Got a houseplant query? Our expert is here to answer them. This week, how to deal with the dreaded mealybugs

What’s the problem?
My indoor lemon tree has sticky leaves and white spots underneath. What is it?

Diagnosis
The sticky residue and white growths suggest an infestation of mealybugs. Mealybugs are notorious for attacking the fleshy leaves of cacti and succulents, but they also thrive on citrus trees, feeding on sap and weakening the plant. Their waxy coating makes them resistant to standard insecticides, and so difficult to treat. They also excrete honeydew, a sticky substance that attracts fungal growth and can worsen the problem.