THE INSECT AND OTHER INVERTEBRATE FAUNA OF ARABLE LAND AT ROTHAMSTED1

Abstract

1. Samples of soil were taken from two of the plots at the Rothamsted Experimental Farm and all insects and other invertebrates were recorded together with the approximate depths at which they occurred. One of these plots (plot 2) has received 14 tons of farmyard manure per acre per annum since 1843; the other (plot 3) has received no manure of any kind since 1839. This difference in treatment had a very marked effect on the number of insects present. Twenty-three samples of soil were examined from each plot, each sample being a cube 9 times 9 × 9 inches. The soil in each sample was removed in five layers, so that it was possible to determine the approximate depth at which the specimens occurred. There were, in round numbers, 15,100,000 invertebrates per acre, of which 7,720,000 per acre were insects, in plot 2, and 4,950,000 invertebrates per acre, of which 2,470,000 per acre were insects, in plot 3. The greatest number, both of insects and of other invertebrates, occurred in the upper three inches of the soil, but some species were found in larger numbers at a greater depth, the greatest number of Elateridae larvae being found at a depth of five to seven inches, and of Symphyla at a depth of seven to nine inches. Some species, such as the larvae of Chironomidae and Trichocera, were practically confined to the plot which had received farmyard manure, plot 2, while other species, such as the Collembola, Onychiurus ambulant and O. fumctarius, although they occurred in both plots, were considerably more numerous in plot 2. Injurious insects, such as the larvae of Elateridae, Tipulidae and Hepialidae, appeared to be little affected by the different manurial treatment of the two plots, and occurred in practically equal numbers in the two plots. Although 198,653 and 164,983 Elateridae larvae per acre occurred in plots 2 and 3 respectively, they did not produce any appreciable effect on the crop. An attempt was made to estimate the amount of nitrogen contained in the bodies of the soil fauna, and it was found to be 7349.6 gm. or 16.2 lbs. and 3409.2 gm. or 7.5 lbs. in plots 2 and 3 respectively. It is unlikely that there is any appreciable loss of nitrogen from the soil due to the migration of winged members of the fauna. The worms, insects and insect larvae are beneficial in loosening the soil and facilitating aeration and drainage. The net results of these observations show that, although the introduction of farmyard manure greatly increases the invertebrate population of the soil, the latter organisms are saprophagous and are not directly injurious to the growing crop. Such injurious organisms as are present occur in approximately equal numbers whether the land be manured or not. The most notable exception to this generalisation is met with in the Diplopoda, whose numbers are increased by about 200 per cent in the manured plot.