The growing plants are thinned and kept weed-free. Harvesting begins in late fall after the first frost and continues through winter. The rows can be covered with straw to enable the crop to be lifted during frosty weather. Low soil temperatures cause some of the starches stored in the roots to be converted into sugars, giving them a sweeter taste. Cultivation problems Parsnip leaves are sometimes tunnelled by the larvae of the celery fly (Euleia heraclei). Irregular, pale brown passages can be seen between the upper and lower surfaces of the leaves. The effects are most serious on young plants, as whole leaves may shrivel and die. Treatment is by removing affected leaflets, whole leaves, or by chemical means. The crop can be attacked by larvae of the carrot fly (Chamaepsila rosae). This pest feeds on the outer layers of the root, burrowing its way inside later in the season. Seedlings may be killed while larger roots are spoiled. The damage done provides a point of entry for fungal rots and canker. The smell of bruised tissue attracts the fly. Parsnip is used as a food plant by the larvae of some lepidopteran species, including the parsnip swallowtail (Papilio polyxenes), the common swift moth (Korscheltellus lupulina), the garden dart moth (Euxoa nigricans), and the ghost moth (Hepialus humuli). The larvae of the parsnip moth (Depressaria radiella), native to Europe and accidentally introduced to North America in the mid-1800s, construct their webs on the umbels, feeding on flowers and partially developed seeds. Parsnip canker is a serious disease of this crop. Black or orange-brown patches occur around the root's crown and shoulders, accompanied by cracking and hardening of the flesh. It is more likely to occur when the seed is sown into cold, wet soil, the pH of the soil is too low, or the roots have already been damaged by carrot fly larvae. Several fungi are associated with canker, including Phoma complanata, Ilyonectria radicicola, Itersonilia pastinaceae, and I. perplexans. In Europe, Mycocentrospora acerina has been found to cause a black rot that kills the plant early. Watery soft rot, caused by Sclerotinia minor and S. sclerotiorum, causes the taproot to become soft and watery. A white or buff-coloured mould grows on the surface. The pathogen is most common in temperate and subtropical regions with a cool, wet season. Violet root rot caused by the fungus Helicobasidium purpureum sometimes affects the