Monarch butterflies are iconic insects, recognized for their striking orange and black wings and remarkable annual migration. Their life cycle begins with a tiny egg. Understanding when and where these eggs are laid is central to supporting this species.
Timing of Egg Laying
Monarch butterflies lay eggs primarily during spring and summer across North America. The first generation lays eggs in late March and April in the southern United States and northern Mexico, laid by monarchs returning from overwintering sites. These butterflies are often 7-8 months old, having survived the previous fall’s migration and winter.
Subsequent generations continue northward, laying eggs as milkweed plants emerge. Adults from these summer generations live 2-5 weeks, mating and laying eggs to produce multiple generations. The final generation, emerging in late summer or early fall, is non-reproductive and migrates to overwintering grounds. Egg laying generally occurs during daylight hours.
Where Monarchs Deposit Their Eggs
Female monarchs exclusively lay eggs on milkweed plants (Asclepias). Milkweed is the only food source monarch caterpillars can consume. The plant contains cardenolide toxins, which caterpillars sequester, making them unpalatable to predators.
Monarchs typically deposit a single egg per milkweed plant or leaf. Eggs are most often found on the underside of young, tender leaves near the top of the plant, or on flower buds and stems. This placement helps protect the eggs from predators and environmental elements like direct sunlight.
Appearance and Quantity of Monarch Eggs
Monarch eggs are small, measuring about 1.2 mm high and 0.9 mm wide. They are pale white or creamy yellow when first laid. Their conical or oval shape features longitudinal ridges.
A single female monarch can lay 300 to 500 eggs during her 2-5 weeks of reproductive activity. Some females have been recorded laying over a thousand eggs. Although one egg is usually laid per leaf, multiple eggs can sometimes be found on the same plant.
Factors Influencing Oviposition and Early Hatching
Environmental and biological factors influence egg-laying (oviposition) and hatching. Temperature plays a significant role; warmer conditions accelerate incubation, while cooler temperatures slow it down. Eggs typically hatch 3-8 days after being laid, with four days being common.
Humidity is another factor, as monarch eggs may not hatch in very dry conditions. The health and availability of milkweed plants are also important. Females prefer to lay eggs on smaller, low-density milkweed patches. Before hatching, the egg may darken slightly as the caterpillar inside becomes visible. The caterpillar’s first meal upon hatching is often its own eggshell, which provides essential nutrients.