Introduction

Anthropogenic disturbance is driving global declines in biodiversity across many taxa, including insects [1–3]. Rapid urbanization is a leading form of anthropogenic disturbance [4, 5], where natural and agricultural lands are rapidly replaced with impervious surfaces and ornamental plants. Urban and residential development dramatically affects many insect taxa, both positively and negatively, because of their highly modified plant assemblages, typical dominance by nonnative and cultivated plant species, higher fertilizer and pesticide inputs, more frequent disturbance, and fragmented habitable areas compared to surrounding natural areas [6, 7]. Although aesthetic enhancement, economic value, and human use functionality are at the forefront of urban landscape design and maintenance decisions, the public is increasingly aware of biodiversity challenges and urban landscapes have become hotspots for wildlife conservation efforts with strong potential added value [8–11].

Recent evidence indicates that urban landscapes can indeed support diverse insect populations if designed and managed appropriately [10, 12–15]. Initiatives such as the National Pollinator Garden Network’s Million Pollinator Garden Challenge™ [16], Monarch Watch’s Monarch Waystation Program [17], and the Pollinator Partnership’s Bee Friendly Garden program [18] all include guidance on creating insect conservation habitat in urban landscapes. However, insect conservation efforts in urban landscapes can be hindered due to high pest pressure [7] and the indiscriminate widespread use of non-native ornamental plants [14, 15]. These challenges may be further complicated when considering conservation of lepidopterans, a popular choice for the public given their charismatic appearance and role as pollinators, which include taxa historically regarded as plant pests as well as species of conservation concern.

Monarch butterfly (Danaus plexippus, L.) populations have declined by over 80% in recent decades primarily due to habitat loss [19–22], leading to the recent listing of monarchs as “Endangered” by the International Union for Conservation of Nature [23]. Monarch larvae are dietary specialists, feeding exclusively on milkweed and close relatives (family Apocynaceae, subfamily Asclepiadoideae). Thus, monarch conservation efforts largely focus on planting milkweed to enhance breeding habitat. Tropical milkweed (Asclepias curassavica, L.) is a species whose use outside of its native range in southern Mexico, Central America, and South America has received widespread criticism. This criticism is due in part to evidence that it can disrupt monarch migration through the U.S. by shifting diapause [24] or encouraging residency outside of their typical overwintering grounds in southern Mexico [25]. In addition, tropical milkweed has elevated cardenolide concentrations compared to other milkweed species [26–29] and recent evidence indicates tropical milkweed may become a less suitable, more toxic host under projected climate warming [26]. Tropical milkweed can also increase pathogen exposure to monarchs by supporting prolonged, high-density residency [30, 31]. Despite these well-documented concerns, tropical milkweed remains by far the most commercially available milkweed species produced in nurseries and planted in urbanized landscapes throughout southern North America due to its aesthetic appeal and ease of propagation. Increased plant availability and consumer engagement with conservation have spurred a multitude of organized efforts and initiatives to create monarch conservation habitats in urban gardens, parks, and schools throughout North America [17, 32–34].

