Seasonal incidence and Chemical Control of Land Snails in Pear Orchards at Alexandria Governorate

INTRODUCTION

Land snails became one of the serious injurious animal- pests of a wide range of agricultural and horticultural crops in world wide which capable of extensively damaging plants including flowers, vegetables and some spherules and trees (Godan, 1983 and Speiser and Kistler, 2002). In Egypt, land snails have been dispersing in different localities especially in the northern governorates. These pests attack numerous orchard fruit trees, field and vegetable crops as well as ornamental plants and cause great damage to all plants in the northern costal belt (Kassab and Daoud, 1964; Bishara et al., 1968; EL-Okda, 1979; Hashem et al., 1992; El-Deeb et al., 1996; Abobakr, 1997; Eshra, 1997and 2004).

The terrestrial snails Eobania vermiculata, Theba pisana, Helicella vestalis and Monacha obstructa were recorded in many Egyptian Governorates attacking various plantations (EL-Okda, 1984; El-Wakill et al., 2000; Eshra, 2013). There are different methods for controlling these pests. In general, chemical control is one of the most effective methods against land snails, particularly over large areas (Moran et al., 2004; EL-Shahaat et al., 2005).The aim of the present study is devoted to monitor three occurring land snail populations and their chemical control in the pear orchards at Alexandria Governorate during two successive years.

MATERIALS AND METHODS

Seasonal abundance of detected land snails in pear orchards: Survey and monitoring of existing land snails were conducted in a severely infested pear (Pyrus communis) orchard at AL-Amriah region, Alexandria Governorate during two successive years, from December 2012 till November 2014.

The established land snails in pear orchards were recorded. The land snail species were identified according to the terminology given by Godan (1983) and El-Okda (1984). The seasonal abundance of the three, more or less, dominant snail species was estimated by counting the collected snail individuals on 15 randomly marked pear trees (3 replicates, 5trees/each) as permanent sampling sites. Every month, land snail species were handly collected in the early morning from all parts of the tree and the area of surrounding soil. The prevailing monthly average temperatures and relative humidity percentages were obtained from the Egyptian Meteorological Authority (General Directorate for Scientific Research).

Chemical control : The initiated or achieved chemical control was performed against the white garden snail, T. pisana in a pear orchard at AL-Amriah region.

The evaluated treatments were applied as prepared baits using each of methomyl (s-methyl carbamoyl oxy thioacetamidate) as Lannate 90% SP and boric acid 99% (trihydroxid boron, H₃BO₃) as shown in Table (1) and Fig1.

(a)                                                                        (b)

Figure(1): Chemical structure of methomyl (a) and boric acid (b).

Table (1): Local bait compositions (% w/w) for laboratory and field evaluation.

Adult snails of T .pisana were collected from AL-Amriah region and allowed to be acclimatized under laboratory condition for 2 weeks and fed only on bran bait. Healthy and active snails were chosen for the evaluation of tested baits. To evaluate each of the prepared baits, ten adult animals were placed in 1-letter glass jar, covered with cloth netting and secured with rubber band. Three replicates for each treatment were involved and dead animals were recorded daily up to 5 days. The check control treatment was performed pesticide-free wheat bran bait. The mortality percentage values were calculated according to Abbott (1925)

The results as LT₅₀ values were statistically analyzed according to Finney (1971).

Field evaluation:  For each bait treatment a quarter feddan cultivated with infested pear trees by T. pisana was exposed to bait application. For each bait treatment, four replicates of three pear trees were randomly chosen. In addition, untreated twelve trees were left as a check control. The treatments were distributed according to a complete randomized block design.

Baits were applied in heaps (100 g) on blue plastic sheets (25 × 25cm) in early morning on the damply soil (after four days from irrigation). The evaluated baits were renewed every 10 days. Numbers of snails on the trees and around them were estimated before treatment and after 1, 3, 7, 14 and 21 days of bait application according to Henderson and Tilitton (1955).The obtained results were statistically analyzed according to the method of Snedecor and Cochran (1967) and the differences between treatments were compared by L.S.D_0.05 measure.

