Acephate

Acephate is one of the most widely used organophosphates in Brazilian agriculture, and is a fundamental tool in the integrated management of pests in economically important crops. Its systemic action and broad spectrum of control position it as a strategic product for the control of various entomological complexes, from sucking bugs to defoliating caterpillars.

Official chemical name: O-(S)-1-acetoxyethyl O,O-dimethyl phosphorothioate or O,S-dimethyl N-acetyl-O-methyl-phosphoramidothioate

Number CAS: 30560-19-1

Chemical formula: C₄H₁₀NO₃PS (molecular mass: 183,16 g/mol)

Common synonyms: known only as "acephate" in agricultural technical circles.

Chemical class: systemic organophosphate, belonging to the phosphoramidate group.

Release year and history: Acephate was developed and registered as an insecticide in the 1970s, quickly establishing itself as an important tool in integrated pest management. Its formulation as a soluble powder allowed it to be widely adopted in economically important crops, and is considered by agronomists as a "joker" product due to its broad spectrum of action.

Mode of action

Specific biochemical mechanism: Acephate acts as an inhibitor of acetylcholinesterase (AChE), an enzyme essential for the functioning of the nervous system of insects. Once absorbed, the active ingredient binds irreversibly to the active site of the enzyme, preventing the hydrolysis of acetylcholine in the synaptic terminals. This results in the accumulation of acetylcholine in the cholinergic synapses, causing hyperexcitation of the nervous system.

IRAC Group: belongs to IRAC (Insecticide Resistance Action Committee) Group 1B - Acetylcholinesterase inhibitors (organophosphates).

Characteristic symptoms of the action: Affected insects initially exhibit hyperexcitation, followed by tremors, progressive paralysis, and death. Symptoms include uncoordinated movements, involuntary muscle contractions, and loss of feeding ability.

Time for symptoms to appear: The first symptoms appear between 2 to 6 hours after exposure, with insect death typically occurring between 12 to 48 hours, depending on the dose and the target species.

Control spectrum

Species generally controlled efficiently:

• Lepidoptera: Spodoptera frugiperda (fall armyworm), Helicoverpa armigera (apple caterpillar), Chrysodeixis includens (false measuring worm)

• Hemiptera: Nezara viridula (green stink bug), Euschistus heros (brown stink bug), Piezodorus guildinii (small bedbug)

• Coleoptera: Diabrotica speciosa (little cow), Cerotoma tingomarianus (bean cow)

• Mites: Tetranychus urticae (two-spotted spider mite), Polyphagotarsonemus latus (white mite)

Partially controlled species:

• Aphis gossypii (cotton aphid) - moderate effectiveness

• Bemisia tabaci (whitefly) - variable control depending on population

• Frankliniella occidentalis (thrips) - effectiveness dependent on developmental stage

Tolerant or resistant species:

• Bed bug populations with a history of continuous exposure to organophosphates

• Some lineages of Spodoptera frugiperda in regions of intensive use

• Two-spotted spider mite populations (T. urticae) with cross resistance

Technical application recommendations

Recommended dose:

• Standard range: 500 to 1.000 g of commercial product per hectare (375 to 750 g ai/ha)

• Special situations: in severe infestations or less sensitive species, up to 1.200 g pc/ha can be used, always respecting the maximum limit registered for each crop.

Ideal time for application:

• Apply at the beginning of the infestation, when the pests are in the first instars

• For bed bugs: from the appearance of the first nymphs to young adults

• For caterpillars: preferably in the first and second instars

• Avoid applications in advanced stages of pest development

Ideal weather conditions:

• Temperature: between 20°C and 30°C

• Relative humidity: greater than 55%

• Wind: maximum speed 10 km/h

• Time: preference for applications in the early morning (6am to 10am) or late afternoon (16pm to 18pm)

• Avoid applications under conditions of severe plant water stress.

Compatibility and mixtures

Acephate has good physical and chemical compatibility with most commonly used fungicides and herbicides. It is compatible with glyphosate, 2,4-D, atrazine and triazole formulations.

Common mixtures:

• Acephate + lambda-cyhalothrin: broadening the spectrum and reducing the application dose

• Acephate + imidacloprid: control of sucking pest complex

• Acephate + tebuconazole: simultaneous control of pests and fungal diseases

• Acephate + adhesive spreader: improved adhesion and coverage

Mixtures to avoid:

• Strongly alkaline products (pH > 8,5)

• Formulations containing copper in high concentration

• Heavy mineral oils (risk of phytotoxicity)

Resistance and resistance management

There are reports of resistance in populations of Nezara viridula e Euschistus heros in regions of the Brazilian Cerrado, especially where there has been continuous use of organophosphates for multiple consecutive harvests. Cases of cross-resistance with carbamates have also been documented.

Recommendations for rotation of mechanisms of action:

• Alternate with Group 3A insecticides (pyrethroids)

• Use Group 4A products (neonicotinoids) in rotation

• Include biological insecticides (Groups 11A, 11B) in the rotation program

• Implement refuge windows using other control tactics

Practical strategies for resistance management:

• Limit use to a maximum of two applications per harvest

• Respect a minimum interval of 21 days between applications

• Constantly monitor efficacy through bioassays

• Implement integrated management with biological control

• Use recommended doses without underdosing

Agronomic efficiency

Conditions affecting effectiveness:

• Rain: precipitation in the first 4 hours significantly reduces effectiveness

• Drought: Water stress conditions can reduce systemic absorption

• Temperature: above 35°C may cause volatilization and reduce effectiveness

• Humidity: low relative humidity (<40%) compromises product activity

Among the advantages of acephate, the following are usually cited: broad spectrum of action; systemic action providing long-lasting control; good selectivity when used appropriately; favorable cost-benefit; ease of application and mixing.

On the other hand, there are limitations such as: potential for development of resistance; its toxicity to mammals requires special care; relatively long grace period; sensitivity to adverse weather conditions.

Strategic positioning in agricultural systems

Soy: positioned preferentially in the control of bedbugs from the R1-R2 stage, when the crop is most sensitive to attack. Fundamental in the management strategy of Euschistus heros e Nezara viridula.

Corn: use in control of Spodoptera frugiperda in the early stages of culture (V4-V8), always in rotation with other mechanisms of action to preserve efficacy.

Cotton: tool for managing pest complexes, especially in controlling Aphis gossypii and defoliating caterpillars during vegetative development.

Sugar cane: application aimed at controlling Migdolus fryanus (sugarcane borer) and other soil pests, taking advantage of the product’s systemic action.

Bean: strategic positioning in the control of cows (Diabrotica spp.) and bedbugs, especially in dense cropping systems where natural biological control is limited.