Due to multiple anthropogenic factors, many sap-feeding insects tend to reach high densities on plants in urbanized landscapes [7]. The oleander aphid (Aphis nerii, Fonscolombe, 1841) is no exception [35] and has become a pervasive pest of milkweed and other Apocynaceae taxa in urban gardens throughout North America [36]. High density oleander aphid infestations of milkweed cause leaf chlorosis, leaf senescence, honeydew and sooty mold accumulation, and even plant death, which may have under-recognized negative impacts on monarch conservation efforts. Elevated sap-feeding herbivore densities can also reduce the fitness of herbivores feeding on the same host by reducing host plant nutritive quality or increasing the efficacy of constitutive defenses [37]. Cardenolides are cardiac glycosides that function as secondary defensive chemicals in milkweed and are highest in tropical milkweed compared to its native congeners [38]. Previous work using two native milkweed species demonstrated that oleander aphids and monarchs interact via plant-mediated defensive responses, but that aphid infestations had a positive or neutral effect on monarch larval development, depending on milkweed species [39]. Tropical milkweed infested with oleander aphids exhibits density-dependent cardenolide fluctuations, reducing cardenolides at low aphid densities and increasing them at high densities [40]. Although monarchs sequester cardenolides to defend themselves [41–45], larvae experience significant trade-offs between high cardenolide protection and survival [26, 45, 46]. Given that tropical milkweed contains much higher cardenolide levels than its native congeners [38] and commonly experiences severe oleander aphid infestations in urbanized landscapes, aphids may reduce monarch colonization and fitness by reducing milkweed plant quality or altering plant defenses [40]. The objective of this study was to determine the effects of aphid infestations on monarch oviposition and larval success on tropical milkweed. We hypothesized that oleander aphid infestations would negatively affect both.

Materials and methods

Results

Oviposition study

Despite high variability in the number of eggs deposited per caged monarch pair, adult monarchs on average oviposited over three times as many eggs on aphid-free than aphid-infested milkweed plants (Fig 1, Z = 13.86, P < 0.001). The mean (±SE) number of eggs per aphid-free milkweed was 64 (±15.6), with a range of 6–131 eggs, whereas the mean number of eggs per aphid-infested milkweed was 19 (±8.8), with a range of 0–77 eggs.

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Fig 1. Mean (± SE) eggs deposited per plant by monarch butterflies when presented with an aphid-free and an aphid-infested tropical milkweed plant (least-squares means, Z = 13.86, P < 0.001).

https://doi.org/10.1371/journal.pone.0288407.g001

Larval feeding study

Monarch larvae fed leaves from aphid-free milkweed consumed significantly more leaf material than larvae fed leaves from either aphid-infested treatment (Fig 2, F_2,24 = 8.27, P = 0.001). Larvae that fed on aphid-free leaves ate an average of 94.3 cm² (± 4.44), with a range of 71.7–109.1 cm², nearly twice the leaf material consumed by larvae fed aphid-infested “dirty” (56.1 cm² ± 8.52 with a range of 12.4–93.9 cm²) and aphid-infested “cleaned” (63.8 cm² ± 7.47 with a range of 21.4–80.8 cm²) leaf treatments.

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Fig 2.

Mean (± SE) leaf area consumed over seven days (A) and final larval weight (B) of monarch larvae fed aphid-free milkweed leaves, aphid-infested “dirty” leaves, or aphid-infested cleaned leaves. Different letters over bars indicates a statistical difference between aphid infestation categories (least-squares means; A: F_2,24 = 8.27, P = 0.001, B: F_2,24 = 10.27, P < 0.001).

https://doi.org/10.1371/journal.pone.0288407.g002

As herbivory was strongly positively correlated with weight (DF = 24, R² = 0.79, P < 0.001), monarch larvae fed aphid-free milkweed leaves weighed approximately twice as much (0.56 g ± 0.04, with a range of 0.37–0.74 g) as larvae fed leaves from either aphid-infested treatment (Fig 2B, F_2,24 = 10.27, P < 0.001). Larvae that fed on aphid-infested “dirty” leaves weighed the least, with an average of 0.26 g (± 0.05) and a range of 0.09–0.44 g. Larvae fed aphid-infested “cleaned” leaves weighed an average of 0.34 g (± 0.05), with a range of 0.03–0.47g.

All larvae that fed on aphid-free milkweed survived and reached fifth (final) instar by the end of the experiment. Two larvae (22%) that fed on aphid-infested “dirty” leaves died before the end of the experiment (one third instar and one fourth instar), and three additional larvae (33%) only reached fourth instar by the end of the experiment. No larvae in the aphid-infested “cleaned” leaves treatment died, but two (22%) only reached fourth instar by the end of the experiment.

