Aphid infestations, which include greenfly and blackfly, can build up very quickly, so it’s important to treat the problem fast and as soon as it is identified.
If you garden organically, treat with pyrethrum insecticide, or try sprinkling leaves with wood ash or talc then hosing them down the next day. You could also plant ‘decoys’, such as nasturtiums, nearby and plenty of flowers to attract ladybirds – aphids’ natural predators.
The best synthetic treatments include deltamethrin and lambdacyhalothrin – look for them in the ingredients list of pesticides in garden centres. Thiacloprid is even more effective as it works systemically – that is, it spreads from where you spray it through the whole plant.
Always take care when applying chemical treatments and follow the instructions to the letter. Treat the undersides of leaves, where aphids cluster most densely.