Sugar beet root maggot

There are four stages in the sugar beet root maggot lifecycle: egg, larva (maggot), pupa and adult. Eggs of the sugar beet root maggot are white, slightly curved and about 1/25 in (1 mm) long (Figure 1).

The larval form (maggot) is legless and has no distinct head or eyes (Figure 2). Larvae go through three different instars (growth stages). When larvae are in the first instar they are tiny, approximately 1/32 to 1/16 in (0.75 to 2 mm) long, and transparent white. This small size and white coloration can make them resemble root hairs. As larvae grow, they gradually become a more opaque white color and eventually reach lengths of up to 1/2 in (1.2 cm). Pupae have a hard, brown casing and are slightly smaller than the largest larval stage (Figure 3).

The adult form is a small, shiny, black fly with wings that are mostly clear, but with an opaque, smoky brown patch on the outside edge of each wing, about 1/3 of the way down the wing (Figure 4-5). Sugar beet root maggot flies are about the size of a house fly, 1/4 in (6 mm) in length, though notably less “hairy” and more shiny.

Biology

There is generally only one generation of sugar beet root maggots each year (Figure 6). They overwinter as fully grown larvae buried about 10-16 in (25-40 cm) in the soil in the previous year’s beet fields. When soil temperatures warm to the mid-40s degree Fahrenheit (6-8 °C) larvae move into the top few inches of soil to pupate. Typically, adult emergence begins by mid-April in western Idaho and late April early to May in eastern Idaho, though exact timing may vary year-to-year based on spring temperatures. Cold and wet conditions delay adult emergence, whereas unusually warm temperatures can accelerate emergence. Adult abundance similarly varies among years and regions, but may be highest during mid- to late May through early June.

Adult females begin laying eggs 3-10 days after their emergence, and each female may lay about 120 eggs in the soil near sugar beet plants. Eggs hatch in 1-3 days, and maggots immediately begin feeding on sugar beet roots. By early to mid-July larvae may reach full size and will stop feeding to enter the overwintering state (diapause). There is some evidence that a small portion of the population may pupate in the late summer and emerge as adult flies in August.

Damage

Sugar beet root maggots feed on the roots of the plant. In seedlings, their feeding can sever the thin taproot and cause significant wilting or death of the young plant (Figure 7-8). Feeding on the tip of the root in young plants can also lead to forking in the mature taproot. In more mature plants, maggots feed on the surface of the root, leaving shallow, oozing, black lesions (Figure 9). These lesions range in size from tiny (about the size of a pinhead) to 3/4 in. (2 cm) long and may cover the entire root surface when infestations are severe. Lesions created by root maggot feeding can be invaded by root-rot pathogens, which can result in severe stand loss, especially in fields with histories of root rot.

Monitoring

Adult flies can be monitored using orange sticky traps (Figures 10-11); fly captures can be used to make insecticide application decisions. Amalgamated Sugar oversees an annual monitoring program across southern Idaho using these traps. If you want to build your own traps, follow the instructions found in further reading.

The following key features can be used to distinguish sugar beet root maggot flies from other commonly captured flies (e.g., anthomyiid family flies including beet leafminer, onion maggot and seedcorn maggot flies):

• Size: 1/4 in (6 mm) long, stout bodied (somewhat similar to house flies)
• Color: shiny, black body, without bristles or stripes
• Wings: transparent, with one opaque, smokey patch along the leading edge, about 1/3 of the distance from where the wing attaches to the body (Figure 4)
• Legs: black, except for yellow-white “knees” and “ankles”

Use this protocol to scout for sugar beet root maggot flies in sugar beet fields:

• Place three or more traps per field along fencerows or ditches adjacent to sugar beet fields. Space traps several hundred yards apart. Make sure that the orange, sticky face of the trap is directed north or east, and that it is 1 ft above the soil surface. Remove weeds that block the trap.
• Check traps at least twice (ideally three times) weekly beginning by April 15 in western Idaho and by May 1 elsewhere. Each time you check traps, record the number of flies on each trap and the average flies per trap in each field. Divide the trap average by the number of days since the last trap check to get the daily average. Keep a record of the trap averages you find across the entire season. For an example on keeping a season-long record.
• To reset traps, either replace the orange sticky surface or scrape off adhesive and reapply.
• Continue checking traps until you see a definite peak in fly captures, normally between late May and early June, though this may be affected by unusual weather.
• Once daily average fly captures have shown a peak, a control decision can be made.* Use Table 1 below or this calculator to determine your economic threshold.
• You can also predict the date of peak seasonal flight using a degree-day model found (click “Quick Start”), though this should not replace sticky trap monitoring.

*Mini-Cassia and American Falls areas may experience heavy fly pressure over extended periods, necessitating split applications of granular insecticides into at-plant and post-emergence treatments. See further reading for more details.

Table 1. Action thresholds for sugar beet root maggot in sugar beet. If the cumulative average number of flies captured per trap from the beginning of the season until peak capture is greater than these thresholds, treatment is recommended. If the running tally on the day of the peak is less than these values, potential yield loss does not warrant the cost of treatment. Also, find an online version of this calculator.

Management

Primary management tactics

Sugar beet root maggots are primarily managed with granular insecticides targeting the larval stage and applied based on scouting and action thresholds. Early planting allows plants to reach a larger, less susceptible growth stage before maggots are present.

Cultural

• Early planting can avoid plants being in vulnerable seedling stage when flies reach their seasonal peak.
• Deep tillage in the fall targets overwintering larvae, exposing them to freezing temperatures and resident predators.
• Frequent irrigation can force larvae toward the soil surface, where their feeding causes less severe damage and where they are more susceptible to insecticides.
• However, keep in mind that overwatering encourages soil-borne pathogens.

Behavioral

Pheromone lures have been developed that improve monitoring traps and may be used in mass trapping, but specific recommendations for these approaches are still forthcoming.

Biological

Avoiding the use of broad-spectrum insecticides can conserve natural predators.

Chemical

• Granular insecticides applied to the soil, targeting larvae, are most effective.
  Post-emergence applications of granular insecticide are more cost-effective than at-plant applications in most parts of Idaho.
• Applications should be applied within 10 days of peak flight, though mini-Cassia and American Falls areas with heavy fly pressure may need to split applications into at-plant and post-emergence treatments (at or near peak flight).
• Seed treatments may be effective if pressure is low to moderate.
• Foliar insecticides targeting adults should be used with caution since they require frequent reapplication and can trigger outbreaks of other pests like aphids and caterpillars.
• Recommendations for pesticides to use in the management of sugar beet root maggot can be found on the PNW Pest Management Handbooks website.

Further reading

Pesticide warning

Always read and follow the instructions printed on the pesticide label. The pesticide recommendations in this University of Idaho webpage do not substitute for instructions on the label. Pesticide laws and labels change frequently and may have changed since this publication was written. Some pesticides may have been withdrawn or had certain uses prohibited. Use pesticides with care. Do not use a pesticide unless the specific plant, animal or other application site is specifically listed on the label. Store pesticides in their original containers and keep them out of the reach of children, pets and livestock.

Trade names — To simplify information, trade names have been used. No endorsement of named products is intended nor is criticism implied of similar products not mentioned.

Groundwater — To protect groundwater, when there is a choice of pesticides, the applicator should use the product least likely to leach.