Discussion

Our results demonstrate that high-density oleander aphid infestations, a common occurrence in ornamental milkweed gardens, have substantial negative effects on monarchs by reducing oviposition, larval weight, and larval leaf consumption by nearly 50%. Aphid infestations cause physical changes (e.g., clustering of aphids on undersides of leaves, buildup of honeydew and sooty mold) in addition to inducing chemical defenses, primarily cardenolides, in milkweed. Many factors influence cardenolide concentrations in milkweed, including temperature [26] and herbivory [39, 55], and plant response to each varies by milkweed species [26, 28, 29]. Importantly, herbivory on highly defended milkweed species like tropical milkweed tends to cause more pronounced effects on induced chemical defenses compared to its lower-cardenolide congeners [39, 42]. This may help explain the difference in our results from previous studies that used other milkweed species. Specifically, Ali and Agrawal [39] observed faster growth in monarch larvae fed aphid-infested A. syriaca and that aphid feeding reduced cardenolide levels in A. syriaca, which likely benefitted monarch larvae. Given that high density aphid infestations induce elevated cardenolide concentrations in the already highly-defended tropical milkweed [40], our findings likely differ because they reflect the consequence of very high cardenolide levels for larval survival and growth [26, 45]. All aphid-infested plants in our experiments had high-density infestations; therefore, these plants presumably experienced aphid-induced cardenolide increases along with sooty mold, honeydew, and aphid carcass accumulation on the leaves. Although we cannot determine whether reduced oviposition, larval feeding, or larval weight was caused by these physical changes to the plant or induced plant defenses, we can infer based on previous research [39, 46] and our findings that both likely played a role.

Oleander aphid infestations can also affect monarchs by reducing host plant quality via decreased nitrogen concentrations [55]. Lepidopterans, including monarchs, typically compensate for decreased nitrogen by increasing herbivory [56], but we observed the opposite behavior. Although our aphid-infested plants had less nitrogen, monarch larvae consumed significantly less aphid-infested leaf material than monarchs fed aphid-free plants. This suggests that induced chemical defenses or physical changes to the plant caused by aphid infestation may have inhibited compensatory feeding. As a result, larvae fed aphid-infested leaves were significantly smaller than larvae fed aphid-free plants and would likely develop into smaller adults, provided they successfully completed metamorphosis. Although we did not explicitly test effects of aphid infestation on monarch larval development rate or survival, we did observe clear differences in both between larvae in the aphid-infested and aphid-free milkweed treatments. For example, due to mortality or delayed development rate, 56% of larvae feeding on aphid-infested “dirty” milkweed did not reach final instar after seven days whereas all larvae feeding on aphid-free milkweed did. Our results likely provide a conservative assessment of plant-mediated effects on monarchs since neonates and earlier instar larvae would likely be more susceptible to plant defenses or reduced plant quality than the third instar larvae we inoculated plants with.

Taken together, the effects of high-density aphid infestations on monarchs may have significant implications for insect conservation efforts in urbanized landscapes. These results suggest that pest-ridden tropical milkweed provides reduced conservation value for monarchs as it recruits fewer monarch eggs, and those eggs will hatch into larvae with worse developmental outcomes. Although the largest swaths of milkweed plantings in North America are composed of species that may have less toxic effects than we observed with tropical milkweed, these occur predominantly in rural or agricultural lands [57]. Participation in pollinator conservation by U.S. residents, over 80% of whom reside in urban areas [58], is booming, with over 41,000 Monarch Waystation gardens registered with Monarch Watch [59] and well over 1,000,000 pollinator gardens registered as part of the National Pollinator Garden Network as of 2022 [60]. In the southern U.S., tropical milkweed is the predominant milkweed species in these spaces. Our results provide further justification for discouraging the use of tropical milkweed in urban and residential gardens due to the ubiquity of oleander aphids in these settings. Our findings also highlight the importance of suppressing aphids on tropical milkweed and, more broadly, suppressing pests on larval hosts of other lepidopteran species. This is especially relevant for highly defended plant taxa and plantings in urban landscapes where public conservation efforts are most prevalent and publicized, but where outbreaks of sap-feeding herbivores and other key arthropod pests are also commonplace and difficult to manage [6].

