The pesticide probably doesn't look very different in the visible spectrum at that low of a concentration - otherwise its presence would be pretty easily detect

The pesticide probably doesn't look very different in the visible spectrum at that low of a concentration - otherwise its presence would be pretty easily detectable to the human eye. If you require detection capabilities exceeding the human eye you're going to need a much more sensitive setup. Most chemicals have characteristic spectra in the infrared, so you'd need sensors going to much larger wavelengths to have significant difference - even then it would be hard to detect against the variation in signal from fruit.