Detecting apple-grass aphid

Of eight species of aphid that feed on apple in the U.K., five are pests capable of causing economic losses. From these five, Rhopalosiphum insertum (Walker) (apple-grass aphid) is an economically important pest of Bramley apple orchards in Northern Ireland (Mowat and Clawson, 1996). Numbers regularly exceed recommended spray thresholds of more than 30 trusses infested per 25 trees during green cluster to pink bud stage (Cross and Berrie, 1994). This aphid is most numerous in the orchards during April and May, and most damage is caused in this period.

It has always been envisaged that natural enemies of aphids within local Bramley orchards could play a valuable role in limiting R. insertum numbers. One such natural enemy is Anystis baccarum (Linnaeus), a large (1.0-1.5 mm in diameter) and commonly occurring predatory mite. Anystis baccarum readily consumes R. insertum in the laboratory (Figure 1). In an exclusion experiment A. baccarum also significantly reduced Aculus schlechtendali (apple rust mite) numbers in some Bramley orchards (Cuthbertson et al., 2003a).

The presence of R. insertum in the orchards may be a valuable food source for A. baccarum, both during the winter, when eggs would be preyed upon, and in early spring as juveniles hatch. Rhopalosiphum insertum is present before other prey species such as Panonychus ulmi (fruit tree red spider mite) or A. schlechtendali have hatched or migrated out from over-wintering sites. The ready food supply of R. insertum in early spring may help to increase A. baccarum populations. Therefore, the feeding habits of A. baccarum in the Bramley orchards may make it a candidate for incorporation into integrated pest management programmes. The aim of this work was to develop a sensitive polymerase chain reaction (PCR) based technique for detecting apple-grass aphid predation by A. baccarum.

Specimens of both R. insertum and A. baccarum were collected from the orchards. Conserved PCR primers for insect mitochondrial DNA (mDNA) were used to amplify a region between the ND1 and large subunit RNA genes. DNA sequencing of the R. insertum ND1-LRNA PCR product allowed R. insertum-specific PCR primers to be designed which amplified a 283bp product from individual aphids (Cuthbertson et al., 2003b). No PCR product was amplified from individual A. baccarum. However, when tested against A. baccarum fed on an individual R. insertum, the diagnostic 283bp product was amplified (Plate 1).

The results show that it is possible to identify the remains of apple-grass aphid in the gut of the predatory mite Anystis baccarum. This same technique can now be used to identify other potential predators of apple-grass aphid. The importance of this study is that predators of the pest can be identified, and subsequent research can then focus on enhancing their numbers (e.g. by micro-habitat manipulation) as part of an overall integrated pest management strategy.

Cross, J.V. and Berrie, A.M. (1994). Sampling and assessment of pests and diseases as the basis for decision making in orchards in the UK. Aspects of Applied Biology 37: 225-236.

Cuthbertson, A.G.S., Bell, A.C. and Murchie, A.K. (2003a). The impact of the predatory mite Anystis baccarum on apple rust mite (Aculus schlechtendali) populations in Northern Ireland Bramley orchards. Annals of Applied Biology 142: 107-114.

Cuthbertson, A.G.S., Fleming, C.C. and Murchie, A.K. (2003b). Detection of Rhopalosiphum insertum (apple-grass aphid) predation by the predatory mite Anystis baccarum using molecular gut analysis. Agricultural and Forest Entomology 5: 219-225.

Mowat, D.J. and Clawson, S. (1996). The need for pest control in Northern Ireland Bramley apple orchards. Crop Protection in Northern Britain 2: 225-230.