lorsban 4E can provide supplemental control to the planting time application. This has been shown to be quite effective in situations of severe root maggot damage.

4. Decisions can be made concerning lay-by treatments and timing for the current year.

a. If the total accumulated catch per trap never exceeds 40 flies, the damage potential is low.

b. If the total accumulated catch per trap exceeds 40 flies by peak fly activity (before trap catches begins to drop off), a significant potential for damage exists and if no planting time insecticide was used, some type of rescue treatment would be in order. Peak fly activity usually occurs between May 20 and June 10 (in Nebraska). Lay-by treatments should be timed according to the timing of significant fly activity. Rescue treatments applied after major larval activity has begun are too late and will be of little use. When using liquid lay-by treatments, timing is critical. They should be applied when the threshold of 40 flies per trap is reached. This may occur before the actual peak fly activity period is noted on the sticky traps. If the period of high fly activity is extended 7-10 days after the first treatment, a second liquid insecticide treatment may be needed to control the later population. These treatments must target control of not only the flies but also a significant residual activity of the insecticide on the soil surrounding the sugarbeet. Chlorpyrifos (Lorsban and other generics) is the only insecticide that has shown consistent effectiveness when applied in this way. Other products will provide only adult control which is not adequate if conditions are favorable for maggot survival.

Sugarbeet Root Maggot Trap Construction:

1. Traps are made from a 2"by2" wooden board that has been painted white and a garden stake, approximately 1"by10", that has been painted a bright (not fluorescent) orange color. (Similar plastic orange stakes are available commercially.) Attach the garden stake to the 2X2 about 1" to 2" from the top of the stake so that a white border surrounds the stake. When the 2X2 is driven into the ground, the bottom of the orange stake should be about 1 foot above the soil surface.

2. Tangletrap, an insect trap adhesive, is placed only on the orange stake in a thin layer. Adding too much adhesive will only lead to a very messy trap, but care must be taken to add enough to be able to catch the flies.

Trap Placement in the Field

1. Traps should be placed out about May 1 and monitored into the second week of June.

2. Four traps should be placed around the perimeter of the current year's sugarbeet field.

a. Traps should be placed at the edge of the field in a fence-row or next to a ditch just out of the range of the cultivator so they will not be knocked over during field operations.

b. Two traps should face north or west and two should face south or east. This arrangement will usually allow two traps to escape being coated with dirt after a strong wind.

c. The orange stake on the trap should face the sugarbeet field or be at a 90° angle to the field.

d. Weeds or grass growing around the trap should be cut or pulled for about a 2 foot radius to maintain trap visibility.

3. Traps should be monitored at least twice a week.

a. Count or record the number of sugarbeet root maggot flies for each trap.

b. The sticky traps do collect flies other than sugarbeet root maggot flies, so correct identification is essential for an accurate control. See the earlier description of the flies.

c. Flies should be cleaned off the trap and fresh adhesive applied. If adhesive remains clean and sticky, dead flies can be picked off and sticky material left for the next trap check. Take care to keep the adhesive material on the trap sticky. Dirt and other insects, if numerous, can limit the fly catch because of limited or no sticky surface to catch the flies. The most common problems in reduced stickiness results from dust storms or high insect numbers, particularly flies near feedlots.

The best decisions for managing the sugarbeet root maggot can only be made when you know what the potential for damage is in your fields. The best information to determine that potential can only be obtained from trapping the maggot flies with the orange sticky stake method. Knowing the potential for SBRM damage is essential to saving money on unneeded insecticide applications while avoiding damage from this insect when populations build to damaging levels.

Product List for Sugar Beet Root Maggots:

Insecticide	Product per Acre	Preharvest Interval, remarks
Counter 15G^R, CR^R	15G: 4.0-8.0 oz/1000 row ft CR: 3.0-6.0 oz/1000 row ft	Applied banded at planting or post emergence; REI 72; PHI 110 days.
chlorpyrifos 15G (Lorsban plus generics)	4.5-9.0 oz/1000 row ft 6.5-9.0 oz/1000 row ft	T-band at planting time. Post plant 2-4 true leaf stage; REI 24 hrs.
	Note: chlorpyrifos 15G is labeled for T-band application at planting, but this application can be significantly phytotoxic to beets when applied in this fashion, especially when used on lighter soils.
Lorsban 4E	band 1.33-2.0 pt./A	Use as primary treatment to control root maggot larvae. See label for timing instructions. PHI 30 days; REI 24 hrs. High rates may be phytotoxic.
Lorsban 4E	broadcast 0.5-1.0 pt./A	Apply based on field trap monitoring for adult fly control. PHI 30 days; REI 24 hrs.
Lorsban 4E	broadcast: 2.0 pt./A band: 0.66-1.33 pt./A	Use as supplemental treatment following an at-plant insecticide treatment for larval control. PHI 30 days; REI 24 hrs.
Temik 15G^R	7.0-14.0 lbs./A	Apply at planting or post-emergence. Potential for groundwater contamination. See label for environmental precautions and restrictions. PHI 90 days; REI 48 hrs.
Thimet 20G^R	3.4-4.5 oz/1000 row ft	Apply at planting, not in contact with seed. REI 72 hrs.
Thimet 20G^R	4.9-7.5 lbs/A	Apply post emergence to dry foliage in band;PHI 30 days; REI 72 hrs.
^RRestricted use pesticide ¹Labeled for chemigation.
"The information herein is supplied with the understanding that no discrimination is intended and that listing of commercial products, necessary to this guide, implies no endorsement by the authors or the Extension Services of Nebraska, Colorado, Wyoming or Montana. Criticism of products or equipment not listed is neither implied nor intended. Due to constantly changing labels, laws and regulations, the Extension Services can assume no liability for the suggested use of chemicals contained herein. Pesticides must be applied legally complying with all label directions and precautions on the pesticide container and any supplemental labeling and rules of state and federal pesticide regulatory agencies. State rules and regulations and special pesticide use allowances may vary from state to state: contact your State Department of Agriculture for the rules, regulations and allowances applicable in your state and locality."

Categories: Insects, Sugarbeets

Date: 04/19/2006