Mineral oil barrier is an effective alternative for suppression of damage by white snails

ABSTRACT

Purpose: Citrus white snail, Helicella candeharica Pfeiffer (Panpulmonata: Helicidae) is one of the most important orchard pests. In this study, the effectiveness of mineral oil was compared with molluscicide baits such as metaldehyde, ferricole (iron phosphate) and a snail-repellent paint in a commercial citrus orchard in northern Iran to reduce access of citrus white snails to citrus trees.

Materials and methods: The number of snails on citrus trees was monitored and counted 10 days after the application of the treatments, and at an interval of 6–8 days up to harvest time.

Results: In the first study, the mineral oil and repellent paint treatments reduced a number of snails best. In the second study, using metaldehyde and mineral oil barrier, again the mineral oil barrier reduced snails best. The cost of each treatment during one season per hectare was calculated at 55, 153, 124 and 120 $/ha for mineral oil, iron phosphate, snail-repellent paint and metaldehyde, respectively.

Conclusions: Mineral oil is an effective alternative for chemical compounds for reducing access by H. candeharica to citrus trees.

KEYWORDS:

• H. candeharica
• iron phosphate
• metaldehyde
• non-synthetic molluscicide
• snail-repellent paint

Introduction

Snails are one of the important agricultural pests around the world, especially in humid regions (Kiss and Magnin, 2003).

Snail control using chemical pesticides is still one of the most effective methods (Heiba et al. 2002; El-Shahaat et al. 2005, 2009). Using molluscicide baits such as metaldehyde and outdated pesticides including carbaryl and lindane are common methods of controlling these pests in citrus orchards (Damavandian 2016). Based on Gimingham (1940), metaldehyde has been the most effective chemical control for snails since introduced as molluscicides bait in 1934.

Many species of predators such as beetles in the families, Carabidae, Staphylinidae, Lampyridae and Silphidae feed on snails (Symondson 2004). The soil-inhabiting beetles (Carabidae) are the principal predator of snails (Helyer et al. 2003). However, due to the high use of pesticides, their residues limit the activity of these predators and ultimately could lead to snail outbreaks (Amiri Besheli 2009). The application of mineral oils is mainly because they are effectively, non-toxic to vertebrates (Beattie and Smith 1996), degrade relatively quickly in the environment (Davidson et al. 1991; Beattie et al. 1995) and have never been associated with resistance or outbreaks of secondary pests (Beattie 1990; Beattie and Smith 1993). Oil spraying is a very important alternative of chemical pesticides for controlling some pests in organic citrus orchards and other organic crops (Davidson et al. 1991; Kiss et al. 2005; Kim et al. 2010). Also, therefore, there is a need for more environmentally friendly methods for control of snails. Here we report the outcome of introducing a low-toxicity alternative to traditional high-toxicity pesticides for preventing damage by snails in citrus orchards in northern Iran.

Materials and methods

A 60 hacitrus orchard, Dasht-e-Naz, Behsat orchard no. 1, located in Sari (36° 42N, 53° 12E), Mazandaran province, in northern Iran was chosen as an experimental orchard. Citrus trees were 5–6-year-old Thomson navel grafted on Citrus aurantium root stock with an inter-row distance of approximately 6 m and the distance between trees within a row about 4 m. The average height of trees was 3.2 m. Two experiments were carried out in two adjacent plots of 1 ha in the mentioned orchard.

Both experiments were conducted in a completely randomised design. In both experiments, all snails were removed from trees before applying the treatments. The treatments included:

1. Mineral oil; plastic foam sheeting was applied around the tree trunk to a height of about 20 cm above the soil surface. The upper edge of the plastic sheeting was tied to the tree trunks to form a cone shape and the lower 10–15 cm of the plastic sheet was smeared with mineral oil (Tehran oil company^®) with a very strong oily smell.

2. Iron phosphate supplied as pellets (Ferricole) (Kimia Sabzavar Company^®): applied at 10 mg/m² as formulated and consisting of 1% iron phosphate, the pellets were scattered around the trees. These pellets were applied three times during periods of rain.

3. Snail-repellent paint (Sabzarang) (Kimia Sabzavar Company^®): contains copper and iron salts and painted in a 10 cm band around the tree trunk.

4. Metaldehyde supplied as pellets (Partonar Company^®): applied at7 mg/m² as formulated, pellets were scattered around the trees. After rains more pellets were applied which necessitates three applications during the study.

