Invasive Brown Treesnake Research Articles

Inferring the absence of an incipient population during a rapid response for an invasive species.

Successful eradication of invasives is facilitated by early detection and prompt onset of control. However, realizing or verifying that a colonization has occurred is difficult for cryptic species especially at low population densities. Responding to the capture or unconfirmed sighting of a cryptic invasive species, and the associated effort to determine if it indicates an incipient (small, localized) population or merely a lone colonizer, is costly and cannot continue indefinitely. However, insufficient detection effort risks erroneously concluding the species is not present, allowing the population to increase in size and expand its range. Evidence for an incipient population requires detection of ≥1 individual; its absence, on the other hand, must be inferred probabilistically. We use an actual rapid response incident and species-specific detection estimates tied to a known density to calculate the amount of effort (with non-sequential detections) necessary to assert, with a pre-defined confidence, that invasive brown treesnakes are absent from the search area under a wide range of hypothetical population densities. We illustrate that the amount of effort necessary to declare that a species is absent is substantial and increases with decreased individual detection probability, decreased density, and increased level of desired confidence about its absence. Such survey investment would be justified where the cost savings due to early detection are large. Our Poisson-based model application will allow managers to make informed decisions about how long to continue detection efforts, should no additional detections occur, and suggests that effort to do so is significantly higher than previously thought. While our model application informs how long to search to infer absence of an incipient population of brown treesnakes, the approach is sufficiently general to apply to other invasive species if density-dependent detection estimates are known or reliable surrogate estimates are available.

Behavioral differences following ingestion of large meals and consequences for management of a harmful invasive snake: A field experiment.

Many snakes are uniquely adapted to ingest large prey at infrequent intervals. Digestion of large prey is metabolically and aerobically costly, and large prey boluses can impair snake locomotion, increasing vulnerability to predation. Cessation of foraging and use of refugia with microclimates facilitating digestion are expected to be strategies employed by free‐ranging snakes to cope with the demands of digestion while minimizing risk of predation. However, empirical observations of such submergent behavior from field experiments are limited. The brown treesnake (Serpentes: Colubridae: Boiga irregularis) is a nocturnal, arboreal, colubrid snake that was accidentally introduced to the island of Guam, with ecologically and economically costly consequences. Because tools for brown treesnake damage prevention generally rely on snakes being visible or responding to lures or baits while foraging, cessation of foraging activities after feeding would complicate management. We sought to characterize differences in brown treesnake activity, movement, habitat use, and detectability following feeding of large meals (rodents 33% of the snake's unfed body mass) via radio telemetry, trapping, and visual surveys. Compared to unfed snakes, snakes in the feeding treatment group showed drastic decreases in hourly and nightly activity rates, differences in refuge height and microhabitat type, and a marked decrease in detectability by trapping and visual surveys. Depression of activity lasted approximately 5–7 days, a period that corresponds to previous studies of brown treesnake digestion and cycles of detectability. Our results indicate that management strategies for invasive brown treesnakes need to account for cycles of unavailability and underscore the importance of preventing spread of brown treesnakes to new environments where large prey are abundant and periods of cryptic behavior are likely to be frequent. Characterization of postfeeding behavior changes provides a richer understanding of snake ecology and foraging models for species that consume large prey.

Evaluating airsoft electric guns for control of invasive brown treesnakes

ABSTRACTFirearms are often used in lethal control of invasive vertebrates, but safety and regulatory aspects limit the circumstances under which they can be used. During August 2016 at the Brown Treesnake Project laboratory on Guam, we evaluated hobby‐grade Airsoft Electric Guns (AEGs)—a lower powered, less‐hazardous, and less‐regulated alternative to firearms—for capture and control of small animals, with specific emphasis on invasive brown treesnakes (Boiga irregularis). Tests of AEGs differing in power with ammunition (plastic pellets) masses ranging from 0.20 to 0.39 g, fired at gelatin blocks from distances of 4, 8, and 12 m, showed that heavy ammunition is of overriding importance for maximizing lethality: 0.39‐g pellets penetrated more deeply at 12 m than did 0.20‐g pellets at 4 m. Inspection of tissue damage in brown treesnake carcasses subjected to fire with the 0.39‐g ammunition from the same distances suggested that injuries sustained by a direct hit from 12 m would often be lethal, and snakes would be unlikely to survive multiple hits from automatic fire discharged at approximately 17/s. Limited trials with live snakes helped us to understand behavioral responses in a snake hit by ≥1 pellets, including distance traveled over time. Based on these factors, we assessed the risk that a snake injured by pellet fire might evade subsequent capture by rapid responders in the proximity. We also discuss ethical considerations and regulatory advantages of using AEGs. Published 2018. This article is a U.S. Government work and is in the public domain in the USA.

