The white Mediterranean snail in tree crops
Savvas Pastopoulos, Agronomist MSc, Pastopoulos Agronomics, Neos Mylotopos, Pella, Greece
The white Mediterranean snail, Theba pisana, becomes visible at precisely the moment when it is not feeding. The white clusters on stems and trunks are animals in summer dormancy, and the damage is done earlier and later, when the weather cools. This text gathers what is documented about its biology and its damage, sets out exactly what is permitted in Greece, and examines the nematode offered as a biological solution using the figures of the studies themselves.
The plant protection approvals cited here are those of the Greek Ministry of Rural Development and Food and are valid in Greece only. They say nothing about what is permitted in any other country. Rates in this text are given per hectare.
shell diameter
up to 5 clutches a year
a recorded density
the laboratory needed
1. A snail that did not come from anywhere else
Theba pisana (O.F. Müller, 1774) is described in the international literature as an invasive species, and that causes confusion. The label applies to Australia, the United States and South Africa. Its native range is the Mediterranean and much of western Europe [1], which places Greece inside the home of the species.
That has two practical consequences. The first is that it is not a quarantine organism and there is no duty to notify. The second, and the more unwelcome, is that there is no eradication programme to wait for. Wherever the population grows, the matter is management and not removal.
The increase in populations observed in Greek orchards has no published quantitative record. It is reported here as a field observation and not as a documented quantity.
Sources: 1
2. Identification
The shell is hard and coiled, 10 to 30 millimetres in diameter, off-white or cream, usually with a brown band around the spiral, and with the umbilicus, the hole at the centre of the underside, partly closed [4]. The partly closed umbilicus is the character that separates it from related white snails, and it is judged with a finger, by turning the shell over.
No other snail of Greek orchards forms dense white clusters high on stems, wires, posts and trunks in the middle of summer. When that is the picture, identification is all but certain even before the umbilicus is examined. The animals are stuck on with dried mucus and do not move even when touched.
3. The cycle, and why it climbs
The species is hermaphrodite. Each individual lays up to 5 clutches a year, with up to 120 eggs per clutch, below the surface of the soil. The eggs are spherical, white, 1 to 3 millimetres across, and hatch in about two weeks. The young mature within a year, the animal can live up to five years, and there is one generation a year. Laying runs from autumn to spring [4].
The behaviour that defines the species is summer dormancy. When the soil surface becomes hot and dry, the snails climb whatever they find, seal the shell aperture with a membrane of dried mucus, the epiphragm, and cease all activity [5]. The epiphragm limits water loss and is the reason the animal boards freight unnoticed and travels a long way [5].
The Australian extension material places dormancy from late October to late February [4]. That is the southern hemisphere, meaning their summer. The equivalent Greek window falls roughly from late April to late August, and it shifts with the season and the district.
What transfers is not the month but the condition: dormancy begins once the soil surface turns hot and dry, and it breaks with the first substantial rain. No corresponding Greek measurement with temperatures and dates was found published.
4. The damage in tree crops
The damage is done whenever there is moisture, and this is the point most often misread. Summer dormancy is not a continuous four-month state. Moisture, from heavy dew or from rain, is enough to make the animals active and bring them down to the soil surface [7]. They feed at night, early in the morning, and in rainy or overcast weather [4].
The Australian and Californian literature describes dryland systems, where the soil surface stays dry for months in summer and dormancy genuinely does not break. An irrigated Mediterranean orchard is a different environment. Every irrigation wets the surface, and to that are added the heavy dews of August nights.
The result is that the animals gain repeated feeding windows through the summer that do not exist in the systems where the figures were measured. Reproduction points the same way: the species is described as an opportunistic breeder, and breeding has been recorded in summer when the summer was wet [7]. No corresponding measurement in an irrigated Mediterranean orchard was found published.
Three distinct kinds of damage are recorded in tree crops. The first is to foliage and tender shoots, with defoliation that becomes severe under heavy infestation [5]. The second is to the fruit: in citrus the snails scar and chew the rind, sometimes through to the flesh, and scar leaves and twigs [4]. The third is not agronomic but costs just as much: the clusters of dormant animals clog harvesting machinery, and livestock reject pasture and hay that are heavily contaminated [1].
Significant damage has been recorded in citrus, almond, olive and grapevine in Australia, as well as in vegetables and ornamentals [1]. In California densities of up to 3,000 snails per tree have been measured [1].
Why the plant is left stunted
Under large populations the picture in the field is of stunted plants, as though the vigour had been drawn out of them. The mechanism is not sap sucking. The snail has no mouth that draws sap; it has a radula, a ribbon of microscopic teeth that rasps and shreds tissue [7]. The outcome, however, arrives at the same place, and it arrives by three routes that add up.
