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Butterflies sometimes need a bit of prompting to lay eggs, especially on cold, dark, and/or rainy days. The secret is to make them think it is a bright warm sunny day. Because thier instinct is to fly to light, their host plant should be placed where they will have to constantly touch it as they go to the light source. Using a timer set to turn off or on every 10 minutes to every hour will constantly be stimulated to move again and will touch the plant again - laying more eggs.