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Abstract Monarch butterflies (Danaus plexippus) have suffered dramatic declines and, as of 2025, are being considered for listing as a threatened species under the U.S. Endangered Species Act. Monarch declines have been explained, at least in part, by the loss of milkweeds (Asclepiasspp.), their larval host plant. Increasing milkweeds in urban gardens may aid in monarch conservation efforts but concerns have been raised regarding negative interactions between monarchs and aphids co-occurring on milkweeds in these gardens. We made 934 observations on approximately 150Asclepias syriaca(common milkweed) stems in garden beds in a highly urbanized landscape in Chicago, IL, USA over two summers. Using generalized linear mixed-effects models, we found that the number of monarch eggs and larvae was either not correlated or positively correlated with aphid abundance, indicating that aphid infestation did not deter oviposition by monarch butterflies on milkweed stems. Implications for Conservation: Planting milkweeds in urban gardens may aid in monarch population recovery, but aphid milkweed specialists often occur at high densities in urban gardens. We did not find that stems with high aphid abundance had fewer monarch eggs and larvae, suggesting that aphids may not reduce the value of native urban milkweeds for monarch oviposition as previously suggested.
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Clonal versus non-clonal milkweeds ( Asclepias spp.) respond differently to stem damage, affecting oviposition by monarch butterflies
Background Oviposition decisions are critical to the fitness of herbivorous insects and are often impacted by the availability and condition of host plants. Monarch butterflies ( Danaus plexippus ) rely on milkweeds ( Asclepias spp.) for egg-laying and as food for larvae. Previous work has shown that monarchs prefer to oviposit on recently regrown plant tissues (after removal of above-ground biomass) while larvae grow poorly on plants previously damaged by insects. We hypothesized that these effects may depend on the life-history strategy of plants, as clonal and non-clonal milkweed species differ in resource allocation and defense strategies. Methodology/Principal Findings We first confirmed butterfly preference for regrown tissue in a field survey of paired mowed and unmowed plots of the common milkweed A. syriaca . We then experimentally studied the effects of plant damage (comparing undamaged controls to plants clipped and regrown, or damaged by insects) on oviposition choice, larval performance, and leaf quality of two closely related clonal and non-clonal species pairs: (1) A. syriaca and A. tuberosa , and (2) A. verticillata and A. incarnata . Clonal and non-clonal species displayed different responses to plant damage, impacting the proportions of eggs laid on plants. Clonal species had similar mean proportions of eggs on regrown and control plants (≈35–40% each), but fewer on insect-damaged plants (≈20%). Meanwhile non-clonal species had similar oviposition on insect-damaged and control plants (20–30% each) but more eggs on regrown plants (40–60%). Trait analyses showed reduced defenses in regrown plants and we found some evidence, although variable, for negative effects of insect damage on subsequent larval performance. Conclusions/Significance Overall, non-clonal species are more susceptible and preferred by monarch butterflies following clipping, while clonal species show tolerance to clipping and induced defense to insect herbivory. These results have implications for monarch conservation strategies that involve milkweed habitat management by mowing. More generally, plant life-history may mediate growth and defense strategies, explaining species-level variation in responses to different types of damage.
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- Award ID(s):
- 1907491
- PAR ID:
- 10216797
- Date Published:
- Journal Name:
- PeerJ
- Volume:
- 8
- ISSN:
- 2167-8359
- Page Range / eLocation ID:
- e10296
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.7717/peerj.10296
