Caterpillars which feed on sugar-beet foliage

CATERPILLARS WHICH FEED ON SUGAR-BEET FOLIAGE W. A. THORNHILL

Some moth caterpillars are well known as pests of agricultural crops including sugar beet. Cutworms, which are the caterpillars of moths such as the Turnip Moth, Agrotis segetum (D. & S.), can cause severe damage to sugar-beet plants below soil level. Much less well known are the moth caterpillars which feed on the foliage of sugar-beet plants.

The life styles of these caterpillars are of two types. Caterpillars of the Fiat Tortrix moth, Cnephasia interjectans (Haw.), bind leaves together, or fo Single leaves of young plants using silk, thereby creating a Chamber within which they feed (Jones & Jones, 1984). Other species, which are usually at their most numerous later in the season than the Fiat Tortrix, roam freely over the foliage. Severe infestations of free-roaming caterpillars may result in complete skeletonisation of leaves, though this rarely occurs in Britain.

To compile a list of Caterpilar species which attack sugar-beet foliage, and to enableraeto identify them in thefield,caterpillars of the free-roaming species were collected from the crop foliage from 1972 to 1983, and reared in the laboratory. They were placed in glass jars containing peat, and fed on sugar-beet leaves. Most species pupated in the peat, the others on the leaves.

A total of 57 caterpillars, collected from several sites in Suffolk, were successfully reared to the adult stage. These comprised: 16 Nutmeg, Discestra trifolii (Hufn.), 12 Bright-line Brown-eye, Lacanobia oleracea (L.); Cabbage Moth, Mamestra brassicae (L.) and 3 Broom Moth, Ceramic (L.) (all belonging to the Hadeninae group), together with 3 Large Yellow Underwing, Noctua pronuba (L.) and 1 Small Square-spot, Diarsia rub (View.) (species belonging to the Noctuinae), 9 Angle Shades, Phlogophora meticulosa (L.) (Amphipyrinae) and 1 White Ermine, Spilosoma lubrici (L.) (Arctiinae). In addition, 2 Dot Moth, Melanchrapersicariae (L.), we reared from caterpillars collected from beet foliage in Norfolk. Some were species not previously recorded feeding on sugar beet.

Caterpillars of at least two other species were found but could not be reared to the adult stage. Dunning and Byford (1982) mention other species which cause damage to beet foliage in continental Europe but do not appear to do so in Britain, though recorded here (Kloet & Hincks, 1972). These are the Beet Moth, Scrobipalpa ocellatella (Boyd) and the Beet Webworm Margaritia sticticalis (L.). Jones and Dunning (1972) also record the Diamond-back Moth caterpillar, Plutella xylostella (L.), as an occasional pest of beet in Britain, but it was not found in this survey.

The numbers of the species in the above list is not an accurate indication of their relative abundance in thefield.Once a species could be identified in the field it was no longer necessary for me to collect it; this stage was reached sooner with some species than with others, if it was reached at all. For example, the markings of the Nutmeg caterpillar were very variable and many specimens needed to be collected before it was possible to be confident of its identity in thefield.From general Observation the two most commonlyTrans. Suffolk Nat. Soc. 23

CATERPILLARS WHICH FEED ON SUGAR-BEET FOLIAGE

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occurring of the free-roaming caterpillars attacking sugar beet were the Bright-line Brown-eye and the Cabbage Moth; though both these species varied in colour their markings were more consistent than some of the others collected.

In the laboratory the Large Yellow Underwing caterpillars differed from the other species listed in that when they were more than half grown they remained in the peat during the day and only fed on the leaves at night.They behave similarly in thefield(Edwards & Heath, 1964), but during the day they can occasionally be found below older sugar-beet leaves which are touching the ground. Large Yellow Underwing caterpillars are generally known as cutworms.

Of the caterpillars found, the Nutmeg has the most restricted ränge of food-plants, feeding principally on species of the Chenopodiaceae (South, 1939), which includes sugar beet and some common agricultural weeds such as Fat Hen, Chenopodium album L. and Common Orache, Atriplexpatula L The Bright-line Brown-eye is often abundant on Goosefoot, (Chenopodium spp.). All the other species feed on a wide ränge of grasses or on low-growing plants and it is not surprising that they are found on sugar-beet foliage.

Free-roaming caterpillars do most damage in late summer and autumn (Dunning, 1975), but by then sugar-beet plants are large enough to withstand attack and the damage is rarely serious. Attacks by them are very seldom reported in current annual surveys (Maughan, 1982) of the crop by British Sugar plcfieldstaffand, even if the species are considered collectively, they are not among the most serious pests of sugar beet in Britain.

References

Dunning, R. A. (1975). Arthropod pest damage to sugar beet in England and Wales. Rep. Rothamsted exp. Stn. for 1974, 2,171.

Dunning, R. A. & Byford, W. J. (Eds. English Translation). Pests, diseases and disorders of sugar beet, Deleplanque & Cie, Maison-Laffitte.

Edwards, C. A. & Heath, G. W. (1964). The principles of agricultural entomology. Chapman & Hall, London.

Jones, F. G. W. & Dunning, R. A. (1972). Sugar beet pests. Ministry of Agriculture, Fisheries & Food, Bulletin 162.

Jones, F. G. W. & Jones, M. G. (1984). Pests offieldcrops. (3rd. Ed.) Arnold, London.

Kloet, G. S. & Hincks, W. D. (1972). A check list of British Insects, Part 2: Lepidoptera. (2nd. Ed. revised) R. ent. Soc. Lond.

Maughan, G. (1982). Specificfieldsurvey (fieldstats). British Sugar Beet Review, 50, 64.

South, R. (1939). The moths of the British Isles. Frederick Warne & Co., London.

W. A. Thornhill, Broom's Barn Experimental Station, Higham, Bury St. Edmunds, Suffolk IP28 6NP Trans. Suffolk Nat. Soc. 23