PLANT PROTECTION 1 – Pests, Diseases and Weeds

PLANT PROTECTION 1 – Pests, Diseases and Weeds
FEEDING AND
PLANT DAMAGE
Slugs are an
increasing problem
worldwide, Some
species are known to
eat 1/3rd of their body
weight each day
Snail and slug numbers
can be fantastic - up to
7 million snails/hectare
have been recorded in
Australia!
Skeletonisation
of gazania leaves
by snails
CLASSIFICATION,
IDENTIFICATION,
DIAGNOSTICS
Do not confuse pest
with beneficial species
DIRECT FEEDING DAMAGE.
Slugs are primarily pests of ground crops, eg annuals, vegetables, potato tubers,
seedling crops, weeds. Snails and slugs may damage emerging crops so much
that re-sowing is necessary and can cost growers million of dollars each year in
lost production. Snails and slugs are minor pests of ornamental crops but
infrequent control may lead to persistent problems.
SEEDLINGS Completely consumed, eg emerging seedlings, cuttings,
new growth can be extensively damaged. The cotyledons are
eaten, killing the young plant before its roots can be established.
LEAVES
Holes chewed (older snails), eg begonia, cabbage
Skeletonization (young snails), eg gazania
FLOWER BUDS
Eaten around edges of petals, eg daffodils, iris
Holes chewed, eg orchids
PASTURE, CROPS Chewing damage, slime
FRUIT, BULBS
TUBERS
TRUNKS,
ROOTS
Holes, eg strawberry
Tunnels bored, eg potato
Chewed bark, eg citrus
Chewing damage, eg roots exposed in soil cracks
INDIRECT DAMAGE.
Slimey trails and coiled threads of spaghetti-like excreta make plants look
unsightly and unsaleable; stock will not eat slime covered grass.
Contaminated stored grain is downgraded because of snail infestation or
increased moisture content because of the crushed snails. May contaminate
harvested citrus fruit. Crushed snails may damage harvesting machinery.
Transmission of plant diseases Overseas, snails and slugs are known to
have transmitted tobacco mosaic virus and various fungal diseases, eg
Phytophthora, Fusarium and rusts of European plants.
Transmission of animal and human diseases
– Introduced helicid snails, eg Cochlicella, Helix, Theba, in SA are commonly
infected with a flatworm that can infect humans.
– Snails may eat rat faeces contaminated with parasitic worms (Angiostrongylus
cantonensis) and may then contaminate vegetation depositing the parasitic worm
in its mucous trails. Rats can be re-infested by eating infested snails (May 1998).
– Snails (Lymnaea spp.), 4-10 mm, long are the intermediate hosts for liver fluke
(Fasciola hepatica) which is a major parasite of sheep, cattle, goats, pigs and
wildlife in south-eastern Australia.
Introduction of decay organisms.
CLASSFICATION AND IDENTIFICATION
Snails and slugs belong to the Phylum Mollusca, Class Gastropoda.
Snail identification is based on the features of the shell and size, shape,
colour and culture of the snail itself. Slug identification is based on size,
shape, colour and culture of slugs.
Identification tools include:
– Pocket Guides are available for both snails and slugs.
– Fact Sheets for local species by State Depts. of Primary Industries.
– Web sites include: Land snails of Australia – Museum Collections
www.environment.gov.au/biodiversity/abif/bat/snails.html
– Lucid key A Key to the Families Non-Marine Molluscs of Quarantine Concern
in Australia www.lucidcentral.org/
DIAGNOSTICS.
Damage by adult snails and slugs is easy to recognize on most plants.
Look for silvery slimy trails and excrement casts which are long and curly
and adhere to feeding sites.
Young snails may skeletonise surface of leaves of plants such as gazania.
Damaged leaves then shrivel up and are then difficult to recognize as snail
damage. Snails and slugs may eat from the edge of the leaf as well as from
within the leaf margins.
Do not confuse snail or slug damage with that caused by chewing insects,
eg caterpillars, beetles, cutworms, or birds on certain plants, eg
Cucurbits
- Snails, leafeating ladybirds, pumpkin beetle
Geraniums - Snails, caterpillars of various moths
Spinach seedlings - Snails and slugs, cutworms, birds
Snails and slugs are not usually seen during the day. Where damage is
unexplained, inspect area after say 10 pm on a mild calm night, or put out
traps (sacks, etc) or baits, to monitor population densities (page 233).
Snails and slugs 229