Apple rust
Dr Andrew G. S. Cuthbertson,* Dr Alan C. Bell and Dr Archie K. Murchie
Applied Plant Science Division, DARD, Newforge Lane, Belfast BT9 5PX, U.K.
*Present address: Dr Andrew G.S. Cuthbertson, Central Science Laboratory, Sand Hutton, York YO41 1LZ, U.K. Tel.: +44 (0) 1904 462201; Fax: +44 (0) 1904 462111; E-mail: a.cuthbertson@csl.gov.uk
The apple rust mite (Aculus schlechtendali) is an economically
important pest of Bramley apples. Severe outbreaks of this pest can cause
bronzing of leaves and often premature leaf fall. Apple rust mite populations in
Ulster orchards often occur in numbers well above recommended spray thresholds
(Cross and Berrie, 1994). However, surveys by DARD indicate that pest damage is
normally inconsequential. It has always been thought that the natural enemy
fauna in the orchards may be capable of limiting rust mite populations during
times when damage is likely. One such natural enemy is the commonly-occurring
predatory mite Anystis baccarum. Research has shown this mite to feed
readily upon the apple-grass aphid (Rhopalosiphum insertum) in local
orchards (Cuthbertson et al., 2003a). The aim of this project was to
evaluate the impact of A. baccarum on apple rust mite populations.
A. baccarum is very mobile (and easily observed) on the branches of the
tree, whereas apple rust mite is sedentary on the leaves. Sticky tape wrapped
around branches, in conjunction with two applications of an acaracide, was used
to remove the predator from certain branches of apple trees. These traps did not
effect the rust mite. Other branches on the same trees were not treated and were
also trap free. Therefore, on the same trees A. baccarum was removed from
some branches, while on others it had open access.
Highest rust mite numbers occurred in early August (Figure 1). Numbers
exceeded recommended spray thresholds. The branches from which the predatory
mite was removed had higher populations of rust mite than the branches on which
it was present (Figure 1). On the date of highest rust mite occurrence (3rd
August) the mean count per one 4 leaf sample was just over 650 rust mites on
branches from which A. baccarum was removed, whereas on branches where
the predator had open access the mean was around 300.
Figure 1. Mean numbers of apple rust mite per leaf.
 Branches where A. baccarum
was present ,  branches where
A. baccarum was excluded. |
This exclusion experiment showed that within the orchard, apple rust mite
numbers exceeded the recommended spray thresholds. However, no visible damage to
foliage was recorded. The research has also shown that exclusion of A.
baccarum from certain branches of the trees allowed the rust mite numbers to
increase greatly (Cuthbertson et al., 2003b). Therefore, it is important
that this predatory mite is recognised as a beneficial species by growers and
steps taken to conserve their populations. Further research has shown A.
baccarum to be compatible with certain fungicides commonly used within the
orchards (Cuthbertson and Murchie, 2003) and therefore offers great potential
for incorporation into integrated pest management programmes within the orchards
for the control of not only apple rust mite, but other invertebrate pest
species.
References / further reading
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. and Murchie, A.K. (2003). The impact of fungicides to
control apple scab (Venturia inaequalis) on the predatory mite Anystis
baccarum and its prey Aculus schlechtendali (apple rust mite) in
Northern Ireland Bramley orchards. Crop Protection 22: 1125-1130.
Cuthbertson, A.G.S., Fleming, C.C. and Murchie, A.K. (2003a). 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
Cuthbertson, A.G.S., Bell, A.C. and Murchie, A.K. (2003b). 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.