An account is given of the damage caused by adults and larvae of Brontispa longissima (Gestro) to the young unopened fronds of coconut palms in the British Solomon Islands. Normally, only young palms less than 10 years old are severely attacked, but this has interfered seriously with replanting programmes. Full-grown palms suffer severe damage only occasionally; reasons for this difference in susceptibility are suggested.
Earlier reports, that seedlings of Malayan type are more readily and severely attacked than those of Samoan or local origin, were confirmed. A fertiliser treatment had no effect on the growth of the palms or the amount of damage by Brontispa.
Laboratory tests showed that the deposit from a spray of 0·2 per cent. p,p′DDT was completely effective for three–four weeks, that chlordane (0·16%) became significantly less effective in the third and fourth weeks, and that dusts of DDT, alone and with BHC, showed signs of diminished efficiency in the fourth week.
In small-plot trials with sprays of DDT or chlordane, applied at various concentrations and frequencies, the effects of treatment were assessed by counts, at suitable intervals, of living and dead examples of Brontispa, by estimating damage to the youngest open frond, and by measuring the growth of seedlings. DDT at 0·1, 0·2 and 0·3 per cent. of p,p′isomer, and chlordane at 0·08, 0·16 and 0·24 per cent. gave very satisfactory control of a heavy attack of the beetle and suppression of damage. Applications at 10-day intervals were more effective than 3-weekly ones. Similar trials with dieldrin at 0·1 per cent. and DDT at 0·3 per cent. against a light attack were completely effective.
In further trials, direct comparisons were made between DDT at 0·25 and 0·025 per cent., chlordane at 0·16 per cent. and dieldrin at 0·15 per cent. applied at both fortnightly and monthly intervals. The lower concentration of DDT was much less effective than the other treatments. DDT at the higher concentration and dieldrin were equally effective when applied at fortnightly intervals, but at monthly intervals the latter was superior in control of immigrant beetles. This is attributed to its greater persistence. It is concluded that, in the formulations used in this trial, dieldrin is superior to DDT and chlordane.
Further trials with dieldrin at 0·15 per cent. showed that palms subjected to only moderate reinfestation were adequately protected by spraying every six weeks, whilst isolated areas of young palms may be left much longer, probably several months, once the initial population has been controlled. The maximum frequency, under the most adverse conditions, might vary between 2 and 4 weeks, initially. Varietal susceptibility in the coconut does not affect the efficiency of control.
Slight phytotoxicity, indicated by retardation of growth and rate of production of new fronds, was caused by frequent applications of high volumes of spray, but the volume necessary to produce such effect is considerably in excess of that required for satisfactory control. Unnecessarily high volumes and concentrations should, however, be avoided.
The spray was applied to each palm, individually, downwards on to the central spike, where the beetle lives. Equipment capable of delivering a fine, low-volume spray controlled by an efficient trigger tap will use only about 16 cc. spray per palm up to 3 years old. With a low-volume atomiser, this consumption can be reduced to about 6 cc.
An imitation aerial spray gave promising but inconclusive results.
With the proprietary formulations of insecticides used, the addition of a wetting agent was unnecessary.
Field applications were tried out in the Russell Islands on extensive areas of young palms that showed heavy beetle attack. Sprays containing dieldrin or DDT, each in two proprietary formulations, gave excellent and satisfactory results, respectively.
It is concluded that the experiments clearly indicate that very satisfactory control can be achieved at low cost, and that a programme of replanting could be safely embarked upon without fear of the severe losses, due to Brontispa, that have been suffered in the past.