Studies on the Coconut Pest, Pseudotheraptus wayi Brown(Coreidae), in Zanzibar.
Authors: Vanderplank F. L. 
Published: July 2009

An undesirable effect of using a dust containing 0·4 per cent, lindane for the control of Pseudotheraptus wayi Brown on coconut palms is the eradication of Oecophylla longinoda (Latr.), a species of ant inimical to this Coreid, and in the search for a more suitable preparation, formulations were tested in Zanzibar that contained DDT, with synthetic resins to prevent its absorption through the treated surface of the palm. One such formulation, prepared by dissolving 23·5 lb. of a proprietary coumarone indene resin and then 235 lb. 80 per cent. p,p′DDT in a mixture of 100 gal. power kerosene and 100 gal. diesoline, heated to 40–45°C., is here termed 10 DRDK and its properties described.

When 10 DRDK was applied by pneumatic hand-sprayers to the centres of palm crowns, small species of ants (Pheidole spp., Crematogaster spp., Paratrechina longicornis (Latr.)) and Anoplolepis longipes (Jerd.) were eradicated from such sites, whereas O. longinoda increased in numbers and other large species (Camponotus spp., Polyrhachis spp., etc.) were unaffected. Although 10 DRDK was lethal to all species when applied as an aerosol in the laboratory, the residual deposit from it killed only the smaller species, apparently because their bodies make contact with the DDT crystals whereas those of the larger species (except A. longipes) are held clear. Under Zanzibar conditions, this deposit, which is at first gummy, gives rise within 2–3 weeks, or following mechanical disturbance, to needle-like crystals of DDT, 10–120 μ long × 2–10 μ wide projecting from the sprayed surface and forming clumps up to 1·5 mm. high.

In field tests, paper bands dipped in 10 DRDK and attached round palm trunks prevented the passage of A. longipes for six months, and of Pheidole for nine months, despite heavy rain; that of the larger species of ants was unaffected. Determinations of the deposit on a sample band selected each week at random showed great variability but fell from 392–478 μg. DDT per sq. cm. initially to 92–176 μg. after 31 weeks. In concurrent tests of similar bands exposed in the laboratory, the deposit fell only to 282 μg. per sq. cm. over approximately the same period.

In tests of 10 DRDK applied topically to the dorsum of Pseudotheraptus wayi, complete mortality of adults was obtained within 48 hours with droplets 85 μ in diameter (containing 0·31 μg. DDT) and the size required diminished progressively, the earlier the stage treated. When individuals of P. wayi were allowed to walk for one minute on glass plates bearing known deposits from 10 DRDK, complete mortality of all stages, from 2nd-instar nymphs to adults, followed within 24 hours of exposure to 2·7 mg. DDT per sq. ft., but not to 1·6 mg., except in the case of recently moulted adults, 5th- or 4th-instar nymphs.

Two experimental areas totalling some 1,000 palms, of which 10 per cent, had Oecophylla nesting in them in October 1952, were sprayed with a commercial formulation containing DDT from November 1952 to June 1953, and the percentage fell to 2·5; the areas were then treated with 10 DRDK by hand-sprayer until January 1954 and by machine for the rest of 1954. The percentage of palms having Oecophylla, in October 1953, October 1954 and February 1955 increased to 7, 15 and 38, respectively. In a control area one mile away, the percentage rose from 23·5 to 36 over the same period. In a third area, 12 applications of 10 DRDK totalling 10 gal. per acre were made by machine during 1953, 12 totalling 33 gal. in 1954 and 9 totalling 30 in 1955, and the percentage of palms colonised by Oecophylla rose from nil to 21·7 in 1953, to 26·1 by March 1955 and to 30 by April 1956, while the percentage colonised by A. longipes fell over the same period from 90 to 1.5. In the control areas, A. longipes remained dominant and no Oecophylla became established.

The addition of lindane to 10 DRDK to give concentrations of 0·1, 0·5 or 1·0 per cent, adversely affected the selective properties of the formulation, and although the highest strength gave a good kill of P. wayi and other insects, apart from Coccids, which increased considerably, it reduced Oecophylla to a greater extent than it did other species of ants. No advantages are obtained by the addition of dieldrin, endrin, pyrethrins or rotenone.