Effects of molluscicides on seedlings of four grassland plant species

Published source details Hanley M.E. & Fenner M. (2007) Effects of molluscicides on seedlings of four grassland plant species. Journal of Applied Ecology, 34, 1479-1483.
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Summary

Pesticides to protect plants are usually applied to reduce herbivory by invertebrates. However, as well as affording protection to plants, there is a possibility that some pesticides may directly affect germination and seedling survival. This study investigated whether two molluscicides did indeed affect germination and survival, as well as reducing herbivory, which might thereby influence experimental results where molluscicides are used to exclude slugs and snails from plant communities.

Molluscicides: Field trials of two molluscicides were undertaken in Hampshire, southern England. The repellants used were:

Fertosan powder - aluminium sulphate based;

Draza slug pellets - methocarb based mixed with an attractant bait.

Treatments: The experiment was a 2 x 2 x 4 factorial design with two application rates of the molluscicides and four common UK grassland herbs: ribwort plantain Plantago lanceolata, meadow buttercup Ranunculus acris, common ragwort Senecio jacobaea and dandelion Taraxacum officinale.

Seeds (100) of each species were sown into 40, 13-cm diameter pots (160 in total) filled with sterile John Innes no.1 potting compost, placed outdoors in March 1993. The use of a sterile medium reduced the risk of the molluscicides influencing soil micro-organism activity which could in turn affect seedling performance.

Species groups were divided into four treatment sub-groups (10 replicates of each):

i) 2.7 g Fertosan dissolved in 170 mL water;

ii) slug pellets only (5/pot equivalent to 0.2 g);

iii) both molluscicides;

iv) no molluscicides (control).

Chicken wire protected the pots from birds and small mammals, and molluscs were excluded by surrounding the pots with slug pellets. The number of seedlings was recorded 50, 75 and 100 days after sowing.

Affect of molluscicides on seedlings: Plantago seedling numbers were significantly reduced (by approx. 20-25%) by Draza (22-30 seedling/plot) compared to controls. Ranunculus numbers conversely appeared stimulated by Draza (1-2 times that of controls), with Draza plus Fertosan (average c.11-15 seedlings/pot) around three to times that of controls (c.3-6 seedlings/pot).

Neither molluscicide had an effect on Taraxacum (average of c. 16-21 seedlings/pot regardless of treatment) or Senecio (average of c. 42-52 seedlings/pot regardless of treatment). The response of seedlings to either molluscicide did not vary significantly with time.

Conclusions: Draza slug pellets reduced Plantago seedling numbers whist those of Ranunculus increased. Neither molluscicide (separately or together) affected Senecio or Taraxacum. Other studies have shown that it is not unusual for a pesticide to be toxic to one plant species but not another.

In an early field study the authors found that Plantago seedling survival was dramatically increased by molluscicide application, despite negative effects on the plants due to the pesticide itself found in this current investigation.

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