5. Control

Suppression of snails with mineral oil, iron phosphate, snail-repellent paint and metaldehyde

The citrus orchard selected for this experiment had 540 trees in 18 rows. The treatments applied in this experiment included mineral oil, metaldehyde, iron phosphate, snail-repellent paint and control (untreated). Each treatment consisted of four replicates and each replicate composed of four trees. Replicates were separated by a buffer zone consisting of two rows of trees. After treatments, all 80 trees and their branches were thoroughly examined for snails, every 7–10 days with a total of five assessments between treatments and harvest time. At each sampling date, snails were removed from the trees after counting, so, snails counted on the next sampling date were those who could pass through the barriers and the treatments did not affect them.

Suppression of snails with mineral oil and metaldehyde

In this experiment, we used a citrus orchard block of 1 ha consisting of 15 rows of trees with 30 trees in each row. One row was used for each replicate. In total, 300 trees were treated and sampled. The treatments included mineral oil, metaldehyde and an untreated control. Each treatment had five replicates (rows) and each replicate consisted of 30 trees. The most common molluscicide for controlling Helicella candeharica in this region is metaldehyde and mineral oil treatment. Since mineral oil is relatively safe for the environment, in this experiment only metaldehyde and mineral oil treatments were applied. At each sampling date, four trees from each row and a total of 60 trees were sampled on each sampling date. On the first sampling date, trees were randomly selected in each row, and then on the next sampling date adjacent to trees previously randomly selected were used. In this experiment, existing snails on the trees were counted only once and then removed, so the results of the sampling date had no effect on the next sampling date.

The cost of each treatment during one season per hectare was calculated as follows:

1. First step: The cost of each treatment per tree ($/tree) = amount of application(s) for each tree (g) × unit price ($)

2. Second step: Total cost ($/ha) = the cost of each treatment for each tree ($/tree) × the number of trees in a hectare (approximately 300 trees) × applications + labor cost

Statistical analysis

Data were analysed using one-way analysis of variance (ANOVA). The means of each treatment for each sampling date and the totals over time were compared using Tukey’s range test.

Results

Suppression of snails with mineral oil, iron phosphate, snail-repellent paint and metaldehyde

The results of the ANOVA showed that there were significant differences among the numbers of H. candeharica in the different treatments at all sampling dates (P < .01). According to the results of the mean comparison of the number of snails in the different treatments on different sampling dates (Table 1), the lowest number of live snails on all dates occurred in the mineral oil treatment, followed by snail-repellent paint. On the final sampling date, the lowest number of live snails after the mineral oil was found in the metaldehyde treatment, although there were no significant differences between this treatment and the snail-repellent paint andiron phosphate treatments. These results indicate that the greatest efficacy was gained with mineral oil.

Table 1. The effect of various chemical repellent on the presence of snails on citrus trees.

Date (day)	Development stage of trees	Treatments	Number of alive snail for each tree
			Minimum	Average	Maximum
0	Fruit enlargement and maturation	Mineral oil	1	3.6b	11
		Iron phosphate	2	26.3b	80
		Snail-repellent paint	1	4.6b	14
		Metaldehyde	5	36.4b	174
		Control	2	83.4a*	252
6	Fruit enlargement and maturation	Mineral oil	2	15.5b	91
		Iron phosphate	4	29.4b	98
		Snail-repellent paint	5	18.4b	56
		Metaldehyde	3	32.8b	125
		Control	4	74.0a	192
14	Fruit enlargement and maturation	Mineral oil	0	0.6b	2
		Iron phosphate	0	9.8b	48
		Snail-repellent paint	0	3.8b	24
		Metaldehyde	0	17.8b	72
		Control	11	41.1a	123
20	Fruit enlargement and maturation	Mineral oil	0	0.3b	1
		Iron phosphate	1	10.0b	39
		Snail-repellent paint	0	2.4b	14
		Metaldehyde	1	17.3b	84
		Control	16	38.6a	106
30	Fruit enlargement and maturation	Mineral oil	1	4.9c	17
		Iron phosphate	3	11.9bc	32
		Snail-repellent paint	4	11.4bc	27
		Metaldehyde	3	21.9ab	54
		Control	5	30.6a	78

*Different letters in each sampling date show a significant difference among treatments at P = .05, one-way ANOVA followed by Tukey’s test.

When considering the total number of snails over the period of evaluation, there was a significant difference among the treatments (P = .0001, df = 4, F = 26.101) (Figure 1). Mineral oil and snail-repellent paint had the greatest efficacy as these treatments had the lowest numbers of snails during the entire period of sampling. Iron phosphate and metaldehyde were less efficacious but were superior to the control.

Figure 1. Mean total number of snails found on citrus trees treated with various snail repellents over the experimental period of 30 days in the first study. Different letters above the columns indicate significant difference among treatments at P = .05, using one-way ANOVA followed by Tukey’s test.

