Controlling Slugs Sustainably
Slugs severely hinder the establishment of new crops such as clover, ryegrass, winter wheat, pasture and even maize, resulting in the increased costs of re-sowing, reduced yields and the economic and environmental costs of control.
The problem has become greater with the trend to reduced or no-tillage, leading to increased crop residues and minimum soil disturbance, which provide ideal conditions for slugs. Baits, grazing and burning give some control but are often insufficient or unpredictable.
Integrated pest management (IPM) is a strategy that focuses on long-term prevention of pests or their damage through a combination of techniques such as biological control, habitat manipulation, modification of cultural practices, and use of resistant varieties. Pesticides are used only after monitoring indicates they are needed according to established guidelines, and treatments are made with the goal of removing only the target pest and not beneficial organisms. Research in Australia has shown that it is possible to control slugs using an IPM approach.
FAR and MAF Sustainable Farming Fund have funded a three-year project to identify the species of slugs that damage crops, other insects that cause similar damage, the natural predators of slugs, and sustainable methods of control. Two years into the project there is a fair bit of information available that is of use to arable farmers and has implications for pastures.
The research has involved monitoring slugs and their predators at six trial sites in Canterbury, plus lab trials to determine which slugs eat the most and which predators are most effective.
Slug species
A number of species of slugs occur in New Zealand, some are natives and do not pose a threat to crops, but a number of introduced species are pests on farms:
These are (in order of pest importance):
Derocerus reticulatum (grey field slug)
Derocerus panormitanum (brown field slug)
Milax gagates (Milax North Island only)
Arion spp. (Arion)
Different species have different life cycles and also cause varying levels of damage according to how much they eat eg. grey field slugs cause about five times more damage than brown field slugs. Knowing what species of slug are present leads to better decide control decisions.
Grey field slug
Up to 40mm outstretched
secretes milky-white mucus when scratched or disturbed
skin often covered in a reticulated pattern,
colour varies from pale cream or pink to dark brown
breeds whenever conditions are suitable, especially under irrigation
probably the most important South Island pest
Brown field slug
Up to 30mm outstretched
secretes clear mucus when scratched or disturbed
Probably a single generation
Milax
generally black
sharp ridge/keel down its back (the whole length of the body
Nth Island only, seen in maize.
Arion
up to 20mm outstretched
yellow foot and yellow-coloured mucus secreted when disturbed
minor Sth Island pest
In Canterbury, the total number of slugs at monitored sites peaked in October and January, with the January peak comprising more adults.
Earwigs
In Australia they have been found cause damage that is indistinguishable from slug damage, and may explain why some slug baits have proved ineffective in preventing damage on some occasions. This underlines the need to monitor the species active at sites and not rely on signs of damage in making control decisions.
Predators of slugs
The monitoring and laboratory trials have identified three main beetle species that prey on at least one life stage of pest slugs. These ground beetles are part of a group known as Carabids. They are generalist predators in both juvenile and adult stages. Adults are active foragers and are typically shiny black or metallic in colour.
A. Megadromus antarcticus Length: 25-35mm
This large beetle is commonly referred to as the metallic green ground beetle, due to its shiny metallic green colour. It is a ferocious predator and ate all slug species in all size classes presented to it during the lab trials.
B. Metaglymma moniliferum Length: 15-20mm
This black medium sized carabid ate small/juvenille sized slugs only (up to 10mm long).
C. Holcaspis angustula Length: 12-16mm
This small black carabid ate only freshly hatched slugs.
Not a lot is known about these beetles but they probably live for more than a year, and over-winter within field boundaries before moving into the crop during spring. It is expected that cropping systems can be made more carabid-friendly so that their populations within paddocks will increase.
It appears that Carabids may die after feeding on slugs poisoned by Methiocarb baits, so this chemical should be avoided. Where baits are necessary in the establishment phase of a crop, EDTA or Metaldehyde baits give as good a slug kill and do not harm Carabids.
Once Carabid populations build up they appear to give adequate control of all stages of slug development.
Monitoring
Large wooden tiles, sacks, used spray containers with water in them, slices of tree trunk these sorts of things can be used to provide a refuge for slugs and beetles, and can be used to monitor populations if left in place for several weeks.
The main messages are:
Monitor slug populations to identify which species are present, their patterns of development and reproduction. Do this on several paddocks so that when you sow a crop you know in advance what sort of slug problem could occur
If circumstances require it, put out baits but ensure that the chemical used wont harm predators
Consider not planting susceptible crops after a substantial build-up in the slug population
More information is available from FAR, and on the MAF website.
