7.2.3 Insects

Mirids

Since the beginning of the 20^th century, cocoa mirids (Sahlbergella singularis and Distantiella theobromae: also known as capsids) were reported in West Africa [1]. These insects have become the most damaging insect pests in the region and are thought to cause annual crop losses in excess of 200,000 tonnes. 

They are an example of ‘new encounter’ pests - cocoa originated in the Amazon region of South America, and having been introduced to West Africa in the 19^th century, became infested with local insects that adapted to a new food source. 

Similarly, a complex of true bug pests (called Hemiptera) adapted to cocoa in South East Asia, including a number of mirid species in the genus Helopeltis.

Both nymphs and adults of S. singularis and D. theobromae cause economic damage to cocoa by feeding on shoots and immature pods by piercing and sucking sap with their needle-like mouth parts. 

Mirid feeding results in lesions on immature stems, branches and pods which can lead to secondary fungal infections and cankers. 

Mature pods do not suffer significant internal damage but the classic dark circular feeding lesions are usually the first symptoms that can be seen, younger pods (less than three months old) have less chance of surviving the damage :

Entwistle’s book [2] remains the best overview of early development of mirid control measures. Insecticide application techniques on cocoa remain essentially based on experiments that were carried out in the 1960s when the organochlorine gamma-HCH (also called BHC and lindane) was the AI of choice. 

Two properties, persistence and fumigant action (vp = 4.4 mPa), helped to overcome inadequacies in application and HCH remained in widespread use until the 1990s. 

Resistance (see section 4.6) to this organochlorine by cocoa mirids was detected in the 1950s and, as with other pests, necessitated the development of an Insecticide Resistance Management (IRM) strategy. 

A successful technique has been to interchange the compound with other insecticides, belonging to different MoA groups, in order to reduce selection pressure on a single biochemical pathway. 

Early screening of chemicals from the 1960s to the early 1990s focused on carbamates (IRAC group 1A) and organophosphorus (OP) compounds (group 1B). 

Examples of widely used AI included the carbamates: propoxur (vp = 1.3 mPa) and promecarb (vp = 1.4 mPa); the OPs: chlorpyrifos (available as methyl and ethyl compounds), diazinon and pirimiphos methyl and the organochlorine (IRAC group 2) endosulfan (vp = 0.83 mPa). Most of the compounds have now been withdrawn and fumigant action is now considered unacceptable in new pesticide development.
 

• Current management is often achieved with regular applications of pyrethroid (group 3) and neo-nicotinoid insecticides (NNI: group 4A) such as imidacloprid and thiamethoxam. The latter are of interest since they have systemic action and relatively low mammalian toxicities, but concerns have been raised about the possible impact on bees and other pollinators with the nitro-substituted NNI (Table 4.1). Spraying should be based on monthly monitoring of mirid damage but probably occurs on a calendar basis.
• Where outbreaks are limited to defined pockets, ‘spot spraying’ can be recommended to reduce the quantity of insecticides used.
• Cultural recommendations are to regularly remove chupons, which provide additional feeding and egg laying sites and to maintain a continuous canopy. Where the canopy is broken, new growth encourages mirid feeding – plantain can be planted to quickly close the open canopy.
• The search for alternative control methods continues, with two current lines of research (i) manipulation of mirid pheromones (mating attractants for better monitoring but not control [3]) and (ii) the use of biopesticides (plant extracts/oils and mycoinsecticides). Pest outbreaks often occur when a species is no longer controlled by its natural enemies (which in the case of Hemipteran insects include specific fungi that are diseases of insects). Mycoinsecticides are often formulated spores of such fungi and can be applied in a similar way to chemicals.
• A 2015 publication from CRIG, Ghana [4] proposes a more integrated/targeted approach for mirid management based on regular monitoring, forecasting and farm-specific recommendations based on levels of mirid damage to reduce the number of insecticide applications throughout the year.