Evaluation of neem seed extracts for the management of some cocoa mirid species

ABSTRACT

Laboratory and field studies were conducted to investigate the toxicity,

antifeedant activity, moulting inhibition property and persistence of two neem seed

extracts against the cocoa mirids Distantiella theobroma (Dist.) and Sahlbergella

singularis Hagl. The study also investigated the effects of neem extracts on nontarget

arthropods. Propoxur (Unden 200 EC) was used for baseline comparison. In the

laboratory bioassays, the neem was applied at the concentrations of 5%, 10%, 15%,

20%, 25%, and 30% weight/volume (w/v) (water extract) and 0.77%, 1.54%, 3.08%,

and 4.62% (neem oil).

The neem extracts acted mainly as antifeedants and stomach poisons against

the mirids. The 20% neem extract provided the greatest antifeedant effect (84.3%) and

gave the highest nymphal mortality (93.3%) 30 hours after treatment. Moulting was

significantly inhibited (>50% inhibition) by the neem oil. LC50 values estimated

from regression equations were 8.17% and 2.67% for the water extract and neem oil,

respectively. LT50 values for the 20% neem seed water extract and 4.62% neem oil

were 16 and 24 hours respectively. LC50 for propoxur is 2.73 x 10'3 % (Marchart,

1971).

In subsequent experiments, the 20% neem seed extract and 0.3% and 3.0%

neem oil were tested in the field. Two application methods, spraying a tree from one

side of the trunk into the canopy (T1 method), or spraying from both sides of the tree

(T2 method), were compared using the "Urgent" GmbH motorised knapsack

mistblower at the 2nd and 3rd nozzle restrictor positions.

When the mistblower was operating at the 2nd restrictor position, the resulting

application rates were 75 and 480 litres/ha for the T1 and T2 methods, respectively.

Corresponding application rates at the 3rd restrictor position for T1 and T2 methods

were 100 and 640 litres/ha. Results from the field study confirmed the short residual

activity of neem observed in the laboratory. Reduction in mirid numbers after 48

hours of treatment at the 2nd restrictor position were 80.3%, 51.7% and 80.4% for the

20% neem seed water extract and 0.3% and 3.0% oil treatments, respectively using

the restrictor 2. Restrictor 3 gave 88.9% reduction in mirid numbers after 48 hours of

treatment. No significant difference in reduction in mirid numbers was found between

the two different restrictor settings (P = 0.05) within 48 hours.

When trees were sprayed from two sides, a relatively better result was

obtained than when they were sprayed from one side only. The addition of 4.5 ml

propoxur per litre of neem extract increased efficacy significantly. The T2 method

gave a more prolonged control of the mirids as the population did not recover even

after two weeks of treatment. The more enhanced spray coverage achieved using the

T2 method was therefore promising. Current trend of pesticide application technology

however, is towards low volumes of application. For this reason, application of neem

and propoxur mixture, using the T1 method (75 liters per ha.) may offer a more

practical approach of mirid pest management especially in small scale farming

systems. Neem was observed to be toxic to some nontarget insects, particularly ants,

and spiders. Further investigation into neem/propoxur combinations and hazardous

effects on nontarget species is suggested.