4.5.1 Insecticides

Insecticides (as opposed to fungicides and herbicides) are perhaps the most controversial of the pesticides. Historically, they have included some of the most toxic substances applied by farmers, but modern insecticides now include substances which can be formulated into products that are in toxicity class III or better (see section 5.1.1). 

The following is a brief description of the IRAC MoA groups, with a summary of the properties of insecticides in current use for cocoa given in Table 4.1.
 

Group 1

Group 1 insecticides inhibit the Acetylcholinesterase (AChE) pathway at nerve junctions. Because the AChE mechanism in insect synapses is similar to that of mammals, many group 1 compounds are extremely or highly hazardous (toxicity class I), although there are exceptions (e.g. malathion, temephos which are in toxicity class III). 

This group contains a number of systemic compounds (e.g. carbofuran, carbosulfan, dimethoate, monocrotophos) and with vp values >1 may have significant vapour action. 

They are divided into two chemical sub-groups:

1. carbamates such as promecarb and propoxur that have been used on cocoa, but are now withdrawn in the EU. Fenobucarb (BPMC) is still widely used against sucking pests in Asia, but not in Europe, so residue tolerances above the Limit of Determination (LOD) for these compounds in the EU are bound to be temporary.
2. organophosphorous (OP) insecticides such as malathion, chlopyriphos and pirimiphos
    

Group 2

Group 2 compounds are called GABA [1] -gated chloride channel antagonists and include two sub-groups:

1. older organochlorine compounds: HCH [2] (hexachlorocyclohexane: of which the purified gamma isomer is called lindane) and the cyclodiene chemical group, that includes endosulfan. Both HCH and endosulfan have historically been very important insecticides in cocoa, but are now obsolete and have been withdrawn. Their fumigant action (high vp: see section 4.4.1) was considered to be a useful property for farmers - substituting for poor application - but is now unacceptable on environmental grounds; in 2009, the production and agricultural use of lindane was banned under the Stockholm Convention on persistent organic pollutants.
2. the relatively new (reported in 1992) group of chemicals called phenylpyrazoles or fiproles, represented by fipronil. Highly potent against a wide range of insects, it can be used at very low rates of application and formulated into products classified as toxicity class III. Nevertheless, fipronil has a toxic sulfone metabolite (MB46136) and, unusually, it has been assigned a MRL of 0.005 (which is below the ‘default’ LOD value). Also, with a known high impact on non-target organisms, it should be deployed with great care and is primarily used for its very effective protection of seedlings (and wooden structures) from termite attacks.

The organo-chlorine compound DDT actually belongs to the same IRAC group (3) as pyrethroids (see Box 2 below) - all these chemicals attack the insect nervous system, but in different ways. 

DDT and most compounds in groups 1-2 represent ‘old insecticide chemistries’ and have been most heavily decimated by regulatory and commercial factors over the past two decades.

The few that remain (mostly OPs) are usually ‘softer’ representatives of their class. They are considered practical and attractive to farmers because they are cheap, fast acting and have a broad spectrum of action. In terms of pest management strategy, they help maintain diversity of MoA for resistance management (IRM), OPs in particular do not build up in the environment and some have such a short persistence that they rarely present residue problems.

Nevertheless, they are suspected endocrine disruptors (see Box 1) and a recent review [3] concluded that “The majority of well-designed studies found a significant association between low-level exposure to OPs and impaired neurobehavioral function” in humans. 

It is therefore probable that OPs are unlikely to remain permitted in most countries beyond the end of the decade.
 

Pyrethroids (IRAC MoA group 3)

Previously the most important insecticides by market share, now the second largest sector of the synthetic insecticide market. They are highly effective against agricultural and public health major pests. 

First introduced thirty years ago by a team of Rothamsted Research scientists led by M. Elliott, they represented a major advancement in activity and relatively-low mammalian toxicity. Their development was especially timely with the identification of problems with DDT (see Box 2) which belongs to the same MoA group (they interfere with sodium transport in insect nerve cells).

