We do backpacking in remote areas in northern Michigan where mosquitoes and black flies can make a trip unbearable. I read about taking Vitamin B6 to prevent bites, tried it and found it completely successful. But one time when backpacking with our daughter, I had forgotten the B6 but was taking Vitamin C up to my bowel tolerance level for a specific medical problem. So my blood and cells were full of Vitamin C. I noticed that my daughter's legs were covered with mosquitoes and black flies and in a few minutes, each bite would swell up and become red. I was surrounded by flies and mosquitoes but either I was not bitten (I did not feel anything) or I did not have any reaction. I think that the mosquitoes do not like the high acid content of the blood (I was taking the acidic form of Vitamin C). In addition, when cell and tissue fluid are saturated with Vitamin C, edemas will be removed by the body very quickly. I think this probably is why bites either do not swell or do not last very long if people are taking Vitamin C.
Vitamin C for Insect Bites and Allergies: I had a spider bite once that swelled on my forearm like a half-egg under the skin. I took 3-4G of ascorbic acid, the cheap Vitamin C from Sams club, about once every hour and a half or so. By dark the swelling was down to maybe 10%. I continued overnight and all day the next day. During that time, while in the yard working, I felt something on my arm. I looked down to see a "fireant" biting feverishly the back of my hand. I assumed there must be something wrong with him as I didn't feel any burning. Another one bit me later with the same results. I only noticed a feeling like something was crawling on me. The wounds never swelled, turned red or itched! It must have been the massive doses of C circulating in my blood.
I estimated I consumed about 40 grams of C over a 24hr period. Normally that much C will give you severe diarrhea and gas! I had neither.
I also take it for severe allergy flareups with great and quick relief, though only last for 2-4 hours, depending on the intensity of the allergen.