The biological Control of Prickly Pear in the Palu Valley, North Celebes.
In view of the fact that abandoned rice-fields and pasture lands in the Palu valley, a district in North Celebes with a very low annual rainfall of 12-30 inches, were being rapidly overgrown by a prickly-pear, Opuntia nigricans, a consignment of Dactylopius opuntiae, Ckll. (tomentosus, auct.) was obtained in December 1934 from Australia, and this Coccid was bred in a greenhouse at Buitenzorg, Java. It completed a generation in 48-63 days on O. nigricans, but required 80 days in one instance on Nopalea coccinellifera, a cactus sparsely distributed throughout Java, and did not survive on Opuntia monacantha, which is common on the northern coast of Java. In March 1935, distribution of infested material in Palu was begun. Care was taken to ensure that the material sent from Java was free from Cryptolaemus montrouzieri, Muls., which was present in the greenhouse [and has been introduced into other parts of Celebes (R.A.E., A 26 337)]. D. opuntiae spread rapidly on O. nigricans in all the areas in which it was liberated, and many plants were dying off by May 1936, while by the end of 1939 the dense cactus vegetation had disappeared from the valley, giving place to a new growth of Leucaena glauca[Leucaena leucocephala] and other plants. Isolated cactus plants in sunny places proved the most difficult to kill as the Coccid appeared to develop more slowly on them. No natural enemies of it were observed in Palu.