Work consisted firstly in identifying the most active components of pyrethrum, extracted from East African chrysanthemum flowers and long known to have insecticidal properties. Pyrethrum rapidly knocks down flying insects, but has a low mammalian toxicity and negligible persistence - which is good for the environment but gives poor efficacy when applied in the field. Pyrethroids can be described as chemically stabilized forms of natural pyrethrum.

The 1^st generation of pyrethroids, developed in the 1960s, include bioallethrin, tetramethrin, resmethrin and bioresmethrin. They are more active than the natural pyrethrum, but are unstable in sunlight. Activity of pyrethrum and 1^st generation pyrethroids is often enhanced by addition of the synergist piperonyl butoxide (which is not itself biologically active). 

After EC 1107/2009, many 1st generation compounds were not re-registered, often because the market was simply not big enough to warrant the costs (rather than any special concerns about safety).

By 1974, the Rothamsted team had discovered a 2nd generation of more persistent compounds notably: permethrin, cypermethrin and deltamethrin. They are substantially more resistant to degradation by light and air, thus making them suitable for use in agriculture, but they have significantly higher mammalian toxicities. 

Over the subsequent decades, these were followed with other proprietary compounds such as fenvalerate, lambda- cyhalothrin and beta-cyfluthrin, but most patents have now expired, making them cheap and therefore popular (although permethrin, fenvalerate and, more recently, beta-cyfluthrin are no longer approved in the EU for use on edible crops under the 91/414/EEC process). 

One of the less desirable characteristics, especially of 2^nd generation pyrethroids, is that they can be irritant to the skin and eyes, so special formulations such as capsule suspensions (CS) have been developed.

Pyrethroids have been widely used against cocoa insects, especially mirids in West Africa (also Helopeltis and cocoa pod borer in South East Asia). They belong to commonly used examples, including bifenthrin, deltamethrin, cypermethrin and lambda-cyhalothrin. Synergized tetramethrin has been applied extensively for control of warehouse pests - partly due to its low persistence and irritancy, but (together with permethrin and bifenthrin) it has not been re-registered. 

First generation pyrethroids have been replaced with natural pyrethrum (usually synergized) and other permitted, 2^nd generation ‘knock-down’ insecticides such as cypermethrin (note that alpha, beta and zeta-cypermethrin are no longer approved for use in the EU). 

These must be used very carefully due to greater persistence and the general risk of insecticide resistance.
 

Neonicotinoid and similar insecticides (IRAC class 4)

Nicotine, the ‘active ingredient’ for smokers, is also a very potent insecticide. Being a natural product, ‘tobacco tea’ was previously permitted for organic pest management, but purified nicotine would be classified as most toxic (class 1) if sold commercially. 

As with pyrethrum and the pyrethroids, the commercialised synthetic analogues, called ‘neonicotinoid’ or ‘nicotinyl’ insecticides (NNI) are more stable than their natural progenitors in sunlight. 

Unlike pyrethrum and pyrethroids but in common with other ‘new chemistries’, NNI typically have relatively low mammalian toxicities compared with their natural analogue, with several products available in toxicity class III.
 

Table 4.1 Properties of some insecticides in current use for cocoa (EU approval status updated Sept. 2025)

There are now about a dozen NNI that have been developed since imidacloprid was introduced in 1991 by Bayer AG and Nihon Tokushu Noyaku Seizo KK. They belong to three chemical sub-groups, of which two are of current interest in cocoa. 

All NNIs are systemic, having a high solubility and log P values of <1 (see Table 4.1). Probably the most controversial aspect with these compounds is the relatively high toxicity of some AI to bees (in spite of having passed through a whole raft of environmental testing before registration). 

In Europe, the problem was managed by engineering controls that greatly reduce drift of spray droplets and dust from seed dressings.

In 2013, a moratorium was placed on three NNI: clothianidin, imidacloprid and thiamethoxam in the EU (see section 4.8). At this stage, we can only speculate on the practical medium to long-term consequences of this moratorium and any further restrictions in cocoa consuming countries. 

