Bertha Armyworm

Sustainable Agriculture Facts
Growing for tomorrow
Technical Information for Conservation Farmers

For more information about bertha armyworm, contact the local representative of the appropriate agency.

The larvae are the only stage of the bertha armyworm to cause crop damage.

Bertha armyworm is one of the most significant insect pests of canola in Canada. It occurs throughout Manitoba, Saskatchewan, Alberta and into the interior of British Columbia. Severe infestations can occur throughout most of this area but are usually limited to the parkland area of the Prairies and the Peace River region of British Columbia and Alberta.

Bertha armyworm (Mamestra configurata) is native to North America and belongs to a group of insects referred to as "climbing cutworms". Also included in this group are the true armyworm and variegated cutworm.

In most years, populations are kept low by unfavourable weather conditions such as cold winters and cool wet weather, and by parasites, predators and diseases. But when these natural regulators fail, populations can increase dramatically, creating the potential for widespread damage to a variety of broad-leaved crops.

In extreme situations, infestations of more than 1,000 larvae per square metre have been reported while densities of 50 to 200 larvae per square metre may be common.

Infestations may be localized or spread over millions of acres. Widespread crop losses can be minimized with insecticides if the infestation is detected early. However, failure to detect infestations early may result in insufficient time to apply the chemicals before severe damage is done. Also, there may be temporary insecticide shortages if suppliers are not aware of the potential outbreak.

Life Cycle

Adults

It is suspected that moths are strongly attracted to canola fields that are in bloom and secreting nectar. Adult moths mate within five days of emergence and lay their eggs on the host plants. Each female moth will lay about 2,150 eggs but numbers as high as 3,500 eggs per female have been recorded.

The moth has a wing span of about 4 cm (1.5 inches) and is active only at night. The forewing is predominantly gray, and flecked with patches of black, brown, olive and white scales (Figure 1). Near the middle of the forewing, towards the leading wing margin (front), there is a prominent, white, kidney-shaped marking defined with a ring of whitish scales. Near the tip of the forewing, there is a conspicuous white and olive-coloured, irregular transverse marking that is characteristic of the species.

 

Figure 1 - Adult moth	Figure 2 - Eggs are white when first laid.

Eggs

Larvae

When disturbed, small larvae may drop off the leaves by a fine silken thread. This behavior makes it difficult to distinguish small bertha armyworm larvae from those of the diamondback moth, which display a similar behavior. Large larvae may drop off the plants and curl up when disturbed, a defensive behavior typical of cutworms and armyworms.

Larvae take approximately six weeks to complete their development, depending upon temperature. During this period, they moult five times and pass through six growth stages. As they mature, their colour becomes variable. Some remain green, but many become brown or velvety black (Figure 4).

At maturity, the larvae are about 4 cm (1.5 inches) long, with a light brown head and a broad, yellowish-orange stripe along each side. The velvety black larvae have three narrow, broken white lines on their backs.

At maturity in late summer or early fall, larvae burrow into the ground and form pupae.

 

	Figure 3 - Newly hatched larvae
	Figure 4 - Larvae colours become variable as they mature

Pupae

Pupation usually begins in mid to late August. All larvae will have pupated by early to mid- September. If the autumn is unusually warm, some pupae may continue their development and emerge as moths in late August or September, only to perish when winter arrives.

Pupae are reddish brown in colour, about 0.5 to 1.8 cm (0.2 to 0.7 inches) in size and tapered with flexible, terminal abdominal segments. Bertha armyworm pupae are indistinguishable from other cutworm pupae (Figure 5).

 

Figure 5 - Bertha armyworm pupae

Host Plants and Damage

Canola, rapeseed, mustard, alfalfa, lamb's quarters and related plants are preferred host plants. Bertha will also feed on a range of secondary hosts including flax, peas and potato.

The degree of crop damage varies with the crop, the plant's growth stage, the growth stage of the larvae and the number of larvae present. Significant crop damage usually occurs within a three-week period between late July and late August, depending on the season and crop location.

Small larvae feed on the undersides of the leaves, chewing irregularly-shaped holes in the leaves. They usually cause little damage at this stage, even when population levels are high. Crop damage occurs rapidly once the larvae moult to the second-last stage. These larvae are about 1.3 cm (1/2 inch) in length. Larvae in the last two larval stages eat about 80 to 90 percent of the plant material consumed during the life of the larvae.

If the plants, especially canola, drop their leaves before the larvae are mature, the developing larvae will feed directly on the seed pods. Seed pods may be "debarked," but more commonly, the larvae chew holes in the pods and eat the seeds. At high numbers, the entire seed pod may be consumed. Even if the pods are only stripped of their outer green layer and not eaten entirely, crop losses may still occur because of premature shattering.

In flax, the larvae eat the flowers and developing bolls. Once the flax bolls are full-size and start to ripen, larvae usually feed on the calyx below the boll. Occasionally, larvae will feed on the green stems of ripening bolls, causing them to drop off.

