The diamondback moth, Plutella xylostella, has historically been a widely dispersed pest in the United States and Canada with elevated populations concentrated in areas where cruciferous crops and weeds such as turnips, broccoli and mustard species are found. Significant increases of winter canola acreage in several areas, including the southern Great Plains in the United States, have proven to be a preferred host crop for diamondback moth. Egg laying and economic damage from larval feeding are not uncommon.
The diamondback moth, believed to be a native species to Europe, is now found worldwide. Its life cycle consists of a complete metamorphosis including adult, egg, larva and pupa.
The adults are very small, grayish-brown moths approximately 1/2 inch in length. When folded, the moth's wings appear to wrap around the body with the posterior end of the wings upswept Figure 1. The male moth contains diamond-shaped markings on its wings, resulting in the name "diamondback."
The adult female lays small (1/64 inch) oval to round flattened eggs, yellowish white in color, normally in groups of 3 to 8. Female moths can lay an average of 150 eggs. Eggs hatch in 5 to 6 days depending on temperature.
Newly hatched larvae pass through 4 growth stages, or instars. Larvae first appear light green and darken as they mature. Fully grown larvae reach approximately 1/3- to 1/2-inch in length, and are pointed at both ends. Large larvae appear similar to a looper type worm, and aggressively thrash back and forth when disturbed, dropping from the plant on a silken strand. They pupate within a loosely spun cocoon attached to leaves or stems of the plant.
Adults can emerge in 6 to 12 days, and a life cycle can be completed in about 32 days, depending upon temperatures. It is not uncommon to find all life stages at the same time in a field.
Larvae of the diamondback moth cause the most plant damage. Upon hatching, young larvae first burrow between the upper and lower leaf surfaces creating "mines." These result in "windowpane" tunnels and scars upon the leaves as pictured in Figure 2. As larvae grow, they begin to feed through larger areas of the leaf creating small irregular holes. Usually leaf damage by larval feeding is cosmetic and only results in minor yield reductions. Occasionally, high populations can lead to entire leaves being eaten, total plant defoliation, and possibly loss of plant stand.
Larvae feeding of greater concern is damage to the crown or growing point of the canola plant in the fall, or damage to flowers and pods in the spring. Larvae that overwinter in the crown of the plant may damage the crown sufficiently to cause plant death, which is often misidentified as winterkill. Crown damage to the canola may also result in irregular growth and development in the spring following fall dormancy.
Larval feeding during the reproductive stages of canola is detrimental to yield. During this stage, larvae may feed on flowers, which can ultimately lead to flower abortion and a potential delay in plant maturity. When immature pods are damaged, failure to produce seed or quality damage to seed may occur.
Diamondback moth larvae are normally detected very early in the canola life cycle and development, up until fall dormancy occurs. Overwintering larvae may be present if control measures in the fall were not adequate, resulting in populations the following spring. Winter canola fields must be monitored closely after planting until dormancy, and can require extensive scouting procedures to accurately determine the presence of larvae and levels of damage that may result.
Scouting should be performed by examining numerous areas of the field and examining individual plants very closely. Digging plants and shaking them within a white bucket assists in visual evaluation of population levels. While no standard thresholds have been established for overwintering larvae, it is recommended that treatments be made if significant defoliation has occurred, or if 2 to 3 larvae per foot of row are found. Control recommendations should be based upon larval counts and canola growth stage.
Diamondback moth populations can increase rapidly under some environmental conditions. Heavy infestations may result in multiple generations of the pest in a single cropping season.
The ability of this pest to reproduce rapidly has allowed it to develop resistance to specific insecticides, especially the pyrethroids class of chemistries. At this time, pyrethroids comprise the majority of labeled insecticides for winter canola. It is recommended that a rotation of chemical classes be used to control this pest, and that extensive field scouting be performed following treatment to determine the level of control that was achieved.