Some Aspects of Mealybug Behaviour in Relation to the Efficiency of Measures for the Control of Virus Diseases of Cacao in the Gold Coast

Abstract

Mealybug populations (Pseudococcus njalensis Laing) originally present in piles of slash infected with the virus of swollen shoot of cacao are usually small. It is suggested that many of the insects leave the trees at an early stage in the development of infection.Slash piles offer conditions which induce mealybugs to remain within them. The piling of infected slash retards the rate of wilting of the cut wood and consequently retards the rate of migration of the mealybug population. The construction of piles protects the mealybugs from direct insolation and rainfall and leads to the persistence of the population on the cut wood. The colonies which survive the longest are those in the lower part of the heaps where death and desiccation of the material is most delayed.Mealybugs continue to live and to reproduce within heaps of slash for as long as nine weeks or more. Reproduction of the mealybugs is stimulated as the tissues of the felled trees become wilted and the resulting first-instar nymphs contribute numerous participants to such migration as does occur. There is evidence from laboratory experiments that these insects are not potential vectors.The concentrating of mealybugs in slash piles produces no increase in the population of Coccinellid or Cecidomyiid larvae within the piles.Mealybugs are capable of walking quite rapidly for long distances over smooth surfaces. They become incapable of movement when starved for a few days and the majority die within a week. A soil surface seriously impedes the progress of mealybugs which leave the slash heaps and this effect is more marked with the nymphal stages. Even on clean-weeded soil the rate of travel is slow and few. if any, insects can travel over appreciable distances in the period during which the virus may persist within them. Results of preliminary experiments were confirmed by the use of insects labelled with radioactive phosphorus.Some of the ants, which become associated with slash piles and which remain in attendance of the residual mealybug population, carry mealybugs, but the number of insects transported in this way is of little practical significance.The establishment of migrating mealybugs on seedlings and mature cacao is poor even in the presence of their attendant ants.All these factors make the possibility of infecting healthy trees from infected slash extremely remote. There is accordingly no reason to believe that the efficiency of “ cutting out ” is seriously reduced by the movement of vectors from the infected trees which are cut out.Without the co-operation of farmers in supporting the removal of contact trees no further recommendations can be advanced to improve the present method of control of swollen shoot.