The problem has become greater with the trend to reduced or no-tillage, leading to increased crop residues and minimum soil disturbance, which provide ideal conditions for slugs. Baits, grazing and burning give some control but are often insufficient or unpredictable.
Integrated pest management (IPM) is a strategy that focuses on long-term prevention of pests or their damage through a combination of techniques such as biological control, habitat manipulation, modification of cultural practices, and use of resistant varieties. Pesticides are used only after monitoring indicates they are needed according to established guidelines, and treatments are made with the goal of removing only the target pest and not beneficial organisms. Research in Australia has shown that it is possible to control slugs using an IPM approach.
FAR and MAF Sustainable Farming Fund have funded a three-year project to identify the species of slugs that damage crops, other insects that cause similar damage, the natural predators of slugs, and sustainable methods of control. Two years into the project there is a fair bit of information available that is of use to arable farmers and has implications for pastures.
The research has involved monitoring slugs and their predators at six trial sites in Canterbury, plus lab trials to determine which slugs eat the most and which predators are most effective.
Slug species
A number of species of slugs occur in New Zealand, some are natives and do not pose a threat to crops, but a number of introduced species are pests on farms:
These are (in order of pest importance):
Derocerus reticulatum (grey field slug)
Derocerus panormitanum (brown field slug)
Milax gagates (Milax North Island only)
Arion spp. (Arion)
Different species have different life cycles and also cause varying levels of damage according to how much they eat eg. grey field slugs cause about five times more damage than brown field slugs. Knowing what species of slug are present leads to better decide control decisions.
Grey field slug
Up to 40mm outstretched
secretes milky-white mucus when scratched or disturbed
skin often covered in a reticulated pattern,
colour varies from pale cream or pink to dark brown
breeds whenever conditions are suitable, especially under irrigation
probably the most important South Island pest
Brown field slug
Up to 30mm outstretched
secretes clear mucus when scratched or disturbed
Probably a single generation
Milax
generally black
sharp ridge/keel down its back (the whole length of the body
Nth Island only, seen in maize.
Arion
up to 20mm outstretched
yellow foot and yellow-coloured mucus secreted when disturbed
minor Sth Island pest
In Canterbury, the total number of slugs at monitored sites peaked in October and January, with the January peak comprising more adults.
Earwigs
In Australia they have been found cause damage that is indistinguishable from slug damage, and may explain why some slug baits have proved ineffective in preventing damage on some occasions. This underlines the need to monitor the species active at sites and not rely on signs of damage in making control decisions.
Predators of slugs
The monitoring and laboratory trials have identified three main beetle species that prey on at least one life stage of pest slugs. These ground beetles are part of a group known as Carabids. They are generalist predators in both juvenile and adult stages. Adults are active foragers and are typically shiny black or metallic in colour.
A. Megadromus antarcticus Length: 25-35mm
This large beetle is commonly referred to as the metallic green ground beetle, due to its shiny metallic green colour. It is a ferocious predator and ate all slug species in all size classes presented to it during the lab trials.
B. Metaglymma moniliferum Length: 15-20mm
This black medium sized carabid ate small/juvenille sized slugs only (up to 10mm long).
C. Holcaspis angustula Length: 12-16mm
This small black carabid ate only freshly hatched slugs.
Not a lot is known about these beetles but they probably live for more than a year, and over-winter within field boundaries before moving into the crop during spring. It is expected that cropping systems can be made more carabid-friendly so that their populations within paddocks will increase.
It appears that Carabids may die after feeding on slugs poisoned by Methiocarb baits, so this chemical should be avoided. Where baits are necessary in the establishment phase of a crop, EDTA or Metaldehyde baits give as good a slug kill and do not harm Carabids.
Once Carabid populations build up they appear to give adequate control of all stages of slug development.
Monitoring
Large wooden tiles, sacks, used spray containers with water in them, slices of tree trunk these sorts of things can be used to provide a refuge for slugs and beetles, and can be used to monitor populations if left in place for several weeks.
The main messages are:
Monitor slug populations to identify which species are present, their patterns of development and reproduction. Do this on several paddocks so that when you sow a crop you know in advance what sort of slug problem could occur
If circumstances require it, put out baits but ensure that the chemical used wont harm predators
Consider not planting susceptible crops after a substantial build-up in the slug population
More information is available from FAR, and on the MAF website.