Three approaches to mitigating the impacts of such pervasive urban plant pests are to use plant species that are less susceptible to key pests, use plants that have less toxic secondary plant defenses, or to employ strategies that suppress aphids without negatively affecting monarchs. When conserving herbivorous insects like Lepidoptera with specific host plants, plant choice may be restricted to a relatively small pool of closely related species [61], and pest susceptibility may be high on all commercially available species or cultivars. For example, of the 35 federally listed endangered or threatened butterflies and moths, 27 have only one or two known larval host plant species [62]. Since many of these larval host plants are uncommon native species or not widely commercially available, their pest susceptibility in urban landscapes may be poorly understood and no viable alternative hosts may be available. The most common approach to aphid suppression is insecticide application, which includes a wide array of products that could have a broad spectrum of negative impacts. As pesticide use, along with habitat loss, is a major contributor to pollinator and biodiversity declines [63, 64], insecticides beyond insecticidal soaps are not currently a sustainable approach to aphid suppression on larval host plants. Although the authors did not measure aphid density or suppression, Nestle et al. [65] found that increasing floral richness in milkweed gardens increased local predatory insect abundance, which could help regulate oleander aphid densities and mitigate plant-mediated effects on monarchs. Leveraging natural biotic or abiotic regulators of aphid populations on milkweed should be investigated. Finally, although we used a non-native, highly defended milkweed species, commercial availability is a driver of consumer planting decisions [14], and tropical milkweed is well established in nursery production and trade. Using an alternative milkweed species (e.g., Asclepias incarnata) may reduce the effects of induced plant defenses [26–29], but oleander aphids also commonly reach high densities on these less toxic milkweed species [36]. Therefore, increasing the production and availability of alternative milkweed species may help mitigate our observed plant-mediated negative effects of oleander aphids, but would not eliminate potential effects of associated pest management inputs targeting aphid outbreaks. Further, the strategies described above do not offset the implications of our findings given the dominant market share currently occupied by tropical milkweed in nursery production and urban gardens.

This study identifies a potential widespread and under recognized impediment to monarch conservation efforts throughout the southern U.S. and sheds light on a need for providing high-quality, pest-free milkweed to support monarch conservation efforts. We also demonstrate a current void in integrated pest management tactics for ornamental plants that are larval hosts for insects of conservation concern. As public interest and effort in biodiversity conservation continues to grow, nursery production of host plants will likely follow. Thus, the relevance of pests and their management on wildlife-friendly ornamentals to biodiversity conservation will also increase [66]. Although it is well supported that some insect herbivores outbreak on urban plants [7], this response can vary by insect life histories and local landscape characteristics [67]. Improving our understanding of how urban landscape characteristics influence multitrophic interactions may help inform future management strategies that suppress pests and conserve beneficial Lepidoptera on host plants in situ without the use of insecticides that may introduce additional risk factors. Using insecticides when managing plants for conservation purposes is generally discouraged, but insecticides are currently some of the few tools that effectively suppress pests and potentially preserve the conservation value of these plants in urban settings. Given that many insecticides used to control key pests are also toxic or of unknown toxicity to insects of conservation concern like non-pest Lepidoptera species [68, 69], future research should address the potential conflicts and synergies between conservation efforts, ornamental plant selection, and pest management to develop best management practices for insect conservation in ornamental plantings. Such work could have widespread implications for the production, selection, and maintenance of conservation plantings in urbanized landscapes.

Supporting information

Acknowledgments

We sincerely thank Marc Godts with Green Isle Gardens Nursery in Groveland, FL for providing milkweed plant material used in this study. We also thank three anonymous reviewers and the editor for their constructive criticism and feedback, which improved the quality of this manuscript.