THE COMPARISON OF SUCTION TRAP, STICKY TRAP and TOW-NET FOR THE QUANTITATIVE SAMPLING OF SMALL AIRBORNE INSECTS

The sticky trap and stationary aerial ‘tow-net’ catch insects which alight or fly on to them or are blown against them by the wind. It would be expected that such traps would be inefficient in light winds or in calm weather; and even though their efficiency should increase with stronger winds, errors of unknown magnitude may occur not only in estimations of density and of proportions of species in the air, but also with comparisons of actual catches. These errors are due to unknown degrees of weighting as the traps sample by means of a variable wind from a changing population density. The suction trap, on the other hand, samples a constant quantity of air in all relevant wind-speeds and does not appear to suffer so seriously from these disadvantages. It also works efficiently in perfectly calm weather when maximum densities of insects are often in the air. The performances of the three traps in the field operating over a range of wind-speeds are described. Particular attention has been paid to aphids, for which the sticky trap and tow-net are generally used and for which the suction trap was primarily designed. Density estimates of these insects in winds below about 3 m.p.h. are much larger when calculated from suction trap catches as compared with estimates from sticky trap and tow-net catches. There is reason for the belief that the suction trap is neither attractive nor repellent to aphids to a significant extent, and that it catches these insects at random by virtue of its air stream alone; weighting of catches due to variable quantities of air being sampled does not occur. It is considered, therefore, that the suction-trap values of density are likely to be the more accurate ones. Sampling of other small insects is also discussed.