4.5.4 Pesticides for vertebrate pests
A range of vertebrate pests, from elephants to smaller rodents and birds, have been recorded as cocoa pests [1]. It is significant perhaps, that vertebrates are probably responsible for most natural sowing of cocoa seed, with the Brazilian kinkajou (Potos flavus) specifically associated with cocoa in its centre of origin.
The most consistently damaging species are probably rats and squirrels, with studies indicating crop losses of between 1% and 20%. Losses in South -East Asia and certain islands appear to be especially high, with anecdotal reports of high damage where cocoa is grown near food crops such as rice; the world average loss may be 5-10%.
For many years, there were essentially two groups of rodenticides: acute and chronic agents, which are by necessity all highly toxic to mammals.
The older, acute toxicants such as zinc and aluminium phosphides could become ineffective due to ‘bait shyness’: where rats learned to associate the food bait with the poison.
Sodium fluoroacetate (‘1080’) is another inorganic acute poison: considered effective for area-wide control operations (including aerial applications), but it has become unacceptable for “environmental, animal welfare and social pressures”.
Anti-coagulants kill by preventing blood clotting, but the first generation of agents (e.g. warfarin) could be subject to bait shyness. They were supplemented with a number of ‘second generation’ anti-coagulant rodenticides (SGAR): that only require a single feed by the pest and have a delayed action.
Anti-coagulants (e.g. bromadiolone, difenacoum and warfarin) are no longer permitted in the EU and are all subject to the default MRL of 0.01 mg/ kg.
Formulated together with the toxicant and a food-bait (often grain), with a warning colorant within a waxy, waterproof matrix: bait block (BB) formulations could simply be tied singly to cocoa trees but are now only for indoor use in the EU, due to the impact on raptors such as owls (see below).
The success of rodent control operations often depends on the scale of treatment and timing: it is usually better to apply over larger areas (e.g. whole villages) when alternative food sources for the pest are most scarce (e.g. the beginning of the field crop growing season).
Very small-scale operations, such as treatments in single houses, may have only a short-term effect and be a false economy; large-scale campaigns should be accompanied with public education about the hazards of baits and supplies of the anti-coagulant antidote (vitamin K1).
A combination of rodenticide resistance and concerns about their toxicity has prompted investigations into alternative methods over the last decade.
A review of these [2] included certain plant extracts and cholecalciferol (calciferol or vitamin D3), which may be efficacious on its own or used in combination with SGARs such as coumatetralyl.
Biological rodent control approaches have included the services of barn owls, with their successful establishment in a cocoa-coconut agro-ecosystem in Malaysia [3].
Rodenticides must be used carefully: UK studies on their impact showed increased presence in birds, with widespread use towards the end of the 20th century, but only 7% of contaminated owls (forming 2% of all owls examined) were judged to have actually died of rodenticide poisoning [4].
A microbial control method uses a product based on the protozoan Sarcocystis singaporensis [5].
