EggsBertha armyworm eggs are laid in single-layered clusters of about 50 to 500 eggs on the lower surface of the host plant leaves. The eggs are sculptured, ridged and pinhead in size (Figure 2). When first laid, they are white but become darker as they develop. At average temperatures, the eggs hatch within a week.LarvaeNewly hatched bertha armyworm larvae are about 0.3 cm (1/10 inch) long. They are pale green with a pale yellowish stripe along each side (Figure 3). Because of their size and colour, they are difficult to see on the underside of leaves.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.
PupaeBertha armyworms survive the winter as pupae in the ground at depths of 5 to 16 cm (2 to 6 inches). A pupa is a pod-like structure the bertha armyworm forms around itself to protect it while it transforms from the larval stage to the adult moth.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).
Host Plants and DamageLarvae are the only developmental stage of the bertha armyworm to cause crop damage. They feed on a variety of crops and weeds.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. MonitoringThe presence of bertha armyworm larvae in a crop one year is not a reliable indicator of what to expect the following year. Bertha armyworm populations fluctuate widely from year to year.Adult populationsBertha armyworm populations can be monitored using traps baited with sex pheromone, which attracts the male moths. The number of moths collected by these traps gives an indication of the bertha armyworm larval populations to follow. Usually, high numbers of moths translate into high numbers of larvae later in the season.Larval populationsEarly detection and regular monitoring of bertha armyworm larvae are critical to minimize crop losses. Larval monitoring should begin about two weeks after peak trap catches and continue until the number of larvae exceed economic thresholds and the crop is sprayed, or until the crop is swathed.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 ActionInsecticide application is recommended when bertha armyworm larvae are abundant enough that the value of the crop they consume is greater than the cost of controlling them. This number is called an economic threshold.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.
ControlIn most years, bertha armyworms are controlled naturally by biological or environmental factors.Environmental controlEnvironmental conditions have a significant impact on bertha armyworm populations, especially on the over-wintering pupae. During harsh winters in snow-free fields, most bertha armyworm pupae die. Bertha armyworm outbreaks appear to be favoured by snow accumulation, which protects pupae from prolonged exposure to temperatures below -10 C.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 ControlA number of diseases and parasites attack the bertha armyworm, including:
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.
Cultural ControlBertha armyworm populations can be manipulated to reduce crop loss. Methods include planting alternative crops, effective weed control, early swathing and fall cultivation.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 ControlChemical control is the producer's last line of defence against the bertha armyworm. For best results, apply an insecticide as soon as economic thresholds are reached.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.
For Best Control
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. Forecast MapsReturn to Pest Management Home
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