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Abstract

Notes on the More Important Insects in Sugar-cane Plantations in Fiji.

Abstract

Sugar-cane in Fiji is generally grown on comparatively small and isolated blocks of land, frequently in narrow strips along the coast or the banks of rivers, large compact areas such as exist in Hawaii being practically unknown. The cultivated soils arc of various kinds, including alluvial flats, which are the most profitable for sugar-cane, red hill-soils, sand, stiff clay and reclaimed salt-marshes. The most destructive pest of sugar-cane on the island is Rhabdocnemis obscura (cane beetle borer) [RAE, A, v, 52-54, etc,]. The life-history and habits of this weevil are described. The percentage of stalks damaged by the borer found in the Fiji mills in 1917 was 14, a high figure when it is remembered that the injury materially reduces the sugar content of the untunnelled portions of infested stalks. Injured stalks arc also liable to fall to the ground, where they quickly rot. Remedial measures that have proved of great assistance in Fiji include the use, for seed purposes, of uninfested cane only, the burning of trash on badly-infested fields immediately after cutting, the ploughing of fields to be replanted as soon as possible after harvesting, and the collection of beetles by means of traps of split canes. For four years attempts have been made to introduce into Fiji the Tachinid parasite, Ceromasia sphenophori, which has proved so successful in Hawaii, but these attempts have been unexpectedly disappointing, the colonies gradually dying out as soon as the breeding cages were removed. It is thought probable that the large jumping spiders and the small brown ants, combined with peculiaritics in agricultural methods and in the habits of growth of the leading variety of cane are responsible for this failure. A less important sugar-cane pest is another weevil, Trochorrhopalus strangulatus, which is much smaller than R. obscura and not very seriously destructive, as it apparently never attacks perfectly sound cane, but generally breeds in rotten or weak stalks. The remedial measures described above Would also be useful against this species. White grubs that attack cane on an extensive scale are the larvae of Rhopaea vestita, and R. subnitida, both of which are natives of Fiji. These beetles never occur together; R. vestita is the more destructive and is found only in sandy soils; R. subnilida occurs in small numbers in alluvial soils, and is sometimes very injurious in red hill-soils. R. vestita deposits from 20 to 30 eggs singly in June and July at a depth of 15 to 20 inches and the grubs emerge in 28 to 34 days and begin feeding on humus and living vegetable matter, being most destructive from January to March. When full-grown they pupate in April or May in earthen cells at a depth of G to 18 inches, the adults emerging after 31 days and remaining on the ground for another week or two. Sugar-cane of all ages is attacked; fields replanted from January to March suffer severely. Enemies of these grubs are a wireworm, Monocrepidius pallipes, and a Scoliid parasite, Discolia ovalauensis. Mites attack the grubs in considerable numbers in captivity, but apparently do not exercise much control in the field. Cane should not be planted in infested fields until April; it will then be free from serious infestation until December or January, when it will be well established and better able to resist attack. Beetles and grubs should be collected by hand when possible, and the frequent ploughing of infested fields will expose many of the grubs to destruction by minah birds. R. subnitidits is very similar in life-history, seasonal occurrence and habits. As the reproductive powers of both species arc similar, the relative scarcity of R. subnitidus is worthy of further investigation; it is probably due to some enemy peculiar to this species that has escaped attention, or to some factor in the soils frequented by the grub. The remedial measures for both arc similar. Adorelus vcrsutus Har. (rose beetle) is not generally regarded as a pest of sugar-cane, but has been found attacking the germinating eyes of recently planted canes. The grubs occur in all types of soil, usually within 0 inches of the surface, and the beetles are found throughout the year. Only slight damage is done to sugar-cane. Scolia manilae, the parasite introduced to control white grubs in Hawaii with so much success, has been imported into Fiji and liberated. While the success of this experiment cannot yet be determined, grubs of R. veslila and A. versutus have undoubtedly been attacked by the parasite and adults were subsequently reared. Wireworms occurring in Fiji cane-fields include Simodactylus cinnamomeus, Lacon stricticollis and Monocrepidius pallipes. The first two species are very destructive to young cane, while the third is