| Locust handbook |
|5. Southeast Asian locusts|
The genus Valanga is probably undergoing a period of rapid evolution and shows considerable adaptability. Some 18 subspecies of Valanga nigricornis have been recognised occurring throughout the area shown on Fig. 130. It is found in forest clearings and shrub thickets, feeding mainly on trees.
The adult is a yellowish-brown, or yellowish or greenish with blue-black markings; the hindwings are a deep rose colour. Males are 45-55 mm long and females 15-75 mm (Fig. 131). There is one generation a year. Eggs are laid in clearings in moist soil. Females can lay 2-4 pods at intervals of 12-41 days, with a mean of 158 eggs per female. Hoppers hatch after 60-75 days in Malaysia but in Java it takes 6-8 months. Males usually develop through six instars, sometimes seven, and females through seven, occasionally eight. In Malaysia, hopper development in cages was 92-125 days for males and 111-136 days for females. Hoppers are light green with black markings.
In areas with marked wet and dry seasons it appears that hoppers hatch and develop through the wet season so that Valanga survives the dry season as an immature adult, e.g. Thailand. In Java however, the dry period is survived in the egg stage. Elsewhere where there are no pronounced seasons all stages can be found together. For example, in west Malaysia there are two peak periods of laying, December-January and June-July. Hoppers hatch about 60 days later and fledging occurs in June and December. The adult stage from fledging to final laying lasts about six months which suggests that there are two physiological races each having one generation a year.
Young hoppers remain close to laying sites. Both hoppers and adults like sunlight and seek out sunny areas for basking, either at the top of vegetation or on open sites. Most feeding is done during the daytime. There are no records of gregarious behaviour.
Most damage done by Valanga occurs when numbers increase locally. There are no records of invading swarms and large-scale movements. Local population increases appear to be related to rainfall. Heavy rainfall causes high mortality in both eggs and hoppers but dry periods allow numbers to increase. In Malaysia it is thought that several consecutive dry seasons are unlikely and as Valanga has only one generation a year then widespread population increase also seems unlikely.
In Java, where eggs remain dormant for 6-8 months, dry weather delays hatching but the arrival of the west monsoon results in the simultaneous hatching of many hoppers. Fledging occurs towards the end of the west monsoon and the young adults soon mature and lay. If the succeeding east monsoon is particularly dry, however, the adults concentrate and if these conditions are repeated the following year then there will be many locusts. The outbreak in 1915 in the teak forests of Java was thought to have followed three seasons of below average rainfall and synchronisation of breeding.
Elsewhere outbreaks are thought to have arisen when forest areas were cleared for cultivation.