The effects of nitrogen levels in sitka spruce needles on Elatobium abietinum (Walker) populations in North-Eastern Scotland.

In the past seven years five serious outbreaks of E. abietinum have occurred in north-eastern Scotland. These were usually characterised by a rapid build-up of numbers from April to June followed by a sudden drop in late June and early July. Rapidly increasing populations were characterised by low adult/first instar nymph ratios, slowly increasing populations exhibited higher ratios. More alatae were present when aphid numbers were high. This resulted in a decreased production of nymphs. The total soluble nitrogen level in Sitka spruce needles decreased from May to August. Most individual amino acid levels also decreased. Several essential amino acids, including methionine, histidine aud iso-leucine, were in short supply for much of this period. Amino acid levels in aphid extracts also decreased from May to July. The size of the adult aphids remained constant throughout April to late June but dry and fresh weight decreased from late April onwards. The lipid proportion of the dry weight decreased from May to August as did the number of adults bearing embryos. From July onwards a small, decreasing population of aphids existed which showed reduced rates of development and reproduction.