beneficial, as it is a formidable enemy of R. vestila. S. cinnamomeus is by far the most abundant species, being found in all types of soil, but reaching its maximum numbers in rich low-lying ll its. Its life-history and habits have been described [RAE, A, v, 182]. While this beetle is very difficult to control, beneficial results have been obtained by continuous planting of a certain proportion of the rows, transplanting being done only in wet weather. Cane on alluvial fiats should be second ratooned, as by this means the annual planting area is reduced; drainage should be improved wherever possible and the importance of clean cultivation cannot be overestimated. L. stricticollis is neither so abundant nor so voracious; its feeding habits arc similar and remedial measures identical. M. pallipes is found only in sandy soils, and has never been known to attack cane. Cirphis unipuncta (sugar-cane army worm) causes considerable damage to young crops, the caterpillars being most abundant during the cooler months; the damage in Fiji has never yet been serious. The caterpillars feed at dusk on the tips of the leaves, feeding at first on tender grasses until they are able to attack the harder tissues of cane leaves. The larval stage lasts three weeks, pupation occurring in the soil and lasting 10 or 11 days. Enemies include the minah bird, a hornet, Polistes macaensis, a Tachinid, Sturmia bimaculata, a Braconid, Apanteles sp., and certain Carabids. These enemies generally hold the pest in check, but, if necessary, poison-sprays could be used with good effect. Cirphis loreyi is occasionally destructive to cane, but its attacks are seldom serious. Prodenia litura (Mauritius-bean army-worm) is present in most plantations, Mauritius beans being used as a covering crop about once in four year, 5, and ploughed in as green manure. The usual food is tobacco [RAE, A, vi, 379]. Remedial measures are seldom necessary, enemies, including the brown ant, Pheidole megacephala, destroying large numbers of the eggs. Brachiyplatys pacificus (Mauritius bean bug) is another pest of Mauritius beans and the weeds of cane-fields and is largely controlled by an egg-parasite, Ooencyrtus pacificus, and a species of Isaria fungus. Trachycentra chlorogramma, is chiefly injurious in the low-lying reclaimed swamps which frequently contain a considerable proportion of sickly canes; apparently healthy stalks of Badila cane are also attacked. The damage by this moth is similar to that caused by R. obscura, but the tunnels are wider and shorter, and are frequently abandoned and a new one begun in the same or another stalk. The larva drags about with it a case in which it can quickly seek protection and pupation occurs in a very tough case made of cane fibre, generally within the larval tunnel. The burning of trash after harvesting and the exposure of tunnelled stalks to the heat of the sun will largely check the increase of this insect. A new species of Cosmopleryx (cane leaf-miner) tunnels in the mid-ribs of cane, especially in young plants. It is kept down to a considerable extent by parasites. A leaf-hopper, Perkinsiella vitiensis, in its young stages feeds on the cane, sucking the sap and excreting honey-dew, on which a black fungus grows; the injury is, however, slight, the hoppers being controlled by the egg-parasites, Ootetrastichus, Paranagrus, and Anagrus. A Stylopid, Elenchus tenuicornis, also attacks both the young and adult stages. Aleurodes comata is occasionally very numerous on the under-surface of cane leaves, but is never an important pest and is largely controlled by a Syrphid larva which feeds on all stages, and is an unidentified species of Xanthogmmma. Pseudococcus bromeliae (cane mealy-bug) is a minor pest of cane. The locusts, Locusta danica and Cyrtacanthacris guttulosa, are sometimes numerous along the edges of cane-fields where they strip the cane leaves to the mid-ribs. The minah bird is a very effective check on these locusts, which constitute its chief food. The hornet, Polistes macaensis, has only occurred in Fiji for the last 15 years or so, but is already one of the commonest insects in the islands. The fertilized females, after hibernating during August and September, begin nest-building in. October. The first generation requires 30 to 40 days for its life-cycle, and by April the neat may be about 12 inches in. diameter. Towards the end of April both males and females appear and mating takes place in June and July. These hornets act as a check on many pests in Fiji, and should be regarded as beneficial, though their sting is very painful to man and necessitates the expense of canvas suits, gloves and veils for the workers that in certain districts are obliged to enter the fields and destroy the nests before the cane can be thrashed. In most localities, however, the cane is cut during the hibernating period so that this difficulty does not arise.