Insects injurious to forest and shade trees

Abstract

An account is given of the bionomics of the principal insect pests of forest and shade trees in South Africa, including the Longicorns, Phryneta spinator, F. [cf. R.A.E., A 8 69] and Phoracantha semi-punctata, F. [17 245], which are widely distributed on willow and Eucalyptus respectively. A list is given of the trees infested by the Saturniid, Nudaurelia cytherea, Cram, which causes severe defoliation. Pinus insignis appears to be the favoured food-plant. The eggs, which are deposited in batches of 100-150 on the young shoots in the higher branches, hatch in 18-19 days. The larvae moult 5 times. Pupation takes place in the soil at a depth of 2 ins., generally near the base of the tree. A loose sandy soil is preferred. The pupal stage lasts 6-7 months, usually starting in early November. The moths emerge in late May and early June and only live for a short time. They are not attracted to baits, and are slightly phototropic. The larvae are subject to a disease and to parasitism by Apanteles maculitarsis, Cam., and two other Hymenopterous parasites destroy about 9 per cent. of the eggs. Pigs will root out the pupae efficiently; in one district they reduced the average number of pupae round the base of each tree from 150 to 0.36. In 1926 and 1927 the Lasiocampid, Nadiasa (Taragama) concolor, Wlk., caused severe local defoliation of Pinus insignis [Pinus radiata] in the western Cape area, and then declined. It has two generations a year. Adults emerge in late June and July. The eggs are laid almost at once and hatch in about 2 weeks. The larval stage lasts 8-10 weeks and the pupal about a month. New adults start emerging in November and are found with eggs and young larvae in December. The main defoliation occurs from January to March. Of the larvae or pupae in cocoons examined, 74 per cent. were killed by disease, and 16 per cent. by insects, including a Tachinid fly, Sturmia dilabida, Villen., 3 Ichneumonids, a Chalcid and the larvae of Myelois ceratoniae [Ectomyelois ceratoniae], Zell., which are usually fruit feeders but were found eating dead larvae. Control by hand-picking proved expensive. In 1929, the Lymantriid, Euproctis terminalis, Wlk., defoliated Pinus leiophylla and P. patula, and to a less extent P. insignis and P. pinaster, in the eastern Transvaal. The infestation has become less severe, but is still quite serious, especially where delayed rains in the spring follow a dry winter. The adults emerge from the beginning of January to the end of February. The eggs, which are laid in batches on the trees, hatch in 2-3 weeks. The larval period lasts for the whole of autumn and winter, but the most serious defoliation occurs at the end of August and the beginning of September. The pupal stage, which is passed 2 ins. deep in the mat of pine needles, lasts from mid-October or November to January. The Lasiocampid, Bombycomorpha pallida, Dist. (pepper-tree caterpillar) although it does less economic damage, sometimes defoliates Schinus molle (pepper-tree) grown for ornament or shade. About mid-October the eggs are deposited in clusters of 180-270 in a band 1 in. long round the stalks of the young leaves or twigs. They hatch in 18-25 days. The larvae, which are gregarious, pass through 5 instars in 46-58 days, and when mature, may travel 20-50 yards in search of a suitable place to pupate. They transform to pupae within 16-22 days after the completion of the cocoon. The adults emerge about the end of January, second generation cocoons are formed from mid-April till the end of May, and adults appear again in mid-October. Parasitism of the larvae by the Tachinid, Sturmia (Argyrophylax) bimaculata, Htg., rises to 30-40 per cent. Infestation may be controlled by hand-picking and destruction of the branches bearing the larvae, or by spraying with 10.25 lb. lead arsenate in 40 gals. water. The Saturniid, Gonimbrasia tyrrhea, Cram., strips the foliage of ornamental willows (particularly Salix babylonica), Pinus insignis [Pinus radiata], Acacia, poplars and oaks. The eggs are deposited in late September and October in batches of 100-150 surrounding the young shoots or twigs, and hatch in 12-21 days. The larvae eat the leaves and the bark of the young twigs. They pass through 6 instars, each lasting 10-14 days, and then may feed for 3-4 weeks before transforming to pupae. The pupal stage is passed 2-3 ins. deep in the soil and normally lasts from January to September, though a small second brood emerges towards the end of January and the beginning of February. Two parasites attack the eggs, and in some districts destroy a high percentage. Hand-picking the larvae is effective in slight infestations, and egg-masses may be destroyed with the branches bearing them. Damage to timber by Lyctus brunneus [Ahasverus advena], Steph., which has only recently been recorded in South Africa [cf. 19 24], is increasing. The life-cycle occupies 7-12 months, and the beetles are normally active from October to February, mostly emerging in November and December. Two or more eggs are laid near one another in the pores of sap-wood and hatch in 2-3 weeks. The larvae feed during the summer and pass the winter when practically mature. The pupal stage lasts about a month, the pupae occurring near the surface of the wood. The attack of these beetles is confined to the sapwood of recently seasoned timber [cf. 16 585]. Control measures recommended include sterilisation by heat and chemical methods [cf. 16 139]. The greenhouse thrips, Heliothrips haemorrhoidalis, Bch., has recently attacked pines, Grevillea robusta and Eucalyptus, causing serious reduction in the size of the crown and a drain on the vitality of the tree. The female lays 10-20 eggs singly in the tissue of the pine needle, and these hatch in 9-10 days. The larvae feed in colonies on the surface of the needle, avoiding the sunlight. The two larval instars together last 2-3 weeks, the pre-pupal stage a few hours and the pupal 4-6 days, so that 12 overlapping generations may occur in the year. The constant humidity and high temperatures, and the subdued light resulting from dense stocking and delayed thinning. are favourable for the development of the thrips. Heavy thinning and pruning of stands of Pinus patula, up to 8 years old, stopped infestations for about 2 years, until the crown canopy had again grown over. Trees of which the crowns have been reduced owing to injury by the thrips show a marked recovery after thinning. Pruning the trees only is not efficient for more than a few months. It is therefore recommended that stands of P. patula should be thinned and pruned when 3-4 years old, and regularly thinned subsequently at 5-year intervals. Heavy showers of rain also destroy infestations.