Q: I planted milkweed last year in order to attract monarchs to my yard. This year I had a bunch of caterpillars, then I found milkweed assassin bugs. I have not seen any caterpillars since. My question is, are the assassin bugs harmful to the monarchs? I read they are beneficial, but they do kill caterpillars. Nothing I can find says they are bad for monarchs, but I’m thinking they are. Please let me know what I should do.
A: There is a common bug found on milkweed that resembles the assassin bug and feeds only on the plants. The species you are seeing is the large milkweed bug (Oncopeltus fasciatus) as opposed to the small milkweed bug that may also be found. I am seeing them on my milkweed now and the caterpillars that were on my plants just finished pupating. I saw an adult butterfly emerging this weekend. So I would bet the life cycles of these insects are giving you the impression there is cause and effect when there is none.
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Peter L. Warren is the urban horticulture agent for the Pima County Cooperative Extension and the University of Arizona. Questions may be emailed to tucsongardensage@gmail.com
