I have one Black Hamburg grapevine in a glasshouse. It is 30 years old. Grape crops were excellent up to 6 years ago when it was attacked by mealybugs brought in on an ornamental plant. How can I get rid of this pest chemically? I have tried control by cleaning off old bark, squashing egg-nests, etc., but the pest lives on despite these efforts.
Organically, you might try spraying thoroughly with vegetable oil 50 ml dispersed by 10 drops of washing up liquid in 1 litre of water. The oil smothers the insect, but you need to be very thorough.
Or use a garden insectide such as Plant Rescue, also very thoroughly.
Repeat every 2 weeks until all mealy bugs are gone.