PLANT PROTECTION 1 – Pests, Diseases and Weeds

PLANT PROTECTION 1 – Pests, Diseases and Weeds
SPREAD
CONDITIONS
FAVOURING
Snails are most active in
moist, cool conditions
so a good time to look
for them is after heavy
dew or irrigation
Slugs are likely
to be greatly
underestimated
as pests as they
shelter in cracks
in the ground, under
litter and are not
usually visible
during the day
SNAILS HAVE CAPACITY TO SPREAD
By their own movement over smooth surfaces, depending on the species
they can travel up to 1 metre a day.
Snails may migrate from scrubby areas in search of food during autumn and
winter to pastures and crops, and can travel from 20-55 metres per month.
Snails move in droves in autumn and winter from roadside vegetation into
more exposed pasture where they feed and reproduce before returning to the
roadside in spring and early summer. They have a good sense of smell and
can travel substantial distances to find choice food.
Hitch-hiking on cars, trucks, containers, pallets, bailed hay, nursery stock,
long distances may be covered. This is how exotic snails arrive in Australia.
Eggs or newly hatched snails may be transported on empty bags or
containers, in soil in deliveries and in pots.
Pointed or conical snails gradually spread north through Vic and NSW.
Shell souvenirs, food carried by airline passengers.
EXACT CONDITIONS VARY ACCORDING TO SPECIES
Wet weather especially during autumn, winter and spring, Big spring
rains followed by a mild damp summer and autumn; but not heavy rain or
wind. Mediterranean climates. Only a problem in higher rainfall districts.
Cool climate areas. Cool moist shady places such as under mulches,
boards, stones, benches, in pots, debris, leafy weeds, broadleaved plants
growing close to the ground as shelter on hot sunny days. Cloudy or foggy
days. Mild temperatures (15-25 o C) and calm periods.
Limey soils increase the fertility of both snails and slugs; population
densities are greatest with a soil pH of 6.3-6.7. White Italian snail (Theba
pisana) only thrives in areas of alkaline sandy soil with a high calcium
content mainly near the coast. Slugs are favoured by heavier soils and do
not survive well in fine, light or compacted soil.
Smooth surfaces on which to move.
Ground cover plants and weeds provide ideal moisture levels and shelter.
Perennial and cover crops between trees and vines allow large populations
of slugs to develop. Often a problem occurs on the edges of crops adjacent
to weedy fence lines and around uncultivated islands within crops.
Preceding crops are important. Damage to crops is usually heavier after
peas, clovers and some Brassicas.
Pet food left unattended in gardens attracts great striped garden slugs.
Large amounts of organic matter, eg dug-in vegetation, remains of
straw or manure, help increase soil moisture and are a source of food.
Conservation Tillage (CT) with minimum/no tillage, stubble retention and
direct drilling practices help slugs survive (increased shelter, moisture, food).
Slugs are listed as one of the major pests that benefit from these practices.
Reduced burning of crop stubble.
Most pest snails and slugs are introduced so few natural enemies.
ENVIRONMENT
Does it favour the crop or the snails?
SUSCEPTIBLE
HOST PLANT
PEST SNAIL
PRESENT
Fig. 131. Pest triangle.
232 Snails and slugs