S. Mahunka szerk.: Folia Entomologica Hungarica 27/2. (Budapest, 1974)
the Department of Agriculture, Stock and Fisheries of the Administration of Papua New Guinea, was asked to visit the Maprik Subdistrict in order to assess whether the outbreak of the pest could have been a result of the extermination of its usual natural enemies wich are most likely small Hymenoptera and Coccinellid beetles. These, as a rule are very sensitive to the hydrocarbon insecticides whilst these chemicals are not very effective against pseudococcids. In Papua New Guinea meinly organic phosphates are used to control mealy bugs. During a relatively short stay of the author in the Maprik Subdistrict in June-July 1959 it could not be proved that an imbalance of parasite and host populations was caused by the spraying. On the contrary it was found that insects which are usually susceptible to chlorinated hydrocarbons, such as ants were present in large numbers in almost every storage house associated with the mealy bug populations. Tiny Hymenopterons and Dipterons were sighted in several storage houses and termites were present in two houses; all these are known to be susceptible to chlorinated hydrocarbon insecticides. Furthermore it appeared that only the inner walls of the storage houses were sprayed by the units of the Malarai Eradication Pilot Project of the Department of Public Health (Administration of Papua New Guinea) magainst the Anopheline mosquitoes resting there; the yam-roots and the insects a sociated with them could not have been seriously affected because the yam roots in the storage houses are never stood agains' the walls but they are always placed on the earthern floors where they lie in neat rows. It was found that mealy bug populations were also present in some storage houses which were not sprayed by the mosquito eradication teams and there was no essential difference between the population density of the pest in sprayed and unsprayed houses. Having discussed the problem of the mealy bug outbreak with older inhabitants of the villages, they said that they remembered a similar outbreak which appeared in the yam storage houses some 15-20 years earlier. This was at a time when chlorinated hydrocarbon insecticides were unknown and the old inhabitants confirmed that no malarie mosquito eradication teams have visited the area during this earlier outbreak. To the best of the author' s knowledge there was no intensive malarie eradication carried out in the Maprik Subdistrict by the Department of Public Helath of the prewar New Guinea administration, in the period between the two World Wars. Digging up some yam roots in a village garden the author found that these were also infested with Planococcus dioscoreae .This garden was at a dustance of 1, 5 Km from the closest yam storage house which was treated with hydrocarbon insecticides. The above-mentioned facts indicate that it is very unlikely that the spraying against Anopheline mosquitoes had anything to do with the outbreak of the pseudococcid. It seems very likely that this pest is kept at a very low level of population density by its natural enemies for a long time and then some extreme weather conditions, such as an extreme draught during the dry season or an extremely wet rainy season, or some other weather factors unknown to us, cause the sudden decraese of parasites and predators which is then followed by the appearance of the pest in east numbers. This could have been the case in 1959 and 15-20 years earlier. Similar observations were made on Coccus viri dis GREEN which appeared in unusually dense populations during the heavy rainfalls of the 1973/74 wet season on Cofea arabica in the Wau Valley. As a native insect of Papua New Guinea, Planococcus dioscoreae has undoubtedly some native host plants in this country (possibly indigenous species of Dioscoreaceae) and it spreads onto cultivated yam during heavy otubreaks only. Gradation, repeated appearance of the outbreaks in
