Anomalous stylet punctures of phloem sieve elements by aphids

A deviating type of potential drop (pd), i.e. the electrically recorded int racellular punctures during the pathway phase of plant penetration by aphid s, is described and discussed. This so-called repetitive pd (R-pd) has been observed in 4 aphid species, two on herbs and two on sycamore. Apart from a regular repetition with short intervals, the most characteristic feature of the R-pd is the middle intracellular part, i.e. sub-phase 2, which is ex tended, making the individual R-pd 2-3 times longer than the normal pd. Dif ferences between the normal and R-pd are described, as well as differences within each pd type between species. The R-pd's seem to reflect repeated pu nctures of phloem sieve elements. Their biological significance is unknown.