Suppression of snails with mineral oil and metaldehyde

There was a significant difference among the numbers of H. candeharica in the different treatments (P < .01). Mineral oil treatments had the lowest number of live snails on the individual sampling dates, opposed to metaldehyde and control with higher numbers (Figure 2).

Figure 2. Mean number of snails found on citrus trees treated with various snail repellents on different dates during the second experiment. Different letters above the columns indicate significant differences among treatments on each date at P = .05, one-way ANOVA followed by Tukey’s test.

There were significant differences among the numbers of live snails per tree in the different treatments over the entire period of sampling (P = .0001, df = 2, F = 21.62). The results of mean comparing of the number of live snails per each tree in the treatments are presented in Figure 3. Mineral oil was more efficient in H. candeharica control than metaldehyde and the control.

Figure 3. Mean total number of snails found on citrus trees treated with various snail repellents in total sampling time course of second study. Different letters above the columns indicate significant difference among treatments at P = .05, one-way ANOVA followed by Tukey’s test.

The calculation of the cost of each treatment during one season showed that mineral oil treatment had the lowest cost and metaldehyde and snail-repellent paint are costlier. Iron phosphate treatment was the most expensive for controlling H. candeharica (Table 2).

Table 2. Cost of each treatment during one season per hectare.

Treatment	Amount of application for each tree	Times of application during the experiment	Price ($*/tree)	Labor ($/ha)	Total cost ($/ha)
Mineral oil	77.6 g + 20 cm of con-like foam	1	0.10	25	55
Iron phosphate (Ferricole)	20.5 g	8	0.35	48	153
Snial repellent paint (Sabzarang)	62.5 g	1	0.33	25	124
Metaldehyde	14.6 g	8	0.24	48	120

*Iran’s currency is Rial and one dollar is 42,000 Rial.

Discussion

Barriers which prevent vertical movement of snails can be chemical or physical (Schüder et al. 2003). The plastic foam sheet in cone-like shape mounted around the tree trunk is an example of a physical barrier. Applying strong smelling mineral oil (Tehran oil^®) to the internal edges of the cone caused snails to quickly withdraw their tentacles when encountering mineral oil and escape (Personal observation).

Mineral oils also have a repellent effect that discourages egg deposition and feeding. The residual film may inhibit insects from attaching to plant surfaces (Trammel 1965). Also, it should be noted that ‘arrested activity’ in insects is one recurrent symptom caused by mineral oil that has been reported directly or indirectly by many authors in laboratory toxicity tests (Taverner 2002; Najar-Rodríguez et al. 2007; 2008; Buteler and Stadler 2011).

Separate analysis of each sampling date between treatments was significant (Table 1). The effect of various chemical repellents on the presence of alive snail on citrus trees in all dates of sampling (Table 1) revealed that mineral oil barrier was the most effective treatment giving the lowest mean number of alive snail followed by snail-repellent paint, iron phosphate, metaldehyde and control.

The lowest mean number of H. candeharica on each tree after five sampling dates in the first study was obtained for mineral oil (4.9) followed by snail-repellent paint (8.4), iron phosphate (20.3), metaldehyde (22.6) and control (53.8) (Figure 1), which is contrary to finding by Amiri Besheli (2009). According to Kheirodin et al. (2012), 30 tons sevin and 40 tons metaldehyde were used to control snails and slugs in citrus orchards in Mazandaran, northern province of Iran.

Mean numbers of snails found on citrus trees treated with various snail repellents on different dates during the second study (Figure 2) indicated that mineral oil barrier was the most effective snail repellent with the lowest mean number of alive snails for each tree followed by metaldehyde and control. The best results were achieved applying mineral oil to internal edges of foam cone mounted around the tree trunk, which confirms the results of Kheirodin et al. (2012), but contradicts those of Gimingham (1940). According to Gimingham (1940), metaldehyde has been the most effective chemical against slugs and snails since introduction as molluscicidal bait in 1934. Pesticide residues from irregular application of pesticides in most citrus orchards of Mazandaran caused pests resistance, natural enemies disruption, environmental imbalance and outbreak of secondary pests (Ahmadi and Arbabi 2000; Helyer et al. 2003; Armsworth et al. 2005; Ahmadi and Hallaji Sani 2006; Damavandian 2007; Singh and Agarwal 2007). So, an alternative solution is the use of mineral oils that are a more natural way of control.

Mean total number of alive snails found on citrus trees treated with various snail repellents in total sampling time of second study (Figure 3) revealed that the mineral oil barrier was the most effective snail repellent giving lowest mean number of alive snails for each tree (4.2) followed by metaldehyde (16.72) and control (27). According to Marcinek et al. (2018), mineral oils have insecticidal, miticidal and fungicidal properties and do not contaminate the soil and do not kill the useful insects (Wilson 1999).

