A Method of distinguishing the larval Stages ofAgriotes sputator(L.) (Col., Elaterid.)

Abstract

Little was known about the composition of wireworm populations until Salt and Hollick (1944) published an account of the complete wireworm populations of soil samples from pasture. Their paper showed for the first time the great preponderance of small larvae in any wireworm population from old grassland. It also pointed out that there is " in general, a very marked correlation between the length of the larvae and their numbers-there are many larvae in the smaller size-groups, and progressively decreasing numbers of the larger sizes ".The question at once arises : In such a wireworm population what is the relation between the size of the larvae and their age ? It is known that the larval life extends over a period of about four years, but apart from the fact that in general the smallest larvae are the youngest, and the largest are the oldest, it is impossible to say what proportion of the larvae in a population is in the first, second, third or fourth year of their life. If the population could be accurately divided into year-groups, or into instars, it would be possible to trace the incidence of favourable or unfavourable factors. For instance, a population containing an especially high proportion of larvae in the second year-group would point to especially favourable conditions for oviposition two years previously, and for survival since.The primary object of the present investigation was to discover a means of distinguishing with certainty the different instars of wireworms as the first step in the analysis of larval populations with respect to age and time. Material.The investigations were confined to Agriotes sputator (L.) from Cambridgeshire and, for the greater part of the work, it was desirable to study a large homogeneous collection of larvae. Such a collection was obtained from a soil sample, of area 1 yard sq. and depth 6 ins., taken from permanent grassland in August 1942. This yielded 1,039 wireworms.The larvae from a second sample were examined to corroborate the data obtained from this collection. This sample, from an area 1 foot sq. and 18 ins. deep, was from, the same field but had been taken in February 1942. The larvae were extracted from the soil samples by the method described by Salt and Hollick (1944). They were preserved in 70 per cent, alcohol.A number of cast skins from larvae reared individually in tubes in the laboratory was also available. Some of these larvae were known to have pupated.The character used to distinguish A. sputator larvae from other species present in the August collection was that given by Gueniat (1934), that is the punctation on the cushion on which the legs are borne. This character was easily seen on the large larvae using a 12-5 ocular and a 6-8 objective, but not on the small specimens ; all but three of a total of 755 large larvae examined proved to be sputator. The rest of the collection were all less than 4-8 mm. in length. It would have necessitated mounting each specimen in glycerine and using a much higher magnification to have determined the species in ea...