Friends have a lemon tree and an orange tree in their greenhouse. They are in blossom now and infested with bugs, probably aphids. Last year they had the same problem, sprayed with Safers End-All ii; they had no fruit.
What should they do to get rid of the bugs and to have fruit (which they have had in the past).
Your friends must enjoy their greenhouse a lot in the midst of a cold winter like this. And although greenhouses can be tricky to maintain correctly, they clearly have had much success in the past. Because of that we will assume that the bug problem is the reason for the trees not setting fruit; but in fact there are many other reasons why the plant might skip a year, e.g. inadequate fertilizer, heat, light and so forth.
Firstly they should look closely to confirm that the bugs are in fact woolly aphids, since mealybugs or spider mites are also common and might respond to different treatments. Safers End All II is an appropriate spray for aphids but it would be possible to burn the plant buds if it is too concentrated, so read the bottle carefully. Spray on a cloudy day to prevent the sun from burning the wet leaves and buds. Most importantly, monitor the trees closely and get ahead of problems as much as possible. By the time there is a full-blown infestation, it’s hard to catch up and the tree is already stressed (aphids are sucking out the sap, so the plant decides to cut its losses and not spend energy on making fruit). Strong sprays of plain water will knock off a few bugs. Hanging pest strips in the area will reduce the numbers.
Here are a few other Master Gardener answers in the online library which contain more resources for your friends: