Nagis Gani - Processors & Growers Research Organisation

The following summary is an extract from the full & final report

1. Slugs are a major concern for the vining pea industry, as they are picked up during harvesting and become incorporating into the processing machinery. This can cause problems for both producers and consumers alike.
2. Previous work has provided some evidence that slugs can detect the odours of their carabid beetle predators and actively avoid areas where beetles have been.
3. The present project aims to determine : (1) whether slugs can detect beetle odours and actively avoid them, (2) the nature of these odours, and (3) the prospect for producing artificial odour formulations which can be used as slug repellents during vining pea harvesting.
4. In an extensive series of laboratory experiments, the common field slug Derocerus reticulatum was shown to actively avoid areas previously occupied by its four most common carabid predators, Pterostichus melanarius, P. madidus, P. cupreus and Harpalus rufipes.
5. Avoidance behaviour was shown by the slugs to the odours of live beetles, beetle extracts and to pygidial gland extract, showing that the most likely source of repellent odour is from the secretions of these beetle defensive glands.
6. The nature of the slug avoidance behaviour was examined. In contact with beetle extracts, slugs tend to stop instantly, raise the anterior body from the substrate, retract the antennae and show pronounced side-to-side 'waving' movements of the head. The slugs also tend to recommence forward motion only after an extreme turn away from the source of beetle extract.
7. The nature of the pygidial gland secretions was examined using GCMS. By far the most abundant compounds were the carboxylic acids, methacrylic and crotonic acids (P melanarius), methacrylic and tiglic acid (P. madidus), crotonic and acetic acids (P. cupreus) and formic acid (H. rufipes), together with a number of alkanes and ketones.
8. Solutions of individual acids and pygidial gland extracts from all four beetle species were tested for slug repellency on pea plants under glasshouse condition. All beetle extracts significantly reduced slug damage to plants over a 5-day period. The individual acids were less effective, only methacrylic reducing damage significantly over 5 days. This may be because the acids need to be blended in the proportions found in the secretions to maximise their effect. Reduced plant damage appears to be a result of delayed colonisation of the plants by slugs. Both P. madidus extract and methacrylic acid significantly delayed colonisation of the plants for the first 2 days, but the effect had waned by day 5. Treatments had no major direct negative effects on the plants themselves.
9. Overall, prospects for the development of beetle-derived repellents against slugs are encouraging. Having established the active compounds involved, the next stage must be to find the most appropriate blends and formulations of acids, bearing in mind safety issues for both the industry and consumers.