Pests
Cassida nebulosa Linnaeus - Beet Leaf Beetle, Beet Tortoise, Tortoise Beetle.
Systematic position.
Class Insecta, order Coleoptera, suborder Phytophaga, family Chrysomelidae, subfamily Cassidinae, genus Cassida.Biological group.
Pest of beet.Morphology and biology.
The Tortoise Beetle easily differs from other pest beetles by the shield-shaped pronotum and elytra, which completely cover flat body and head of the beetle and make them invisible from above. The beetle length is 6 to 7 mm. Imagoes are rusty-brown or greenish from above with black irregularly shaped spots, and black from below. Elytra are striated, with longitudinal rows of large punctures. Beetles emerge in April and May and settle on goose-foot, then pass on to beet. Soon they mate, and females lay eggs on leaves of goose-foot and beet. The females lay 2 to 16 eggs and coat them with a turbid-white slime, which hardens fast and forms a semi-transparent film. The oviposition begins usually in the middle of May and lasts 10 to 40 days. Female of the Tortoise Beetle is capable to lay to 200 eggs. Yellowish-green grubs with long spikes arranged laterally emerge in 5-7 days. The grubs pupate on leaves of beet or weeds in 12 to 25 days. Pupae are green, wide, flat, with five serrate blades arranged laterally. The pupa stage lasts 5 to 8 days. Young beetles emerge in July and begin to reproduce in 10 to 15 days. Two generations develop during the summer. The whole developmental cycle of the first generation of the Tortoise Beetle usually comes within 30 to 35 days. Beetles of the second generation emerge in August and spend the winter without laying eggs. They hibernate under the plant remnants and fallen leaves in sparse forests and forest belts, in weed bushes, etc. In the forest-free areas, to 120 beetles per one square meter may crowd to overwinter in rare tree plantations. In the steppe zone, the hibernation is observed in long-fallow lands as well. Conditions of the beetles overwintering are virtually always favorable, and their death is insignificant during the winter. The lower temperature threshold for feeding grubs and beetles is +17°C, and the upper one is +40°C. The most intensive nourishment occurs at 21 to 28°C in grubs and at 20-30°C in beetles.Distribution.
The northern area boundary of the Tortoise Beetle lays in latitude 64-68°N. The species is widely distributed in all beet growing regions of Europe (Denmark, Great Britain, Germany, Switzerland, Poland, Czech Republic, Slovakia, Austria, France, Spain, Italy, Hungary, Romania, Croatia, Serbia and Montenegro, Bulgaria, Albania, Greece, Lithuania, Belarus, Ukraine, Moldova), North America, Japan, northeast China, and the former USSR. It is found in all zones from taiga forests to meadow-steppe region of middle mountains of Tien Shan, with the exception of deserts and alpine belt.Ecology.
The Tortoise Beetle is a narrow oligophage since the beetles and grubs feed on several species of plants of the goosefoot family, namely Frost Blite (Chenopodium album L.), Oakleaved Goosefoot (C. glaucum L.), Garden Orach (Atriplex hortensis L.), Sugar Beet, and Forage Beet. Amaranth, buttercup, turnip and other plants seldom serve as a forage reserve. In the beetle or grub stages, the pest passes on to the beet usually after eating up weeds. Therefore, the higher the pest numbers, the earlier they begin moving to the culture plants. The grubs conduct the settled way of life and tend to wander sometimes. The grubs of the older instars are the most mobile. They can pass from plant to plant especially during heavy drought and lack of food. Many entomophages are the natural enemies of the Tortoise Beetle, including Entedon ovulorum Ratz., Trichgramma evanescens Westw., Tetrastychus cassidarum Rizb., etc. The most abundant pathogen is Beauveria bassiana (Bals.) Vuill.Economic significance.
The Tortoise Beetle has double economic significance. On one hand, this is a pest significantly damages the beet; on the other hand, it is a useful species controlling the development of weeds of the goosefoot family. The beet is damaged by both the beetles and the grubs. The grubs of fourth and fifth instars are the most voracious, consuming about 87% of the total food volume needed for the whole developmental period. The beetles are the most voracious after emergence from pupae and after hibernation before oviposition. They eat away the round through holes in a leaf plate, not damaging veins; wheras the grubs scrape off the leaf pulp on the lower side between veins, leaving the upper rind untouched. The severely damaged leaves gain lacy appearance. The highest damage is from July to the middle August; i.e., in the period of the most intensive growth of leaf plates and roots. The intensive damage results in the further decrease of the yield of roots and beet tops. High damage of the Tortoise Beetle was not observed during the last ten years. The following agronomical measures are important for the Tortoise Beetle control: a) the first weeding before the sprouts emerge, b) the second weeding when beetles finish oviposition and grubs emerge, c) techniques providing the optimal conditions for growth and development of beet, thus increasing plant tolerance. If the weeding is overdue and the pest number is large (more than 20 individuals per one square meter), then insecticide application and successive weeding in 2-3 days are necessary.Reference citations:
Badulin, A.V., Matis, E.G. 1964. Tortoise Beetle. Zashchita rastenii 3: 43 (in Russian).Fedorenko, V.P., Shushkivs'ka, N.I. 2003. Cassid beetles. Kiev: UAAN IZR, 28 p. (in Ukrainian).
Chuvakhin, V.S. Tortoise Beetle (Cassida nebulosa L.). In: Anon., ed. Manual for control of pests and diseases of agricultural crops. V. 8. Moscow: Sel'khozgiz, p. 457-459 (in Russian).
Kokovikhin, A.I. 1972. Tortoise Beetle Cassida nebulosa L. (Coleoptera, Chrysomelidae, Cassidinae) in Western Siberia. In: Zubenko, V.F., ed. Main conclusions of research studies of the Sugar Beet in 1968. Kiev: All-Union Scientific Research Institute of Sugar Beet, p. 427-429 (in Russian).
Kosmachevskii, A.S. 1957. Pests of agricultural crops and their control measures. Krasnodar: Krasnodarskoe knizhnoe izdatel'stvo, p. 115-116 (in Russian).
Makarenko, E.E. 1962. To enhance the control of Tortoise Beetle. Zashchita rastenii 5: 35-36 (in Russian).
Matis, E.G. 1966. Tortoise Beetle. Zashchita rastenii 12: 28 (in Russian).
Matis, E.G. 1970. On harmfulness of Tortoise Beetle. In: Kurmanov, G.Sh., ed. Vestnik sel'skokhozyaistvennoi nauki 7. Alma-Ata: Kainar, p. 94-98 (in Russian).
Matis, E.G. 1971. Ecology and biology features of Tortoise Beetle and its economic significance. In: Kolychev, N.G., ed. Organization and economics of plants protection in RSFSR. Voronezh: Tzentral'no-chernozemnoe knizhnoe izdatel'stvo, p. 76-87 (in Russian).
Minoranskii, V.A. 1973. Leaf Beetles (Coleoptera, Chrysomelidae) damaging beet in Northern Caucasus. Entomologicheskoe obozrenie 52(3): 527-541 (in Russian).
Shchegolev, V.N. 1949. Plant protection from damage by insects and other pests. Moscow-Leningrad: Gosudarstvennoe izdatel'stvo sel'skokhozyaistvennoi literatury, p. 295 (in Russian).
Zubkov, A.F. 2003. Experimental essay on the Sugar Beet pests in Western Siberia and modern plant protection views. St. Petersburg: VIZR, 204 p. (in Russian).