In central Finland there is a group of moths whose common name is bagworm moths because the larvae, or caterpillars, live in bags that they make from leaves and twigs. What’s even more noteworthy about these moths is that for many species males are unnecessary.

Bagworms belong to the family Psychidae which includes species living side-by-side in Finland that reproduce either sexually or asexually. This make these moths ideal for studying the ecological and evolutionary benefits of parthenogenesis—when a female’s eggs develop without being fertilized by a male, or asexually.

A brown moth larva next to a case made of forest debris
A wingless female bagworm moth (Dahlica fennicella) and its homemade case. Right, pupal remains. Image: Veronica Chevasco, © Veronica Chevasco

Sexual reproduction increases genetic variation and is thus central in evolution. However, what the parthenogenic bagworms are revealing is that asexuality may not be as limiting to a species’ survival as some have thought.

In fact, my research with bagworms shows that asexuality can be an adaptive advantage rather than an evolutionary dead end.

For example, by being parthenogenetic it appears that the bagworm species Dahlica fennicella is less vulnerable to wasp parasitism than related sexually reproducing moths.

It could be that behavioural differences between sexual and parthenogenetic species help D. fennicella avoid parasite attacks. It’s also possible that D. fennicella is successful because its offspring are not genetically identical clones, and with four sets of chromosomes, the multiple gene copies might overcome harmful mutations and even some parasitism.

A small brown moth seen from above.
Male bagworm moth (Dahlica fennicella). Image: Veronica Chevasco, © Veronica Chevasco.

Sexual and parthenogenetically reproducing moths look remarkably alike and so species must be identified using molecular methods, such as DNA barcoding. This molecular identification of sexual species shows that the parthenogenetic D. fennicella moth is a truly an asexual species, not a hybrid produced through the cross-breeding of two species of sexually reproducing bagmoths.

Four side-by-side images. A brown larva next to a brown case. An adult female moth. Two moth larvae. A wasp with long antennae and a long ovipositor.
(A) Bagworm larvae and their cases made from leaves and twigs. (B) A wingless, sessile female rests on her pupal case waiting for a mate. (C) A sessile, parthenogenetic bagworm moth. (D) A wasp that parasitizes bagworm moths. Image: Veronica Chevasco, © Veronica Chevasco

These bagworm moth findings don’t minimize the importance of sexual reproduction; parthenogenetic species generally evolve from sexual species and inherit diversity from their ancestral forms.

But the results do show that, among bagworm moths, parthenogenetic species appear to avoid the costs of sex while getting benefits from an asexual life.