Withdrawal from use in the EU could result in diversion of products to secondary markets (with possible consequent ‘price competitiveness’ or ‘dumping’ depending on the viewpoint). We could also expect cyano-substituted NNI to be promoted, justifiably, as ‘more bee-friendly’ or similar; the table above shows that they are more than 2 orders of magnitude less toxic to bees than the nitro-group, especially via the oral route. 

However, an even higher current priority in cocoa is residue management (see Appendix 3). There needs to be more information in the public domain of dosage, concentration level of AI and consequently, whether existing field application practices (and pre-harvest intervals) risk residue levels downstream exceeding of MRLs.
 

Other insecticidal modes of action

The insecticides described above all act on biochemical pathways in the insect nervous system and are thus be grouped as ‘neurotoxic’ or otherwise active on insect coordination. As understanding of the effects of insecticides on target biochemical pathways improves, updates are made available by IRAC [7]. 

Research-based agrochemical companies continue to explore new markets for their proprietary AIs and these are listed here in Appendix 3C, as information is made available to the authors. Companies have recently emphasised the ‘natural origin’ of a number of MoA groups: for example, groups 5 and 6 consist of fermentation products, with relatively large complex molecules called ‘macrocyclic lactones’. 

These were derived from Saccharopolyspora spinosa and Streptomyces avermitilis respectively. There is considerable interest in the latest MoA group (28), the diamides or ryanodine receptor modulators, which are synthetic analogues of water-soluble extracts of the tropical shrub Ryania speciosa; exposed insects exhibit general lethargy and muscle paralysis leading to death, but mammalian toxicity is very low.

There are also reports of limited use of nereistoxin analogues (group 14) in cocoa: a small group of commercial alkaloid pro-insecticides derived from Nereis spp. (marine ragworms). 

Examples are cartap hydrochloride, thiocyclam and thiosultap-sodium: like NNI and spinosyns they affect, in this case block, the nicotinic acetylcholine receptor (NAchR) channel in insect nerve synapses. 

Although available in Asia and Africa, they cannot currently be recommended since MRLs have yet to be established in the EU and elsewhere.

Several of the ‘newer chemistry’ active substances are especially attractive since they have low mammalian toxicities, thus helping to overcome one of the major criticisms of insecticide use. 

Older MoA groups, often of lower toxicity to both mammals and non-target organisms (IPM compatible) have included non-neurotoxic compounds that specifically target insect biochemical pathways. These include various mechanisms in the formation of insect cuticle, regulation of ecdysis (moulting) and other endocrine functions unique to insects and other arthropods. 

Usually slow acting (e.g. taking more than 2-3 days to show activity in the field), non-neuro- active products have proved more difficult to sell, involve greater levels of farmer training and may encounter difficulties at the registration stage (see section 4.7). 

Nevertheless, the need to find effective control measures against pests such as cocoa pod borer and maintain a diversity of MoA for resistance management, may yet establish a role for insecticide groups 15, 18 and possibly others. 

The tetronic acid spirotetramat (group 23) was the first insecticide to exhibit downward (basipetal) translocation, making it very effective against certain sucking insects; it is undergoing evaluation against the mealybug (Pseudococcidae) vectors of cocoa swollen shoot virus disease (CSSVD).

It is important to mention here the potential for microbial control agents (MCA) including entomopathogenic fungi (e.g. Metarhizium and Beauveria spp.) and viruses. These have yet to be assigned MoA groups by IRAC, but the bacterium Bacillus thuringiensis, the most important biopesticide world-wide, has been assigned into group 11A: ‘microbial disruptors of insect midgut membranes’. 

It has been suggested that the ‘cry’ proteins that generate this action could be expressed in the cocoa husk and efficacious against pod borer [9], but genetic modification in this crop is considered highly controversial, even in the Americas.

In most cocoa-growing countries, insecticides constitute the greatest number of registered products: ‘newer chemistries’ are now being registered (see Appendix 3). 

However, the diversity of MoA remains limited and the market is dominated by NNI and pyrethroids, with mixed AI products increasing.