Besides contaminating the environment, metaldehyde did not control the snails effectively during the study. According to our results, metaldehyde was even less effective to control H. candeharica as iron phosphate which is contrary to findings of Amiri Besheli (2009), Kheirodin et al. (2012) and Speiser and Kistler (2002).

It is well known that iron phosphate controls snails (Jackel 1999; Koch et al. 2000) and is not harmful to other organisms in the environment (Roberts et al. 1990; Clark 1993). This compound is also useful as a nutrient for plants (Rae et al. 2009). When snails and slugs eat the pellets, the iron phosphate interferes with their calcium metabolism in their gut, causing the snails and slugs to stop eating almost immediately (Shmuel et al. 2004). Ingestion of the iron phosphate bait, even in small amounts, will cause snails and slugs to cease feeding, although it may take several days for the snails to die (Amiri Besheli 2009). Besides all its good properties, the only negative is that iron phosphate is somewhat more expensive than metaldehyde (Table 2).

Preventing snail’s movement by barriers of copper material is reported by several researchers (Barker 2002; Amiri Besheli 2009; Kheirodin et al. 2012). In this study, snail-repellent paint which contains copper and iron salts caused an overall decrease in the number of H. candeharica and was more effective than metaldehyde and iron phosphate (Figure 1 and Table 1) which is contrary to findings by Amiri Besheli (2009). He reported that the metaldehyde and iron phosphate were more effective than snail-repellent paint.

Ryder and Bowen (1977) indicated that copper salts can be absorbed by snail’s body and caused damage. However, after rain, the copper salt concentration can decrease so much that control can be ineffective. According to Kheirodin et al. (2012), the copper material around the tree trunk can control Helix aspersa Muller for more than seven months.

Various factors are involved in snails’ movement and activity but the most important factor that caused a reduction in snail activity is cool temperature (Riddle and Miller 1988; Ansart et al. 2001; Ansart and Vernon 2003). In our study, during sampling dates, the average temperature was always more than 10°C. Under these conditions, the environmental moisture and raining probably affect the snail activity. Bedford et al. (1998) stated that H. aspersa activity generally becomes more in raining seasons.

On the one hand, results from the first and second experiment, clearly showed that mineral oil was the most effective treatment (Table 1, Figures 1–3) and the most cost-effective (Table 2). On the other hand, many researchers stated that mineral oils are effectively non-toxic to vertebrates and degrade relatively quickly in the environment (Beattie and Smith 1996; Helmy et al. 2012; Damavandian and Kiaeian Moosavi 2014). In the other words, they are environmentally friendly. So, it can be concluded that this new method based on a combination of movement barrier and mineral oil is a very powerful physicochemical barrier for several reasons. The most important reasons include the highest rate of pest control and less environmental pollution. Snail-repellent paint and iron phosphate due to their lower efficacy and metaldehyde due to more environmental pollution in controlling H. candeharica are prioritised as the next choices. Meanwhile, other reasons such as no environmental pollution and cost reduction (Table 2) make mineral oil as the first priority in controlling H. candeharica.

Acknowledgements

We would like to thank Prof. Eddie Ueckermann to edit this manuscript grammatically and thank Mr. Doolatyar who provided citrus orchards for this study.

Disclosure statement

No potential conflict of interest was reported by the authors.

Notes on contributors

Mehraneh Sepasi is achieving MSc degree in agricultural entomology from Sari Agricultural Sciences and Natural Resources University, Sari, Iran. She is an agricultural expert in Agricultural Jihad Organization of Mazandaran Province, Iran.

Mohammad Reza Damavandian has an MSc in Agricultural Entomology awarded from Shahid Chamran University of Ahvaz, Iran in 1993; he achieved a PhD in Agricultural entomology seven years later in 2000, from Stellenbosch University, South Africa. He is Associate Professor of Department of Plant Protection in Sari Agricultural Sciences and Natural Resources University. Mohammad Reza has authored or co-authored over 35 scientific publications, of which around 25 were published in journals listed in Journal Citation Reports by Thompson Reuters.

Behnam Amiri Besheli achieved a PhD IN Agricultural Toxicology in 2000, from Reading University, Reading, England. His main research area is agricultural pesticides and primary pest control metabolites. He is Associate Professor of Department of Plant Protection in Sari Agricultural Sciences and Natural Resources University.

ORCID

Mohammad Reza Damavandian http://orcid.org/0000-0002-4403-569X

Additional information

Funding

This work was supported by the Sari Agricultural Sciences and Natural Resources University [968070932].