Fungus gnat adults got stuck on the sticky trap and this allowed the lettuce to flourish after being stunted by the feeding gnats.
I occasionally hear people expound about the virtues of growing plants in greenhouses and inside other structures. “There are no weeds, insects or diseases to deal with” is a universal message from novice greenhouse growers. That could not be further from the truth! The reality is that growing plants indoor amplifies those problems and they are difficult to control, especially insects.
It is almost impossible to limit the introduction of insects to the indoors. Plants that we bring in every winter, carry with them eggs, larvae and adult forms of insects just waiting for the right conditions to flourish. Even special potting soil mixes that are purchased can contain some insects. Many tomato and pepper growers know that cool temperatures and over watering encourages damping off, a stem disease of seedlings. Certain insects help exasperate this problem by spreading the disease and weakening the plants by feeding on the roots.
A pesticide-free method that may help you reduce flying insects associated with indoor plants are sticky insect traps. Many varieties of flying insects found around indoor plants spend some of their time in the soil as dormant insects, eggs and larvae.
Sticky insect traps come in many shapes and sizes. You can hang them or stake them in place. They are very sticky, so it is easiest to handle them with the protective paper on until you have them set.
You may see these as aphids and gnats feeding on the leaves of plants and leaving behind a sticky residue as plant sap goes through their gut. However the real damage might be unseen damage done to root systems of the plants by immature insects or larvae in the soil.
Sticky insect traps that act like flypaper can catch these insects in their adult form. This reduces the number of breeding adults. You can also use sticky insect traps to monitor for insects by simply checking the traps to see if any bugs are stuck to them.
Sticky insect traps are available on-line and in some garden centers. I think this is a solution to your indoor insect problems that you can get stuck on!