 Soil nutrient deficiencies can cause a variety of plant problems that range from malformed plants to a lack of head growth . Many insects are attracted to lettuce , including cutworms , which cut seedlings off at the soil line ; wireworms and nematodes , which cause yellow , stunted plants ; tarnished plant bugs and aphids , which cause yellow , distorted leaves ; leafhoppers , which cause stunted growth and pale leaves ; thrips , which turn leaves gray @-@ green or silver ; <unk> , which create tunnels within the leaves ; flea beetles , which cut small holes in leaves and caterpillars , slugs and snails , which cut large holes in leaves . Mammals , including rabbits and groundhogs , also eat the plants . Lettuce contains several defensive compounds , including sesquiterpene lactones , and other natural phenolics such as <unk> and glycosides , which help to protect it against pests . Certain varieties contain more than others , and some selective breeding and genetic modification studies have focused on using this trait to identify and produce commercial varieties with increased pest resistance . 
