Certified GKH Gardening Expert
Neem oil is effective against these pests as well. Here is more information: https://www.gardeningknowhow.com/plant-problems/pests/pesticides/neem-oil-uses.htm
Webbing usually indicates spider mites. If you hold a piece of white paper under the webbed leaf and tap or shake the leaf, you should see tiny (the size of a period at the end of a sentence) reddish bugs fall off. You don't say if your begonia is inside or out. Inside, safest thing is to wash off the webbing and mites with a soap solution - 1 tsp mild dish liquid in spray bottle of water. Spray especially the undersides of the leaves, until the plant is dripping. Repeat treatment weekly for 1 month. Outside, use a hose to wash off as many bugs as possible,every day if you can; or you can use the soap solution as above.