- The removal of leaf area is cumulative. Each feeding window removes a little. With thousands of animals per tree and windows that repeat at every irrigation, the sum over a season is chronic defoliation, and that is exactly what leaves the tree without reserves for the following year.
- They target the new tissue. Feeding is heaviest on tender, young growth [7], which is precisely the part that would build next year's framework and crop. A tree that loses every new flush does not grow, however much it is fertilised.
- The remaining green area loses its function too. In extreme cases the loss of green leaf area is such that the plant can no longer take up even the foliar applications it is given, with substantial yield loss [7].
The distinction is not academic. If the cause were sap sucking, one would look for a contact insecticide and examine the trunk. Because the cause is repeated removal of leaf area by many animals, the measure that counts is reducing the population before the damage accumulates, and the moment to intervene is whenever the animals are active, not whenever they are visible.
5. What applies in Greece
Unlike many other pests, here the Greek register names the species. Theba pisana (O.F. Müller) appears as a target in the database of approved plant protection products of the Ministry of Rural Development and Food [6].
The target Theba pisana appears in fourteen crop entries, among them the general heading "Trees", and one entry covering all crops [6].
The only active substance appearing for this target is ferric phosphate, as a soil bait [6]. No other active substance holds an approval for this species.
Products, rates and conditions of use change and are not given here. They are checked before every intervention in the database, at fyto.plantprotect.gr, which is in Greek and covers Greece only.
It is worth noting that the database holds separate entries for other molluscs, such as Helix species and slugs of the genus Arion, with a different range of crops [6]. An approval is judged on the particular species and the particular crop, not on the category "snails".
Sources: 6
6. The nematode, what the studies actually show
The nematode Phasmarhabditis hermaphrodita has been on the market since 1994 in fifteen European countries as a product against slugs [3]. The juveniles enter through a pore at the back of the mantle, establish in the shell cavity and reproduce inside the host, which dies 4 to 21 days after infection [3].
For Theba pisana there are two peer-reviewed studies, from the same group, and their results have to be read together.
| Study | What was tested | Result |
|---|---|---|
| 2020 [1] | Three Phasmarhabditis species on juvenile snails, at 150 infective juveniles per square centimetre | Significant mortality, all three species equally effective, comparable with the chemical control |
| 2022 [2] | The same three species on adult snails, at 150 and then at 90 and 30 per square centimetre | At 150, death within two weeks. At 90 and at 30 it did not kill them effectively, not even the most virulent species |
The recommended field rate of the commercial product is 30 infective juveniles per square centimetre. It follows independently from two directions: the studies state it explicitly as the reference rate [1] [2], and the 2023 review gives a field rate of 3 billion juveniles per hectare [3], which is exactly 30 per square centimetre.
The laboratory therefore killed Theba pisana only at five times the label rate, and at the label rate it did not kill it. Anyone buying the product expecting it to solve a snail problem in an orchard will, on the published evidence, be disappointed.
Two further limitations weigh heavily under Mediterranean conditions. The first is thermal: survival of the nematode falls sharply at 25 and at 35 degrees, and its growth optimum lies around 17 degrees [3]. The second is that many snails are resistant, and in some the nematodes are trapped and encased within the shell [3].
The season when the snail is visible and troublesome, that is summer, is precisely the season when the nematode does not survive. The season when the nematode would have the temperature to work, that is cool damp weather, is also the season when the snail is feeding and therefore reachable. The window exists, but it is narrow and it is not the summer. The authors themselves conclude that research is needed on the method of field application [2].
A note on appraising the sources: the 2022 paper declares that one of its authors is a co-inventor on a patent for a mollusc-killing biopesticide [2]. This does not invalidate the results, but it is recorded here, as the journal itself records it.
7. What can be done in the field
The reasoning, before any intervention
- No intervention on dormant animals. A sealed snail neither feeds nor moves, so no bait reaches it. The moment is whenever the animals are active, and that does not mean autumn alone: in an irrigated orchard the window opens within the summer too, in the morning after irrigation or after a heavy dew [7].
- Straight after irrigation or rain. Baits perform better when the soil is wet and the animals are active [4].
- The herbaceous cover is the refuge. The animals go dormant on the dry stems of grasses, as in Figure 1. Managing the sward inside and around the planting removes both the refuge and the ladder to the tree.
- A physical barrier on the trunk. The Australian extension material recommends copper bands on the trunk to prevent climbing [4]. The measure is mechanical and does not touch the beneficial fauna.
- Collection while they are concentrated. The same summer dormancy that puts the animals beyond the reach of baits makes them visible and gathered together. On small areas, mechanical removal at that moment is the simplest measure there is.