RESULTS AND DISCUSSION

1-Sesonal abundance of land snails in pear orchards in AL-Amriah region.

 Our obtained results revealed that three common land snail species, i.e., the White Garden Snail (WGS), T. pisana (Müller), the Small Sand Snail (SSS), H. vestals (Pfeifer) and the Clover Snail (CS), M. obstructa (Férussac) were found prevailing investigated pear orchard at AL-Amriah region, Alexandria Governorate. The fluctuating abundance of each species, on the examined pear fruit trees during both the successive years was investigated.

              Data presented in Tables (2&3) indicated that the monthly estimated numerical densities of T. pisana (Müller), H. vestalis (Pfeifer) and M. obstructa (Férussac) on the trees of inspected pear orchard during the two subsequent years of 2012/2013&2013/2014 greatly varied according to the studied snail species, temperature, and relative humidity; and showed remarkable high or low fluctuating seasonal patterns. T. pisana snail was the dominant land snail species in pear orchards during the two following years, whereas its calculated general mean number comprised 120.6 snail/tree in the first year and 128.6 snail/tree in the second one. The results also indicated that T. pisana snail was the most abundant snail species. The highest counts of the snail (195 snails/tree) and (243.3 snails/tree) were recorded during May 2013 & 2014 in the first and second year, in respect, while its lowest counts were recorded in December 2012 in the first year (81.7 snails/tree) and in November 2014 in the second one (21.6 snails/tree) (Tables 2 and 3). T. pisana is distributed throughout the Mediterranean, as well as Atlantic Islands, Western Europe, southern Africa and Australia (Anonymous, 1982). It is considered a greatly damaging pest to a wide variety of fruits, vegetables, and ornamental plants in most of the areas of its distribution (Harpez and Oseri, 1961). In Australia, T. pisana is a significant pest of cereals (Baker, 1986). The snail was reported as a pest of seed alfalfa in France (Cairaschi and Lecomte, 1973). Furthermore, it is an intermediate host of significant parasites of cattle and sheep like the lung worm, Muellerius capillaries (Mueller), and many other nematodes (Baker, 1986).

              The second abundant snail was H. vestalis was less abundant than T. pisana snail. Data in Tables (2 and 3) illustrate the numerical densities of H. vestalis on the inspected pear trees at Alexandria Governorate during the two consequent years. It was cleared that the determined infestation rate of pear trees by H. vestalis was high in the two subsequent years during spring months (March to June).The lowest mean number was recorded on pear trees during winter months (January and February) in the first year and September and November in the second one.

              M. obstructa snail was, comparatively, the least abundant species on the pear trees where mean total number of the snails was 66.0 and 70.9 snails/tree in 2012/2013 and 2013/2014 respectively (Tables 2 &3). The highest counts of the snail were recorded in the spring months (March to May) in both years. Unlike the previous two species, the peak count was recorded in April in both years. Generally, T. pisana snail was the most abundant snail in the orchard of pear trees at AL-Amriah region during the two years of study followed by H. vestalis and M. obstructa.

              The obtained results are in agreement with that reported by Bishra et al.(1968).They recorded H. vestalis, T. pisana, Cochlicella acuta and Eobania vermiculata in the northern delta (Alexandria and EL-Behera Governorates).The same species were recorded also by EL-Okda et al.(1980 and 1984) at Alexandria Governorate. Nakhla et al. (1993) reported that E. vermiculata, H. vestalis, C. acuta, and T. pisana were the prevailing land snails in banana orchards at EL-Behera Governorate. Eshra (2013) recorded five land snail species at Abees region in Alexandria Governorate on grape orchards including E. vermiculata, T. pisana, H. vestalis, M.obstructa and Oxychillus alliarius and three land snails, T. pisana, H. vestalis and M. obstructa on navel orange and apple trees in Kafr EL-Dwar center at EL-Behera Governorate.

