Structure and Growth of Mite-Induced Galls of 
Hoheria sexstylosa Col. 
B. C Arnold 1 
The first account of mite-induced galls of 
Hoheria populnea A . Cunn was published by 
Lamb (1952). Attention was devoted chiefly 
to a description of the gall mite Eriopkyes 
hoheriae, and no details of the anatomical struc- 
ture of the gall were given (Lamb, 1952). Galls 
of Hoheria sexstylosa Col. caused by the same 
mite were reported eight years later (Lamb, 
I960). 
The present study is concerned only with 
Hoheria sexstylosa, an evergreen tree widely 
cultivated in gardens in Christchurch, Canter- 
bury, and often seen to bear galls of varying 
size and age. 
Young galls are greyish-green in colour, 
shaped like a top, and attached to the smaller 
branches by a prominent stalk. Ln older speci- 
mens the gall-stalk is generally obscured as the 
continued growth of the gall encompasses the 
twig which bears it (Fig. 1). Galls arise also 
on leaves but this is of less common occurrence. 
The salient characteristic of all galls of Ho- 
heria sexstylosa is the uneven, creased surface, 
tufted with grey epidermal hairs, below which 
the mites live in numerous small cavities. 
As the result of insect attacks, dead greyish 
brown galls are common, but many galls grow 
vigorously for several years and are readily dis- 
tinguished by their greener hue from moribund 
specimens. This perennial habit of prolifera- 
tion which produces galls up to 4 cm in diame- 
ter is often associated with the loss of leaves 
on the infested branches and a consequent dis- 
figurement of the tree. 
Nevertheless trees vary greatly in their pro- 
pensity to gall formation and it has been ob- 
served that two trees may grow close together 
for several years with branches interlaced, and 
only one of the trees bears evidence of galls. 
1 Department of Botany, University of Canterbury, 
Christchurch, New Zealand. Manuscript received 
April 30, 1964. 
The basis of this variation in susceptibility is 
at the moment unknown. 
The chief aims of the present investigation 
were to find which of the plant organs of Ho- 
heria sexstylosa are transformed into galls, to 
examine the structure of the gall, and to ac- 
count for the perennial mode of growth. 
METHODS AND MATERIALS 
Galls, flower buds, and leaves were fixed at 
various times throughout the year in formo- 
acetic- alcohol Selected specimens from five dif- 
ferent trees were embedded in paraffin, and 
serial sections. 10 /x in thickness were cut and 
stained in safranin and fast green (Johansen, 
1940). 
Numerous freehand sections of living galls 
were also made. 
Three unsuccessful attempts were made to 
induce galls on immature leaves and stem tips 
of seedlings of Hoheria sexstylosa . Twenty- 
four seedlings about 9 inches high were raised 
in pots in the glasshouse and used for these 
experiments. ■■ 
In winter (July) small slices of living galls 
containing moving mites and eggs .were in- 
serted into three tender, freshly opened leaf 
buds in each of 10 plants. 
A second -experiment involved a crude gall 
extract. Thirty grams of fresh galls were macer- 
ated in 200 ml of 30% glycerol (Parr, 1940) 
in a domestic blender at room temperature. A 
fluid containing the mites was obtained by ex- 
pressing the macerated preparation through silk. 
The fluid was smeared on 30 freshly cut soft 
stem tips on each of five seedlings. Thirty stem 
tips of five other plants were smeared with 30% 
glycerol only. 
With the remaining four plants a tap water 
extract of macerated galls containing moving 
mites was smeared on freshly cut stem tips in a 
similar manner except that glycerol was omitted. 
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