Insects affecting cabbage and related cole crops
Authors: Dr. Alton “Stormy” Sparks, Jr. and Dr. David G. Riley - University of Georgia
The term "Cole crops" refers to plants in the crucifer family within the Genus Brassica (more specifically, varieties of the species Brassica oleracea) that have similar insect pest complexes. These crops include Cabbage, Collards, Mustard greens, Turnip greens, Radish, Broccoli, Brussels sprouts, Cauliflower, Chinese cabbage, and Kale. Even related field crops like Canola and Brassica napus can have similar insect pests.
The insects described here are common in Georgia and the southeastern USA. It is important to note that there are crop variety-specific considerations, such as foliar pests being more important where leaves are harvested, as in collard greens, and root pests are more important where roots are specifically harvested, as in radish. Similarly, in cabbage where the head forms from the apical meristem, insects that attack this plant structure, such as webworm, can be more economically damaging than insects that just feed on leaves.
The format we use to discuss insect pests is arranged by the type of damage that they do to the crop in early, middle and late growing seasons. Pest descriptions (hyperlinked in the text) are included in the following sections and the levels of injury expected from an insect species are are noted by crop variety. For specific commercial control options, please refer to the cole crop section of the UGA Pest Management Handbook. Additional outside information can be obtained from the University of California's "Integrated Pest Management for Cole Crops and Lettuce" or http://www.ipm.ucdavis.edu/ and John Capinera's "Handbook of Vegetable Pests" from Academic Press.
Insects Attacking Cole Crop Seedlings (Early Season)
Cabbage, Collards, Cauliflower and Broccoli are typically transplanted in Georgia while Mustard, Kale, and Turnip greens are often direct seeded. Consequently, some seedling pests are avoided through transplanting. Even so, young seedlings, and particularly the apical meristem where heads will eventually develop, are very susceptible to damage. Typical pests of cole crop seedlings are Flea beetles [species: tobacco, southern tobacco, pale striped] that cause small shot-holes in leaves, Seedcorn maggot that attack the stem and roots causing the plant to wilt, and Cutworms [species: black and granulate] that clip the plant off at the soil line. Additional, foliar feeders such as Diamondback moth can attack the leaves at an early stage causing reduced plant growth if the defoliation is great enough. Occasionally, small insects can be seen tunneling within the leaves of young plants [primarily Cabbage leafminer and Pea leafminer], but this seldom causes economic yield loss. An insect that can occur in summer and fall planting that attacks the apical meristem is the Cabbage webworm. This damage is more severe in cabbage because it can cause multiple heads that renders the plant unmarketable. The treatment timing for soil insects, such as Seedcorn maggot, is usually pre-planting, for Cutworms and Webworms it is at the time that damage is first detected, and for defoliators it is at 10% defoliation. Early infestations of webworm should be treated when detected.
Insects Attacking Cole Crops During Vegetative Growth (Mid-Season)
In crops such as leafy greens where the harvested portion of the plant are the leaves, control of pests that feed on leaf tissue becomes increasingly critical as the season progresses since even a few holes in harvested leaves make the greens unmarketable. In Cabbage, control of foliage feeders is less critical in young plants, but becomes more critical at the cupping stage when the head begins to form. In general, significant yield loss from Lepidoptera larvae can be prevented during mid-season using a treatment threshold of three larvae (caterpillars or moth larvae) per 10 plants. In other words, if numbers of larvae exceed three in every 10 plants scouted, significant yield loss is likely to occur. The main species of Lepidoptera that attack cole crops in Georgia include the Diamondback moth, Cabbage looper, Imported cabbageworm, Cross-striped cabbageworm and Cabbage webworm. Other insects that can reduce the quality of foliage in this growth stage are Aphids [species: cabbage, green peach, and turnip] which secrete honeydew promoting the presence of Sooty mold on leaves, Sweetpotato whitefly that can transmit Geminiviruses that stunt plant growth, Thrips that can scar the leaf surface when they occur in large numbers, and other foliage feeders such as the Yellowmargined leaf beetle. Stink bugs can often be found on the foliage of cole crops, but economic damage from these pests is not reported in Georgia. Similarly, Cabbage root aphid can be often found on developing turnip roots, but economic yield loss has not been significant enough to warrant soil treatments in the fall of the year when this pest establishes itself on the crop root system. The main damage would be from stand loss. Root aphids have been shown to reduce yields in Texas.
Insects Attacking Mature Cole Crops (Late Season)
Once the cabbage heads or cauliflower kurds are formed, the tolerance for damage or insect contamination in the harvested portion of the plant dramatically declines. Additionally, the pre-harvest interval for many insecticides limits the use of chemicals just before harvest. Marketing and regulation pressures can impact the level of damage or insects allowed in the field at the time of harvest. For example, if the market for Cabbage is flooded with produce, even slight damage to the cabbage head can downgrade the farm gate price. Insect contamination in processed cole crops, such as Grasshoppers, Aphids or even beneficial insects such as Syrphid larvae and Lady beetles in leafy greens that are mechanically harvested, continues to be one of the most difficult insect problems facing producers and processors of these crops. Also, regulations on products exported out of the country, such as zero tolerance of live diamondback cocoons attached to broccoli stalks, can greatly affect insect control intensity late in the season. Insect contamination has the unique problem of being difficult to control with insecticides because killing the insect often does nothing to reduce the contamination (dead insects being just as bad or worse in the processing line). The bottom line for late season insect control in most cole crops is to prevent insects from even being a problem at the time of harvest. Where seeds or seedpods are being harvested, as in the case of cole crops grown for seed production, there is one insect pest of additional importance, Cabbage seedpod weevil, which can attack the seeds in the fruiting structure where cole crops are grown for seed.