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With a little practice and orchid pest information,

orchids can be grown generally free of

serious pests. But in recent years there has been an upsurge

in problems caused by a root-feeding pest called the

orchid snail or bush snail, Zonitoides arboreus (Say).

Description

Adult orchid snails are yellow-brown and about an eighth

of an inch in diameter. The body of the snail is dark grayish-

blue. They damage plants by feeding on the thick, corky

roots that grow from the base of the plant (Fig. 1). Ordinarily

these roots grow down into the media, holding the

plant firmly within the pot. However, plants infested with

snails become unstable. They cannot be sold until the pests

have been controlled and the roots have re-grown. It only

takes two or three adult snails in the medium of a 4-inch

pot to cause serious root damage.

The Orchid Snail as a Pest of Orchids in Hawaii

Robert G. Hollingsworth1 and Kelvin T. Sewake2

1U.S. Pacific Basin Agricultural Research Center, Agricultural Research Service, USDA

2Department of Plant and Environmental Protection Sciences, CTAHR

Pest status

In a 1999 survey of commercial orchid growers in Hawaii,

44% of the 64 respondents reported this snail species as a

pest in their greenhouses, costing them an average of $503

per year in control costs and $5,708 in lost sales during the

previous 12 months (unpublished survey data). Growers

reported damage on nine different types of orchids, particularly

in the genera Oncidium, Dendrobium, Cattleya,

Phalaenopsis, and Vanda. Orchids in the Oncidium alliance

were mentioned most frequently as being susceptible

to attack (42% of the instances reported). The disparity

between the amount of money these growers spent to control

the pest and the value of lost sales indicates the insidious

nature of the damage. In many cases, by the time the

grower discovers the problem, serious damage has already

occurred.

It is not clear why problems with orchid snails are increasing,

because this snail species is not new to Hawaii.

According to Dr. Robert Cowie of the Bishop Museum,

the orchid snail has been present in Hawaii since at least

1928. In its native range (North America and Central

America), this snail is commonly found under the loose,

Figure 1. Two orchid snails near a new root emerging from the

base of an orchid plant. The hollowed-out root (arrow) was

damaged by snails.

Photo by Scott Bauer, USDA Agricultural Research Service.

Figure 2. An orchid snail and eggs on a piece of coir.

MP-1 The Orchid Snail as a Pest of Orchids in Hawaii CTAHR — Aug. 2002

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moist bark of decaying trees, or in decaying wood or vegetable

matter. Pest interception records kept by quarantine

inspectors indicate that potted orchids are the most common

commercial host for this snail. Orchid snails have also

been reported as a pest of sugarcane roots in Louisiana.

The recent increase in snail damage in Hawaii may be

related to the rapid growth of the commercial potted orchid

industry since 1990. Most potted orchid growers use bark

or coir media, which provide a moist environment conducive

to snail growth. Previously in Hawaii most orchid growers

were cut-flower producers and used cinder or basaltic

rock as the growing medium. Some growers have suspected

that purchased bark or coir media may be the source of

their snail problem. However, we were unable to recover

any snails from several types of bark and coir media that

we purchased and tested for the presence of snails. Further,

we discovered that fresh snail eggs allowed to dry for a

week will not hatch, even when re-wetted.

In addition to the damage they cause, snails are a concern

because shipments of potted orchids may be rejected

if snails are found by plant quarantine inspectors. Although

the orchid snail is widely distributed on the mainland United

States and in other parts of the world, this would not necessarily

prevent quarantine inspectors on the mainland from

taking quarantine action if snails were found.

Biology

In laboratory and greenhouse studies, we have discovered

that orchid snails develop slowly, despite their small size.

A snail completes its life cycle in about three months. Adults

lay small white eggs (about 1⁄25 inch in diameter) in clutches

within the pots of orchid plants (Fig. 2). These hatch in

about two weeks. No one has measured how long these

snails live outdoors or under greenhouse conditions. In the

laboratory, orchid snails have lived more than 16 months.

Their shells contain calcium, and laboratory-reared snails

fed lettuce or cabbage grow faster when given supplemental

calcium in the form of crushed oyster shell.

Control

In general, this snail species is difficult to control with

chemicals, and the best method of control is prevention—

sanitation and exclusion. Snails usually come into the

greenhouse with infested plant material and then spread to

other plants. Growers should keep snail-infested plants on

separate benches from plants known to be clean (such as

those from flasks). The supports of greenhouse benches

should be treated to prevent the spread of snails by attaching

copper bands or applying molluscicides. Re-potting

plants into clean media after bare-rooting them is a timeconsuming

but effective method of reducing the pest problem

in extreme cases.

Commercial pesticides registered for slug and snail

control in Hawaii include formulations containing

metaldehyde, iron phosphate (Sluggo®), and methiocarb

(Mesurol®). Although some growers have recommended

dust applications of diatomaceous earth, these proved ineffective

in our tests. We did not do any greenhouse tests

using iron phosphate pellets, but in the laboratory they did

not attract or have an effect on snails.

Orchid snails are susceptible to products containing

metaldehyde and methiocarb. Regardless of the product

used, a single application is unlikely to kill all of the snails.

The key is to use repeated applications at regular intervals

(every three to four weeks) until all the snails have been

killed. Products containing metaldehyde may be more effective

if watering is withheld for a while after treatment.

This is because metaldehyde products cause slugs and snails

to secrete excess mucus, causing their dehydration and

death. Growers must decide for themselves what period of

time water can safely be withheld without risking injury to

the plants.

The effectiveness of pesticide applications can be determined

by removing and searching the planting medium

from a number of orchid plants that were known to be infested

previously. Place the collected shells in a clean jar

with a few drops of water or with a piece of moist paper

towel. Living snails will readily emerge from the shell and

begin moving about. In our studies, the three products that

were most effective against orchid snails were Slug-fest®,

a liquid concentrate containing 25% metaldehyde, Durham®

7.5% metaldehyde granules, and Mesurol®, a wettable powder

containing 75% methiocarb.

Surprisingly, none of the metaldehyde-containing baits

we tested actually attracted snails. Those that had some

effect did so because snails accidentally contacted the baits.

Growers are cautioned that baits containing metaldehyde

products are very attractive and highly toxic to dogs, and

Mesurol is a potent nerve poison and thus is classified as a

restricted-use pesticide.

Disclaimer: Mention of a trade or product name does not imply approval or recommendation of the product to the exclusion of others that

may also be suitable. The description of a pesticide use is not intended as a substitute for restrictions, precautions, and directions given on

the product label. Users of pesticides are responsible for making sure that the intended use is included on the product label and that all

label directions are followed. Neither the University of Hawaii, the United States Department of Agriculture, nor the authors shall be liable

for any damages resulting from the use of or reliance on the information contained here, or from any omissions to this publication.