Dear Adrian,

Without actually 'seeing' the beetle its hard to determine.
I have looked up a few of my garden books on stock and this is the result.

Jack Frenchs Natural Control of Garden Pests :
' Soldier betteles and their larvae eat codling moth as well as other larvae
and a wide range of sap-suckers. They range in color to dull orange to
bluish black.'
Right color but doesnt sound like your bettle.

It could also be a christmas beetle or the asian bettle.
Christmas bettle or the subfamily Rutelinae inclues colorful christmas
beetles which feed on grass roots as larvae and can defoliate tress as
adults. Dysnastinae species are usually black in color and many males have
prominent horns. The largest group, rhino bettle found in the wet tropics.
The largest subfamily is Melonlonthinae, commonly called chafers. They are
usually brown to black in colour, often feed on grass roots as larvae and
are mostly nocturnal.

Info found in ' A field Guide to Insects in Australia'

As I dont use pesticides on any of my vegetables I cant advise you what to
'spray' it with other than 'glue spray'
This will dry on small pests, suffocating them. It also works on larger
pests like harlequin beetles and christmas beetles (which I suspect you
have) Spray every 5 to 7 days. It is no use in wet weather. Remember to
spray under the leaves and in crevices as well as on top of the leaves.
Mix 1 cup of white flower and 1 cup of boiling water. Add 4-6 cups of cold
water. Strain. Spray thickly at once.

Hope that helps.

"Adrian" ay-at-liquisol-com@ wrote in message
...

A hoard of small black beetles are destorying our crop of plums.
They are about 3-5mm long, and burrow right into the stone of the
fruit.
Any ideas on i) what they are, and ii) what do do?

Thanks!