Slug control

October 2007
Integrated pest management (IPM) aims to maximise the ecosystem services that beneficial predators can provide, to minimise unnecessary pesticide use and to ensure that where chemicals are deemed necessary that only those specific to the pest in question are used (broad spectrum insecticides can interfere with the control of other pests and cause unwanted population explosions).

Monitoring is the key to all IPM strategies. All the pests need to be identified in order to understand the best management practice for control. The correct control selection will depend on the relative numbers of pests and beneficials, the types of beneficials, the stage of the crop, the pesticide history and the life stages of pests and beneficials.

IPM looks at all pests in a crop simultaneously as there is no point in spraying for one pest while killing all the beneficials which control another pest.

Abie Horrocks, soils research associate and slug specialist at Crop & Foods Research at Lincoln, has conducted a three-year project investigating IPM for slug control in arable crops, funded by the Foundation for Arable Research (grower-funded) and the MAF Sustainable Farming Fund.

An Australian company, IPM Technologies, was also included in the project.

In 2006 Abie was awarded the FAR (foundation) researcher of the year.

The trend towards minimum tillage for crop establishment, for mainly soil structure reasons, means that slug control will become more important.


Monitoring has established there are three main predator beetles which are capable of contributing to slug control. These are native carabid species called M. antarcticus, H. angustula, and M. monoliferum. They may be present in the field at all times of the year or build up when the slugs are numerous.

The project involved extensive monitoring of the four slug species found in Canterbury arable crops, and they have distinct characteristics, making it possible to tell them apart in the field. The grey field slug is the most common, followed by the brown field slug. The keeled slug and the arion species are less common. Not all slugs are equal in terms of damage. Grey and keeled slugs in a laboratory trial on clover did four times more damage to emerging clover than the brown field slug and arion species. In Australia the keeled slug is an important and aggressive pest. Before this project it was thought that the keeled slug was only p[resent in the North Island.

The grey field slug is the biggest pest. It is very responsive to changes in moisture and will become active extremely quickly when conditions become favourable, Abie said.

Abie established that two commonly used baits for slug control do not then kill the beneficial beetles EDTA and Slug Out. This is to avoid secondary poisoning of predator beetles. To protect the beetles from broad-spectrum insecticides, aphids should be controlled with seed treatments and selective aphicides such as Pirimor.

Recommendations to farmers

Monitoring doesnt need to be time-consuming, as often it comes down to observations farmers are doing every day. However, these key points are essential:

Knowing which crops are susceptible to pest damage.

Knowing which areas on the farm are problematic.

Recognition of the conditions that lead to pest numbers increasing rapidly.

The activity of slugs and ability to do damage is dependent on moisture and temperature. They are most damaging in wet weather. High levels of surface trash also provide an ideal habitat.

For aphids, warmer temperatures over winter are likely to mean that a more intensive spray programme will be needed. Cold, wet and windy conditions slow aphid development and reproduction.

Slug monitoring starts at drilling and continues until the crop is out of danger. Just because numbers are low at drilling does not mean that there wont be population explosions within the window of vulnerability (6 to 8 weeks after drilling).

Slug populations can recover very quickly. Slug pellets are effective in killing slugs active on the surface but the reservoir of slugs in the soil is largely unaffected. Just because baits have been laid once, it cant be assumed that the problem is solved. Slug numbers may outnumber baits.

Slugs may not all be active yet and by the time they are the baits may no longer be effective. Baits will never kill all slugs, so if they are knocked back early they could build up again within the window of vulnerability.