Comments on Lawson et al., 2021 “The puzzle of monarch butterflies (Danaus plexippus) and their association with plants containing pyrrolizidine alkaloids”

Nancy Lawson and collaborators (2021) posed intriguing questions on the interactions of Danaus plexippus (Linnaeus), the monarch butterfly, with plants containing pyrrolizidine alkaloids (PAs). The authors also pointed out the importance of developing research using, not only monarchs but also their relatives from the USA and Latin America. In agreement with these authors, since 2007 a team of Brazilian researchers including the authors of this comment has been conducting studies on the Neotropical Danains Danaus erippus (Cramer), the southern monarch butterfly, and the queen butterfly Danaus gilippus (Cramer). These studies include examining behaviours like approaching and scratching leaves from PA plants as well as using flowers from PA versus non-PA plants. Preliminary observations took place in private as well as protected areas from southeastern Brazil located at the Cerrado and the Atlantic Forest Biomes. From these initial observations, studies that combined field surveys, screening field experiments, and insectary experiments were then carried out (e.g., Ramos et al., 2019, 2020). These studies were mentioned in Lawson et al. (2021), so these comments aimed to contribute to this debate. In the text section ‘scientific natural history is needed for insight’, Lawson et al. (2021) pointed out the need to perform simple screening experiments using self-made butterfly baits. The authors also mentioned the importance of recording monarch visits in the context of milkweed gardens and wildlife habitats, and that other species in addition to the monarch butterfly should be included in these studies. Both butterfly visit surveys and simple screening experiments were carried out in Ramos et al. (2019) and (2020), with these studies focused on southern monarchs and queens, and so it was surprising that no mention of these surveys and experiments was made by Lawson et al. (2021). The study sites at which the survey and experiments mentioned above were carried out consisted of wildlife habitats typical of the very speciose Atlantic Forest. In southeastern Brazil, no sufficient population has been found with which to perform southern monarch or queen surveys at urban gardens, and no milkweed gardens are known to this region of South America as suggested by Lawson et al. However, the field sites at which Ramos et al. (2019) and (2020) were conducted consisted of open pasture areas and associated trails that, together with forest patches that surround them, form a mosaic landscape typical of wildlife areas from the Atlantic Forest Biome (e.g., Ferreira & Rodrigues, 2021; see Smith, [2014] for examples of pastured areas around the world at which non-grazed PA and cardenolide plants are used by Danaus butterflies). In addition, in Ramos et al. (2020) self-made butterfly traps were offered to southern monarchs both under insectary as well as field conditions to examine leaf scratching. The PA plant used for making the baits was Crotalaria spectabilis, whose PA concentration is high compared to other species of Crotalaria (Martins et al., 2015). In agreement with our field data, no butterfly visits were recorded at these baits (Ramos et al., 2019). Migration is probably a factor determining the different abundance patterns seen in southern monarchs from southeastern Brazil compared to the temperate monarchs. There is no evidence that southern monarchs possess a migratory habitat as well as migratory routes in Received: 30 December 2021 Accepted: 25 January 2022