Finally, there's the problem of the toxic milkweed cardenolides. Contrary to the story often told about monarchs and milkweed, the evidence suggests that monarch caterpillars can and do suffer the effects of consuming cardiac glycosides. Different species of milkweeds, or even different individual plants within a species, can vary significantly in their cardenolide levels. Caterpillars feeding on milkweeds with high levels of cardenolides have lower survival rates. Studies have shown that female butterflies generally* prefer to oviposit their eggs on milkweed plants with lower (intermediate) cardenolide levels. If the ingestion of cardiac glycosides was wholly beneficial to their offspring, you would expect females to seek host plants with the highest toxicity