Reviews of Environmental Contamination and Toxicology

Monocrotophos (C₇H₁₄NO₅P) is a systemic insecticide and acaricide of the vinyl phosphate group. It was introduced in 1965 by Ciba AG and Shell Chemical Co. U.S.A.

Toxicological data that were generated in the early 1960s met the state of the art for that time. New tests were performed as science and regulatory requirements developed. The aim of this presentation is to reflect today’s assessment of the existing data. As a rule, only the latest studies in each field have been included. The reference list contains titles of preceding reviews in which the bibliography of early studies, rendered obsolete through changed regulatory requirements, is accessible. All experimental data pertinent to assessing the safety of monocrotophos to users and consumers have been included.

For monocrotophos and its formulations, there exist Food and Agriculture Organization (FAO) specifications (FAO 1988). According to these specifications, the content of monocrotophos in the technical material should not be less than 750 g/kg. When monocrotophos was first launched in the 1960s, its purity was below 700 g/kg. Since then the purity has been continuously improved and is now normally close to 800 g/kg.

Monocrotophos has a high acute oral toxicity. Rat LD₅₀ values assign the technical active ingredient (a.i.) to World Health Organization (WHO) class Ib (WHO 1992). The material is hydrophilic and is on the market mainly in aqueous liquid formulations. Because skin absorption of hydrophilic materials is generally slow, there is a wide gap between oral and dermal toxicity (see Skripsky and Loosli 1994, this volume). Therefore, dermal exposure during work (splashes, spray mist) is clearly less hazardous than oral uptake (careless handling, suicide).

Various aspects of the metabolic fate of monocrotophos have been investigated in rats, goats, cows, rabbits, pigs, insects, and man after oral, intravenous, intraperitoneal, and topical administration.

Monocrotophos is a vinylphosphate insecticide with contact, systemic, and residual activity (Corey et al. 1965). The aim of this chapter is to describe the behavior of monocrotophos in plants, i.e., its penetration and translocation into new growth, its rate of dissipation, and routes of degradation.

Monocrotophos (C1414, SD 9129), the active ingredient (a.i.) of Nuvacron^® (trademark of Ciba-Geigy) and of Azodrin^® (trademark of Shell), is a broad spectrum organophosphorus insecticide with systemic, residual, and contact activity against a wide range of sucking and chewing insects and mites. It is registered for more than 20 crops in over 50 countries and is extremely well tolerated in all crops for which its use is recommended. Cotton is the most important crop, followed by soybeans and rice. In addition, monocrotophos is also used in wheat, potatoes, groundnuts, maize, sugarcane, tobacco, and some vegetables. Application rates for the different uses vary and range from 150 to 1200 g ha^-1. The purpose of this chapter is to summarize research information regarding the environmental fate and toxicity of monocrotophos. The published scientific literature forms the primary source, but results from proprietary, industrial research reports are also included.

Since monocrotophos was introduced for use in crop protection some 30 years ago, many new groups of insecticides have become available, ranging from traditional synthetic chemicals to natural and biological products. Nevertheless, monocrotophos has remained one of the preferred insecticides for farmers in the developing world. Its low cost and versatile activity on a wide range of pests make it the third most widely used insecticide in the world. It is currently registered and sold in more than 50 countries, including seven that are members of the Organization for Economic Cooperation and Development (OECD), of which four are also members of the European Union.

In industrialized and developing countries, modern agriculture is subject to contradictory trends: in the wealthy countries, society demands yield to be secondary to complete environmental compatibility. In poor countries, by contrast, conservation and optimization of yield is mandatory if sufficient food for the increasing population is to be provided. “Insects are to be controlled by biological means, if possible, exclusively”—this statement would be a modern postulate in the industrialized world. However, the reality in tropical and subtropical agriculture is different. There, plant protection by chemicals is indispensable if volume and continuity of agricultural production are to be safeguarded. For research, this means that new insecticides should be environmentally compatible, safe according to all possible criteria, and superb control agents for all pest species concerned, resistant or not. If these postulates are rigorously followed, the chances for development of a new-age insecticide are practically nil. As a consequence, there has been a steady decline in enterprises with the willingness and capacity to invest in research for new insecticides, and this decline will continue. In this situation, it is mandatory to prolong the useful life of proven insect-control agents by any means. This review deals with monocrotophos, a representative of the enolphosphates, and its biological performance. Over the past two decades, a great amount of work has been done to elucidate the pros and cons of monocrotophos. A cross section of characteristics of monocrotophos is presented to which many workers all over the world have contributed.

The majority of insecticides currently used worldwide can be characterized as “broad spectrum,” belonging to organophosphate, pyrethroid, carbamate, or organochlorine chemistry. For reasons of price, efficacy, spectrum, and organochlorine availability, the dominance of these compounds is likely to continue until well into the next century. Therefore, they must be made useful in IPM systems, if this is at all possible (Horn 1988).

Monocrotophos in formulations may be identified using gas chromatography (GC) or high-performance liquid chromatography (HPLC) (see below) by comparing its retention time with that of a monocrotophos reference standard. In ambiguous cases, the use of two dissimilar columns or specific detectors (e.g., a nitrogen-phosphorus detector for GC) may be necessary.

Since the introduction of monocrotophos approximately 30 years ago, numerous methods for the determination of residues have been developed, most of them in the years before 1970. This review summarizes the methods that either are or were used for the quantitative determination of residues of monocrotophos and its metabolites. Some newer methods that were not published before are described in detail. Most methods that were developed before 1970 are now of historical interest only; they are well covered in the literature and, with a few exceptions, will not be dealt with here.

Monocrotophos is a systemic organophosphorus insecticide and acaricide with a broad spectrum of activity against a wide range of sucking and chewing insects and spider mites. With the exception of some northern and central European countries and the U.S., it is used worldwide on more than 30 crops.