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 underside 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. 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. At high numbers, the entire seed pod may be consumed.