- Checking propagation material. The species travels dormant on plants and on materials [5]. A young tree with sealed snails on the trunk is an imported population.
No published Greek measurement was found for population densities, for damage in Greek crops, or for the start and end dates of summer dormancy under Greek conditions. The gap is reported as a finding. Anyone holding field records holds data that exist nowhere else.
8. Sources
The numbers in the text refer to the list below.
- [1] Tandingan De Ley, I., Schurkman, J., Wilen, C. and Dillman, A.R. (2020). Mortality of the invasive white garden snail Theba pisana exposed to three US isolates of Phasmarhabditis spp. PLoS ONE 15(1): e0228244. Source for the native range of the species in the Mediterranean and western Europe, for densities of up to 3,000 snails per tree in California, for damage to citrus, almond, olive and grapevine in Australia, for the clogging of machinery and the rejection of pasture, and for the test of the three nematode species on juvenile snails at 150 infective juveniles per square centimetre. Full text, open access.
- [2] Schurkman, J., Tandingan De Ley, I. and Dillman, A.R. (2022). Dose dependence of Phasmarhabditis isolates on the mortality of adult invasive white garden snails (Theba pisana). PLoS ONE 17(7): e0270185. Source for adult snails also being killed within two weeks at 150 infective juveniles per square centimetre, and for the most virulent species NOT killing them effectively at 90 and at 30 per square centimetre. Source also for the size of the animals, juveniles of 0.25 gram in the first study and adults of 0.4 to 1.2 gram and 11.5 to 15 millimetres in the second, and for the declaration of competing interests. Full text, open access.
- [3] Rae, R., Sheehy, L. and McDonald-Howard, K. (2023). Thirty years of slug control using the parasitic nematode Phasmarhabditis hermaphrodita and beyond. Pest Management Science. A review. Source for the product being on the market since 1994 in fifteen European countries, for the field rate of 3 billion juveniles per hectare, for the route of entry and the death of the host in 4 to 21 days, for the sharp fall in survival at 25 and 35 degrees with an optimum around 17, and for many snails being resistant and encasing the nematodes in the shell. Full text from the institutional repository.
- [4] NSW Department of Primary Industries. Snails in citrus. Official extension factsheet, New South Wales, Australia. Source for the shell diameter of 10 to 30 millimetres, the brown band and the partly closed umbilicus, for eggs of 1 to 3 millimetres in clutches of up to 120 below the soil surface, for up to 5 clutches a year from autumn to spring, for hatching in two weeks, maturity within a year, a life of up to five years and one generation, for damage to the rind of citrus, for activity at night, in the early morning and in rain, for applying baits onto wet soil, and for copper bands on the trunk. The life-cycle figures are given in the factsheet for snails collectively and not for Theba pisana alone.
- [5] Deisler, J.E. and Stange, L.A. White Garden Snail, Theba pisana (Mueller). Featured Creatures EENY-197, University of Florida, revised 2002. Source for the epiphragm and its role in conserving water, for the animal being carried in freight while dormant, and for severe defoliation in citrus and ornamentals.
- [6] Greek Ministry of Rural Development and Food, database of approved plant protection products. Source for Theba pisana (O.F. Müller) appearing as a named target in fourteen crop entries, among them the heading "Trees" and one entry for all crops, and for ferric phosphate being the only active substance approved against this target. Source also for the separate entries of Helix species and the genus Arion. Checked against the data release of 7 October 2026. The database is in Greek, the approvals are valid in Greece only, and they change; they are checked before every intervention.
- [7] Cesar Australia, Pest notes: white Italian snail. Source for moisture, from heavy dew or from rain, making the snails active and bringing them down to the soil surface, for the species being an opportunistic breeder with breeding recorded in wet summers, for feeding by rasping with the radula and the preference for young growth, and for extreme loss of green leaf area preventing uptake of foliar applications with substantial yield loss. Australian extension material for broadacre crops. The qualitative behaviour transfers, the crops and the dates do not.
This text is informative and educational in character and does not replace on-site agronomic assessment. It is not a recommendation to apply plant protection products, and no rate is given as a recommendation. Plant protection products are used only under the approvals of the Greek Ministry of Rural Development and Food and the instructions on the label. Those approvals are valid in Greece only and say nothing about what is permitted in any other country.
The results for the nematode come from laboratory trials outside Greece and are not evidence of efficacy in a field. A nematode-based product is used only where it holds an approval for the crop and the target.
Every intervention is at the user's responsibility, following on-site assessment by a licensed agronomist. Pastopoulos Agronomics accepts no liability for the use of the above information.
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