Movements of the vectors of virus diseases of cacao in Ghana. II.—Wind movements and aerial dispersal
Authors: Cornwell P. B. 
Published: July 2009

The aerial dispersal of the Pseudococcid vectors of virus diseases of cacao in Ghana and the possible influence on it of wind speed was studied at Tafo. Seasonal variations in wind speed in Ghana are slight, particularly inland in the cacao-growing areas. Observations at Tafo during 1955 showed that air movement was maximal between 12.30 and 3.0 p.m. and minimal during the hours of darkness. Daily wind speeds in the open, from April to November 1955, averaged 1·1 m.p.h., 4 ft. above ground. About 80 per cent, of wind speeds measured in one-minute periods on two days in February 1955 were within the range 1–5 m.p.h. and 11–12 per cent, between 5 and the maximum recorded speed of 7 m.p.h.

In cacao, wind speed is reduced by a factor of 10–20 times; daily speeds averaged 0·09 m.p.h., at 8 ft. above ground during April and May 1955. The highest daily averages under a closed canopy (400–650 ft./hr.) were recorded 2 ft. above ground-level; speeds fell to a minimum (25–125 ft./hr.) in the canopy at 15–25 ft., and rose again above the canopy at 30 ft. to a speed (250–350 ft./hr.) comparable with that just below the branches at 10 ft. At breaks in the canopy caused by the removal of diseased trees, daily averages were lowest (125–150 ft./hr.) 2 ft. above the ground. They rose to a peak (400–600 ft.) on a level with the lower branches of the canopy, dropped markedly (250–450 ft./hr.) at the level of the middle canopy and rose to a maximum (650–1,000 ft./hr.) at 30 ft.

All instars of Pseudococcus njalensis Laing walking on pieces of cacao wood in the laboratory withstood removal at air speeds of 10 m.p.h., but these and corresponding stages of three other species could be dislodged by gently tapping the wood. The late nymphs and adults of Ps. njalensis and Ps. gahani Green were more easily removed than their ‘crawlers’, though this difference was not found between the developmental stages in Planococcus citri (Eisso) and Ferrisiana virgata (Ckll.). Amongst the four species tested, first-instar nymphs of Pl. citri were most easily dislodged, and those of Ps. njalensis or F. virgata more so than those of Ps. gahani.

Airborne mealybugs were caught on adhesive traps, on bait twigs pinned to mature trees, and on cacao seedlings. Eight vector species became established on cacao after dispersal by air currents.

Under a closed canopy, more airborne mealybugs were caught at two and ten feet above ground than at levels in and above the canopy. At breaks in the canopy, catches averaged about 13 per cent, of those obtained under a continuous canopy and were insufficient to show changes in aerial density with height. The distribution of catches over the surface of traps would suggest that mealybugs drop from the branches, are carried by air currents when falling, and become laterally dispersed at levels a few feet above the ground.

In a clearing where cacao had been removed to simulate conditions following the routine cutting out of virus-infected trees, airborne mealybugs became established on seedlings at a distance of 45 ft. from infested cacao trees. The ratios of boxes of seedlings which became infested by aerially dispersed Ps. njalensis at increasing distances from infested standing cacao, in relation to those beneath it (unity), were: 0·86 at 10–20 ft., 0·57 at 30 ft. and 0·14 at 40–100 ft. Under conditions of high insolation, the maximum recorded distance of mealybug aerial dispersal from surrounding vegetation to cacao seedlings was 340 ft. Aerial catches on seedlings 40 to 165 ft. from cacao showed an over-all decrease with distance.

Aerial dispersal is more pronounced during dry conditions, particularly during the main dry season, December–February, and to a lesser extent during the brief dry period experienced in July or August.

The infestation rate of cacao seedlings by windborne mealybugs (predominantly Pl. citri and F. virgata) was increased by 50 per cent, when plants were protected from weather by artificial shade. These traps, insulated from ants, failed to become infested by Ps. njalensis.

Out of 64 young cacao trees, 22 per cent, became infested by airborne vectors during the five-month period May to September 1955; when, for similar trees, normal dispersal was augmented by an initial artificial colonisation with Ps. njalensis, the corresponding figure was 41 per cent. The infestation rate, after either augmented or natural dispersal, was not significantly affected by attempts to establish on the trees nests of the ant, Grematogaster striatula Emery, or by affording protection from the weather in the form of artificial shade. It was evident, however, that the presence of mealybug-attending ants is almost essential for infestations of Ps. njalensis to develop, but there must be other limiting factors, since establishment failed on 58 per cent, of trees on which coccidophilic species were present.

The part played by airborne vectors in extending infection by radial and ‘jump spread’ is discussed, together with the possible use of insecticidal measures to prevent their establishment on the trunks of healthy cacao.