Biochemical and histochemical studies on the digestive gland of Eobania vermiculata snails treated with carbamate pesticides

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Abstract

In this study the land snail (Eobania vermiculata), one of the most abundant gastropod of Egyptian farms, was investigated to determine the lethal toxic action of methomyl and methiocarb and to identify the biochemical and histochemical changes as a function of sublethal dose or concentration on the digestive gland of this snail under laboratory conditions, using topical application and baiting techniques. The results showed that methomyl exhibited greater efficacy than did methiocarb against the snails in both techniques. However, higher mortality rates were obtained in the topical application technique than in the poison bait experiments. Biochemical and histochemical examinations revealed that treatment of the snails with methomyl and methiocarb either by topical application or toxic baits caused significant decrease in carbohydrate, lipid and protein contents. This decrease was also more obvious after topical application than after baiting technique, and methomyl was found to be more toxic than methiocarb.

Introduction

Mollusc species constitute a major threat to sustainable agriculture. This threat is associated with cultivation of new crop, intensification of agricultural production systems and the spread through human trade and travel species adapted to these modified environments [1], [2]. The land snail, Eobania vermiculata (Müller 1774), belongs to Helicid family (Helicidae) and is distributed all over the world especially in the Mediterranean region. This species is hermaphroditic and usually inhabits in all coastal parts, fields, gardens, agricultural crops and vineyards. It climbs up trees, palms, bushes and fences on hot daytime. In Egypt, this snail is one of the most common species and is becoming a serious agricultural animal pest to several vegetations including orchard trees, vegetable crops as well as ornamental plants, causing considerable damage to all plant parts [3], [4].

The chemical control of snail populations through the application of molluscicides remained the most effective method, particularly over large areas. Conventional pesticides especially carbamates are successfully used in Egypt as well as in many other countries, as sprays or baits to control these pests [5], [6], [7]. However, their use in higher concentrations has a harmful effect on non-target species like vertebrate animals and human beings and leads to pollution of environment. Therefore, application with lower concentrations in snail control is thus expected to avoid such a hazard.

Topical application and toxic bait methods have been reported effective against terrestrial gastropods. However, the bait should be less repellent to them allowing more effective ingredients to be consumed before termination of feeding [8]. In topical application technique, the molluscicides can affect molluscs by absorption through the skin.

The digestive gland (hepatopancreas) of gastropod molluscs is the key organ of metabolism and it is concerned with production of digestive enzymes, absorption of nutrients, endocytosis of food substances, food storage and excretion [2]. It has been found to be the major site of metal accumulation, and is the organ in which cellular alterations are most evident after exposure to organic and inorganic pollutants [9], [10]. Consequently, cytological and cytochemical changes in digestive gland cells of these animals, can be used successfully as biomarkers in toxicant-exposed animals [11], [12]. Pollutants accumulated through different routes are transported by blood cells to the digestive gland. As the digestive gland represents the main target organ for molluscicide impact, the reactions following application of molluscicide should be investigated. Up to the present, little information is available about the biochemical changes [6], [13], [14] and the histochemical changes [9], [10], [15] in the digestive gland tissues of land gastropods induced by molluscicides.

Therefore, the objectives of the present study were to determine the lethal toxic action of methomyl and methiocarb and to identify the biochemical and histochemical changes as a function of sublethal dose on the digestive gland of E. vermiculata snails under laboratory conditions, using topical application and baiting techniques.

Section snippets

Chemicals

Methomyl (Lannate) 90% SP, S-methyl-N-[(methylcarbamoyl)oxy]thioacetimidate were provided by Dupont de Nemours Co. and pure sample of methiocarb (98.85%), [3,5-dimethyl-4-(methylthio)phenolmethylcarbamate] was obtained from Bayer AG, Leverkusen, West Germany. All other chemicals used in this study were purchased from Sigma or BDH Chemical Companies and they were of the highest purity grade available.

Animals

Specimens of the herbivorous land snail, E. vermiculata (Müller) (20 ± 1.4 mm in shell diameter and

Effect of methomyl and methiocarb on the mortality of E. vermiculata snails

Using topical application technique, the mortality percentages increased by increasing the dose level of both compounds. Methomyl killed 93.33% of test snails at 375 μg/snail, while methiocarb killed 40% at the same applied dose. Probit analysis of the results indicates that the 48 h LD50 value of methiocarb (413.2 μg/snail) was higher than that of methomyl (89.6 μg/snail). This indicates that methomyl is more toxic than methiocarb against the tested snails (Table 1).

Concerning bait technique, the

Discussion

In the present study, it was clearly shown that methomyl exhibited greater efficacy than did methiocarb against E. vermiculata snails in both techniques (topical application and poison baits), which may be attributed to differences in absorption, chemical structure, solubility, and affinity to the biological target and/or other factors. The obtained results are in agreement with those reported by Radwan et al. [6] who showed that bran methomyl bait (0.5% w/w) had the most potent molluscicidal

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