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Have you measured the screen hole size to see if some insects will get through? I've seen many ants on my citrus trees, but they are mostly there to tend to the aphids or the scale. I've never seen a photo of an ant covered with pollen.
Fruit Grower you don't say what type of citrus you have under your screen. However, many, even most citrus varieties are self pollinating and do not require help from bees & etc.