Indirect effects of pesticides on birds

The UK’s Joint Nature Conservation Committee (JNCC) has produced a report that provides further and more detailed evidence that pesticides have indirect effects on farmland birds by reducing the abundance of their invertebrate insect food source.

The JNCC report covers ecological monitoring carried out by the Game Conservancy Trust (GCT) over an area covering 62 km2 in West Sussex, Southern England, between 1970 and 1995. GCT collected information concerning crop types, weed occurrence and insect abundance from about 100 cereal fields in the third week of June in the same way each year of the survey.
    The researchers collated data on the use of herbicides, fungicides and insecticides on the arable crops and related this to measures of weed, invertebrate and bird abundance on a field-by-field basis. They considered 15 groups of weed plants that form avian diets and included two broad measures of abundance, one for broad-leaved weeds and one for grass weeds. Likewise they checked the abundance of five groups of invertebrates that are eaten by birds: spiders and harvestmen; ground beetles and click beetles; sawfly, butterfly and moth caterpillars; leaf beetles and weevils; and non-ahpid bugs and hoppers. This information was related to the average densities on three farmland bird species grey partridge, corn bunting and skylark. Previous research has already confirmed that grey partridge populations have declined because of the indirect effects of pesticides on their diet, and there is strong evidence in the case of corn bunting. This new report sheds more light on exactly how these bird decreases occur.
    Some main findings from the report are:

• The abundance of four out of five invertebrate groups decreased as the application of synthetic pyrethroid (SP) insecticides increased, and all showed a negative relationship with the use of organophosphates. None of the invertebrate groups were affected by the carbamate insecticide, pirimicarb.
• Numbers of invertebrates in all five groups showed a decrease after insecticide use in the previous year. This particularly important finding was additional to any effect attributable to pesticide use in the current year, and took into account crop type in the current and previous years.
• Grey partridge density in August was inversely related to the number of herbicide applications, and positively related to the number of broad-leaf weed groups. 
• Corn bunting and skylark densities in June were low where the number of herbicide applications was high, and for corn buntings where the number of fungicide applications was high.

Conclusions
The apparent ‘good news’ for pirimicarb may lead bird-lovers to recommend its use over the SPs. This would be of concern because pirimicarb belongs to the anticholinesterase nerve-poisons that has caused adverse health effects to users, and is currently under special review by the Pesticide Safety Directorate.
    The carry-over effect is a surprising result in light of the ‘patch-work’ nature of British agriculture. Average field sizes are small, especially when compared to farms in say North America. The perceived wisdom has always tended to be that invertebrates have the facility to recover in the UK because the area of any one spray or group of sprays is relatively small, as it corresponds to a small field size. (DB)

Pesticide use, avian food resources and bird densities in Sussex, JNCC, Natural History Book Service, Tel. +44 (0)1803 865 913, 1999, £15, 103pp.

[This article first appeared in Pesticides News No. 46, December 1999, page 21]