Aphid Infestation and Its Effects on Translocation in Polytrichum commune

Abstract

Polytrichum commune Hedw. is susceptible to attack by Myzodium modestum Hottes, a moss aphid. Scanning electron micrographs indicate that aphids feed by insertion of needlelike stylets into moss gametophores. Liquid scintillation analysis indicates that aphids draw upon labelled tracers following 14C-sucrose treatment of leaves on infested plants. Between 17 and 38% of translocated label can be detected in 2-15 feeding aphids within 4 hr. Localized infestation on a single gametophore within otherwise uninfested plant groupings reduces the flow of label to remote sites (i.e., the shared underground rhizome plus connected neighbors, where allocation falls from 4 to 1% or lower), and alters labelled allocation patterns near the infested site. These data indicate that infesting Myzodium aphids 1) divert nutrients from the food conducting tissues of moss gametophores, and 2) alter the normal source-to-sink flow within moss turfs. Host-specific aphids are widely distributed in the plant kingdom. Most feed on the phloem of flowering plants (Hodkinson & Hughes 1982). A few show specificity for mosses as primary or secondary hosts (Gerson 1982; Moran 1989; Miiller 1973; Patch 1938). The mosses that these aphids attack usually possess an internal food conducting tissue system similar in structure and function to phloem (Eschrich & Steiner 1967; H6bant 1977; Reinhart & Thomas 1981; Thomas et al. 1988). Unfortunately, the procedures so far employed for collection of aphids specific for these mosses provide little information about feeding behavior or the plant tissues from which nutrition is derived. The only reported observation of aphid-moss feeding is that of Melaphis rhois on gametophores of Haplocladium microphyllum (Moran 1989). Feeding was inferred by the presence of droplets of honeydew. One of the objectives of this study was to determine whether Myzodium modestum, an aphid that infests Polytrichum commune gametophores (Miiller 1973), taps nutrients from the translocate flow of its bryophyte host. Scanning electron microscopy was employed to determine the means by which these aphids feed. Radioactive tracers were employed to follow the movement of nutrients from the translocate stream. Another objective was to determine whether aphids can significantly alter translocation patterns in mosses. Aphid infestation of flowering plants is capable of preventing translocate from reaching host roots and of inducing changes in the partitioning of assimilates in affected host shoots (Hawkins et al. 1987; Wu & Thrower 198 1). These changes are presumably of benefit to aphids, but can also markedly alter growth and physiology of the plants that they infest. Our study was designed in part to determine whether infestation of P. commune by M. modestum aphids alters translocation patterns in ways comparable to those observed in flowering plants. MATERIALS AND METHODS Colonies of Myzodium modestum Hottes on Polytrichum commune Hedw. turfs were observed between July and September 1982-1988, in a beech-maple-white pine forest near Webster, Maine. Quick-frozen, critical-pointdried, and gold-coated specimens were observed under an AMR-1000 scanning electron microscope set at an accelerating voltage of 5 kV. Some specimens were preserved in ethanol and sent to Dr. Manya Stoetzel, Beltsville Ag0007-2745/91/1-4$0.55/0 This content downloaded from 157.55.39.208 on Fri, 14 Oct 2016 04:16:38 UTC All use subject to http://about.jstor.org/terms 2 THE BRYOLOGIST [VOL. 94