Table(2): Monthly counted individuals of the detected common land snails species in pear orchard in AL-Amriah region at Alexandria Governorate during 2012-2013.

Values with similar superscript (s) are not significantly different at (P> .05)

Table (3): Monthly counted individuals of the inspected common land snails species in pear orchard in AL-Amriah locality at Alexandria Governorate during   season of 2013-2014.

  Values with similar superscripts are not significantly different.

2- Chemical control

`The problems caused by terrestrial snails, particularly the white garden snail (WGS), T. pisana, have remarkedly increased, as illustrated by the 70-fold increase of molluscicide usage over the last 30 years. These gastropods are a serious pest of worldwide economic importance (South, 1992) as they have adapted well to the various environments to which they have been introduced around the world.

Tables (4 and 5) and Figure (2) show the results of made laboratory and field evaluations of the molluscicidal activity of methomyl, boric acid, and their mixture against the (WGS) T. pisana. It was indicated that methomyl and / or methomyl/boric acid mixture were more toxic than boric acid, whereas the deduced mortality percentages for methomyl and for its mixture with boric acid were higher than that assigned for boric acid alone at all intervals of experimental period. In the laboratory evaluation, the calculated median lethal time (LT₅₀) values of (2.1days) for methomyl and (1.7 days) for methomyl/boric acid mixture indicated that both of them were significantly more rapid in killing snails than in baits of boric acid alone (9.2 days).

The molluscidal efficacy of the laboratory tested baits was also evaluated in pear orchard as shown in Table (5). The reduction percentage values of the mature specimens of (WGS) snail amounted to 31.2, 53.8, 68.3 and 79.3 after 3,7,14, and 21 days with methomyl bait, respectively. While, these % values of infestation reduction comprised 15.9, 23.0, 34.2 and 46.7 with boric acid bait treatment. Referring to tested bait of methomyl-boric acid, the recorded % values of infestation reduction were 41.1, 72.05, 80.2 and 88.7 after 3,7,14 and 21days, respectively. The highest toxic molluscidal effect expressed as general mean of the all evaluated baits was detected after 14 days (60.02%) and 21 days (71.18%). Moreover, the highest determined toxic effect according to the calculated general mean values of infestation reduction showed that the highest toxic influence was 57.12% with the mixed chemicals of 3^rd bait, while the lowest effect is related to boric acid bait (25.41%) (Table, 5).

Edible toxic baits are the most important way of delivery of molluscicides in terrestrial gastropod control programmes (Bailey, 2002). Miller et al. (1988) performed laboratory and field cage tests to evaluate the toxicities of some molluscicides against T. pisana. of 19 commercial bait formulations tested, Deadline Bullets, 4% metaldehyde bait and Slug-Geta, 2% methiocarb bait were the most effective.

Table (4): Efficiency of laboratory evaluated methomyl and boric acid baits against Theba pisana.

    *Values with similar superscripts are not significantly different.

Table (5):Toxic efficiency of field evaluated of prepared baits against Theba pisana snails in pear orchard in El-Amryah at Alexandria Governorate

      *Values with similar superscripts are not significantly different.

Figure (2) :Toxicity lines and median lethal times (LT₅₀) of methomyl, boric acid, and their mixture   against Theba pisana under laboratory condition.

The high molluscicidal activity of methomyl bait was confirmed by many investigators (Radwan et al., 1992; EL-Shahaat et al., 2005; Eshra, 2013). Moreover, referring to that reported before, boric acid and sodium borate are pesticides since 1948 for controlling of many insects by acting as a stomach poison affecting the insects' metabolism, besides its dry powder is abrasive to the insect exoskeleton (Wong et al., 1964; Weir and Fisher, 1972; Reigart and Roberts, 1999). The present study proved that boric acid could not be used alone as molluscicide to control the terrestrial snail T. pisana. However, it could be mixed with methomyl to increase its efficacy and shorten the time of its molluscicidal action.