Monitoring

Adult populations

Larval populations

It is important to monitor larval numbers in each field. Adjacent fields may have very different larval densities, depending upon how attractive the crop was when the moths were laying their eggs. Adjacent crops may also have different-sized larvae, depending on when the eggs were laid.

For accurate larval estimates in a crop, sample at least three locations a minimum of 50 metres apart. Do not sample headlands (20 metres wide) and areas within the crop that are not representative of the field.

At each location, mark out an area of one metre square and beat the plants growing within that area to dislodge the larvae. Push the plants aside or remove them and count the number of larvae in the square metre area. It is important to take your time while counting larvae. Carefully search the soil and leaf litter. The larvae are difficult to see and may be hidden underneath clumps of soil, in cracks or within curled leaves. Use the average number of larvae at the sites surveyed within each field to determine if the economic threshold has been exceeded and an insecticide is necessary.

Economic Thresholds - When to Take Action

The economic threshold for bertha armyworm varies with the cost of the insecticide, the method of application and the crop's value. Using current crop value ($/bushel) and application costs ($/acre), Table 1 indicates the larval density (larvae/square metre) at which an insecticide treatment in canola would be warranted. Twenty larvae per square metre in canola can reduce yields by 1.16 bushels/acre.

Table 1 - Economic thresholds for bertha armyworm on Argentine canola. For example, assuming a crop price of $8.00 per bushel and a spraying cost of $9.00 per acre, the table indicates an economic threshold of 19 bertha armyworm larvae per square metre. Spraying would not be economical if larvae counts are less than 19 per square metre.

Spraying Cost - $/acre	Expected seed value - $/bushel
	6	7	8	9	10	11	12
	Number of larvae/square metre
7	20	17	15	13	12	11	10
8	23	20	17	15	14	13	11
9	26	22	19	17	16	14	13
10	29	25	22	19	17	16	14
11	32	27	24	21	19	17	16
12	34	30	26	23	21	19	17

Control

Environmental control

The trend toward reduced tillage and stubble conservation results in more snow accumulation on infested fields and could favour bertha armyworm survival, especially in years with early snowfall.

Newly hatched larvae are especially vulnerable to inclement weather and diseases.

Biological Control

• a nuclear polyhedrosis virus (Figure 6)
• an ichneumonid wasp (Banchus flavescens)
• a tachinid fly (Athrycia cinerea)

However, these natural enemies often do not destroy larvae until after considerable crop damage has occurred. They have their greatest impact on bertha armyworms produced a year or two after the peak of an outbreak. This is probably why severe infestations only last two or three years.

Their presence in a crop does not, however, indicate that control measures are unwarranted.

 

Figure 6 - Nuclear polyhedrosis virus is one natural enemy of the bertha armyworm.

Cultural Control

Fall cultivation can kill many bertha armyworm pupae by mechanical damage. Tillage can also reduce the amount of snow trapped on a field by removing or flattening stubble and exposing pupae to sub-zero temperatures over the winter. This practice may be effective for individual fields but is not likely to be effective unless it is adopted by all producers in an area. Adult moths are strong flyers and can easily move to adjacent fields. Fall cultivation should not be used on light- textured soils susceptible to erosion.

Effective control of weeds such as lamb's quarters and wild mustard can reduce bertha armyworm infestations in flax, peas, lentils and sugar beets. Larvae will first feed upon these weeds and then move onto these crops after the weeds have been destroyed.

Chemical Control

A single, well-timed application of any registered insecticide applied with aerial or high clearance ground equipment is usually effective. Table 2 summarizes the insecticides, crops, rates of application and preharvest interval.

Table 2. Insecticides registered for the control of bertha armyworm. Indicated are the recommended application rates and the number of days that must elapse between application and harvest. Always read the pesticide label or enclosure for proper application rates, mixing instructions, and safety precautions.

Product	Rate (product per acre)	Preharvest Interval (days)
		Canola	Mustard	Flax
Decis*	40-60 ml/acre	14	14
Lorsban	305-405 ml/acre	21		21
Lannate	360-500 ml/acre	8		8
Monitor	230-500 ml/acre	10
Ripcord*	28 ml/acre (ground)	30
	35 ml/acre (air)	30
Cymbush*	80-113 ml/acre	30
Pyrinex	305-405 ml/acre	21

* Do not apply at temperatures greater than 25 C.

For Best Control

• The larvae should be at least 1.3 cm (about 1/2 inch) long.
• Apply the insecticide early in the morning or late evening when the larvae are actively feeding.
• Do not apply during warm afternoons.
• Use enough water to ensure adequate coverage.
• Use high water volumes in crops with dense canopies such as canola.
• Use the higher label rates of application when a range is indicated.

To avoid killing foraging honeybees, delay insecticide applications until after the crop has finished blooming. If this is not possible, select Decis to control the bertha armyworm larvae, and apply during the evening.