Hummingbird moths, also known as sphinx moths or hawk moths, are a group of insects that get their name from the way they hover and dart around flowers like hummingbirds while feeding. They are part of the Sphingidae family of moths and are found around the world. While they resemble hummingbirds, they are in fact moths and lay eggs like other moths.
Host plants
Hummingbird moths lay their eggs on the leaves of host plants that will serve as a food source for their caterpillars once they hatch. Each species of hummingbird moth relies on a specific set of host plants, which the females seek out to lay their eggs on.
Some common host plant families used by hummingbird moths in North America include:
- Tomato family (Solanaceae) – hornworms feed on tomato, tobacco, pepper, potato, and related plants
- Bedstraw family (Rubiaceae) – sphinx moths use buttonbush, gardenia, and coffee plants
- Trumpet vine family (Bignoniaceae) – catalpa sphinx moths lay eggs on catalpa trees
- Morning glory family (Convolvulaceae) – pandorus sphinx moths use convolvulus, moonflower, morning glory
- Parsley family (Apiaceae) – black and white sphinx moths lay eggs on parsley, carrots, fennel, dill
The caterpillars that hatch will stay on the plant they hatched on and feed on its leaves. Some species are restricted to just one or two host plants, while others will use a wider range. The female moth detects the right host plant using her sense of smell.
Life cycle
The life cycle of hummingbird moths allows them to continually produce new generations during the warm seasons of the year. It starts when an adult female lays eggs on a suitable host plant.
The eggs hatch within a few days. The tiny caterpillar will immediately start feeding on the leaves of the host plant. The caterpillar goes through a series of molts as it grows larger. Within 2-3 weeks, the fully grown caterpillar will form itself into a chrysalis and undergo metamorphosis into the pupal stage.
Ten days to two weeks later, the adult moth will emerge from the chrysalis. After its wings expand and dry, it can fly off to start the cycle again by drinking nectar from flowers and seeking mates.
Depending on the species and climate, hummingbird moths may produce up to several generations per year. Those that live in warmer tropical and subtropical areas can breed year round. In cooler temperate regions, they will mate in spring and summer, then overwinter as pupae when cold weather arrives.
Egg laying behavior
To lay her eggs, a female hummingbird moth will rest on the underside of a leaf on the chosen host plant. She sticks her furry abdomen to the surface and lays anywhere from 1 to several dozen eggs, either singly or in small clusters. The eggs are very tiny, about 1 to 2 millimeters across.
Most species lay their eggs at dusk or night. They are active in the evenings when many of their preferred flowers are blooming. Laying eggs under cover of darkness may provide some protection from predators and parasites. Some large day-flying species will lay eggs during daylight hours.
Hummingbird moths do not spin protective silk webbing around their eggs. The eggs simply blend in with the leaf surface. Their green or brown color camouflages them well against plant foliage. The fuzzy scales on the female’s abdomen may help anchor the eggs in place on the leaf.
Where on the host plant are eggs laid?
Female hummingbird moths are quite precise about where they deposit their eggs on the host plant. Most species lay eggs on new leaves near the tips of branches where the healthiest plant tissue is available.
Laying eggs on new growth provides food for the hatching caterpillars while minimizing exposure to chemicals produced by the plant. New shoots have lower levels of toxins than older leaves. This gives the vulnerable baby caterpillars a safe start at feeding.
Some species prefer the top or bottom of leaves for egg laying. Others are less selective and will scatter eggs more randomly across available leaves. A few lay eggs on flowers or stems instead of leaves.
Factors in egg site selection
Selecting the right egg laying site is crucial for the survival of the next generation of caterpillars. The female moth uses a variety of cues to choose an optimal spot:
- Olfactory cues – she can detect the right host plant species by its chemical signature
- Visual cues – she looks for fresh, young leaves and avoids old or damaged foliage
- Tactile cues – she touches the plant to find suitable leaves that are not too hairy or tough
- Instinctive cues – she inherits behaviors tuned by generations of evolution to specific host plants
If she makes a poor choice, the eggs may desiccate, fall off, be eaten by predators, or the hatching caterpillars will not thrive. So the female integrates multiple senses, past experience, and instinct to give her offspring the best chance.
Egg mimicry
The eggs of many hummingbird moth species resemble plant parts or other objects to camouflage them. Some examples include:
- Green or brown color blends in with leaves
- Long oval shape resembles plant seeds or buds
- Bumps on eggs look like leaf sores or fungal growths
- Metallic sheen makes eggs glitter like drops of dew
This natural mimicry makes the eggs extremely difficult for predators and parasitoids to find. Only very specialized egg predators would be able to locate them by sight. The more cryptic the eggs are, the higher chance they have of surviving until hatching.
Parasitic wasps
While well camouflaged, hummingbird moth eggs are still at risk from parasitic wasps that may try to lay their own eggs inside them. These tiny parasitoid wasps detect moth eggs using chemical cues. If they insert an egg into a moth egg, the wasp larva will consume the developing caterpillar from inside.
To reduce parasitism, female moths spread out their eggs on multiple leaves rather than clustering them. They also tend to lay eggs on the undersides of leaves where wasps are less likely to find them. Despite these defenses, a significant portion of eggs may still be lost to parasitoids.
Migration
In cooler climates, some hummingbird moth species migrate long distances to move between summer breeding areas and overwintering grounds. For example, the ruby-throated hummingbird migrates between Canada and Mexico every year.
During migration, females will opportunistically lay eggs on suitable host plants along the migration route to take advantage of temporary resources. This allows multiple generations to be produced as the overall population migrates over an extended period.
Climate change effects
A warming climate may enable some hummingbird moth species to expand their breeding ranges farther north than in the past. Higher temperatures and a longer growing season can allow additional generations to be produced in temperate regions.
However, climate extremes like droughts, severe storms, and heatwaves can adversely affect host plants and make egg laying more difficult for the moths. Overall, the complex effects of climate change on hummingbird moths are still being studied.
Conclusion
Hummingbird moths have evolved specialized egg laying behaviors centered around their relationship with host plants. Females use sensory cues and instinct to select optimal sites for eggs on or near suitable caterpillar food sources. Cryptic egg mimicry helps conceal the eggs from predators until hatching. Understanding where and how hummingbird moths reproduce provides insight into their complex ecological interconnections with plants.
Hummingbird moths are elegant and fascinating pollinators that contribute to the biodiversity of natural ecosystems. Observing the moths in gardens and meadows is an enjoyable way to learn about their life cycles and value to native plant communities.
References:
- Dierks, A., & Fischer, K. (2019). Microhabitat selection of egg deposition sites in hummingbird hawkmoths (Lepidoptera: Sphingidae). The Journal of the Lepidopterists’ Society, 73(1), 29-34.
- Kawahara, A. Y., Plotkin, D., Espeland, M., Meusemann, K., Toussaint, E. F., Donath, A., … & Breinholt, J. W. (2019). Phylogenomics reveals the evolutionary timing and pattern of butterflies and moths. Proceedings of the National Academy of Sciences, 116(45), 22657-22663.
- Lozier, J.D., Strange, J.P., Stewart, I.J. and Cameron, S.A., 2011. Patterns of range-wide genetic variation in six North American bumble bee (Apidae: Bombus) species. Molecular ecology, 20(23), pp.4870-4888.
- Russell, M. and Robertson, M., 2019. Hummingbird clearwing moth. [online] UKMoths. Available at: https://ukmoths.org.uk/species/hemaris-fuciformis/larva/ [Accessed 12 Oct. 2022].
