Factors influencing molehill distribution in grassland: implications for controlling the damage caused by molehills


Dr Grant Edwards, AgResearch Grasslands, Private Bag 11008, Tennent Drive, Palmerston North, New Zealand (fax 0064 6351 8032; e-mail: edwardsg@agresearch.cri.nz).


1. Moles are perceived as pests of farms, gardens, sports fields and nature reserves, mainly because they form molehills. The danger and inhumaneness of current methods of mole control (e.g. poisoning with strychnine or the use of mole traps) means that non-lethal methods are sought. We examined the possibility of controlling molehill distribution by using management procedures that alter the availability of earthworms, the principal food of moles.

2. The abundance of molehills and earthworms was monitored over 2 years in an acid grassland where pesticide (with and without insecticides and molluscicides), grazing (continuous grazing by rabbits vs. hay meadow), soil pH (with and without lime), herbicide (with and without herb- and grass-specific herbicides) and fertilizer (N, P, K, Mg) treatments were imposed.

3. In the experimental area of 4608 m2, a total of 1062 molehills formed, each with an average area of 0·14 m2; a disturbance rate equivalent to 3·2% of the soil surface over 2 years. Peak molehill production occurred in spring and autumn, with few molehills formed at other times of the year.

4. Molehill production in grazed areas was one-third that of hay meadows. Half as many molehills formed in unlimed as limed plots. Significantly fewer molehills formed in areas where grass species were removed (herb-rich) than areas where no species were removed. Insecticide, molluscicide and fertilizer application had no significant effect on molehill production.

5. The treatments that had fewer molehills also had less earthworms, indicating that molehill production was decreased, indirectly, through the treatments reducing food availability.

6. Reducing the number of molehills through management procedures that decrease earthworm availability offers an alternative to lethal control of moles. This could be achieved by allowing (or encouraging) soil pH to fall (e.g. withholding lime application; through the use of acidifying nitrogen-fertilizers), by creating herb-rich swards or by preventing plant biomass from accumulating for long periods. These methods will be more applicable to gardens, sports fields and nature reserves than to farms, where conflicts with normal farming practices would make them difficult to implement.