Habitat type and structure affect trap capture success of an invasive snake across variable densities

AbstractDetection represents an important limitation of accurately estimating population size, abundance, and habitat suitability for wildlife, which can be especially true for cryptic animals. Moreover, for reptiles, juveniles are often less likely to be detected than later life stages. In the case of invasive species, preventing false negatives early in the invasion process can be critical for improving outcomes of control measures. We evaluated habitat structure in relation to catch per unit effort (CPUE) and mean size of trapped invasive brown treesnakes (Boiga irregularis) on Guam. We used a 5‐ha enclosure containing a known, closed population of brown treesnakes to identify key habitat variables that related to CPUE and mean size of trapped snakes over six years. We then tested the relationship of those variables to CPUE and mean size of trapped snakes at three sites with suppressed snake populations as a proxy for low‐density populations anticipated to occur during early detection of invasive populations. We found that a coarse measure of habitat structure represented by three forest types correlated with trap detections, as well as finer measures of habitat structure, such as distance to nearest branch and the type of trap support structure used. On average, smaller snakes were captured in traps placed higher in the tree canopy. Some, but not all, habitat variables identified as predictive of CPUE and mean size within the enclosed population pre‐suppression were also predictive at the snake‐suppressed (low‐density proxy) sites. Habitat structure around the sampling unit (a trap) affected detection probability and the size of detected individuals independently of the demographic structure of the population. Measuring wildlife‐habitat relationships of invaders in their novel environments may be one method to improve early detection during invasive species management.

Quantile regression of microgeographic variation in population characteristics of an invasive vertebrate predator.

Localized ecological conditions have the potential to induce variation in population characteristics such as size distributions and body conditions. The ability to generalize the influence of ecological characteristics on such population traits may be particularly meaningful when those traits influence prospects for successful management interventions. To characterize variability in invasive Brown Treesnake population attributes within and among habitat types, we conducted systematic and seasonally-balanced surveys, collecting 100 snakes from each of 18 sites: three replicates within each of six major habitat types comprising 95% of Guam’s geographic expanse. Our study constitutes one of the most comprehensive and controlled samplings of any published snake study. Quantile regression on snake size and body condition indicated significant ecological heterogeneity, with a general trend of relative consistency of size classes and body conditions within and among scrub and Leucaena forest habitat types and more heterogeneity among ravine forest, savanna, and urban residential sites. Larger and more robust snakes were found within some savanna and urban habitat replicates, likely due to relative availability of larger prey. Compared to more homogeneous samples in the wet season, variability in size distributions and body conditions was greater during the dry season. Although there is evidence of habitat influencing Brown Treesnake populations at localized scales (e.g., the higher prevalence of larger snakes—particularly males—in savanna and urban sites), the level of variability among sites within habitat types indicates little ability to make meaningful predictions about these traits at unsampled locations. Seasonal variability within sites and habitats indicates that localized population characterization should include sampling in both wet and dry seasons. Extreme values at single replicates occasionally influenced overall habitat patterns, while pooling replicates masked variability among sites. A full understanding of population characteristics should include an assessment of variability both at the site and habitat level.

Effects of an invasive predator cascade to plants via mutualism disruption

Invasive vertebrate predators are directly responsible for the extinction or decline of many vertebrate species, but their indirect impacts often go unmeasured, potentially leading to an underestimation of their full impact. When invasives extirpate functionally important mutualists, dependent species are likely to be affected as well. Here, we show that the invasive brown treesnake, directly responsible for the extirpation of forest birds from the island of Guam, is also indirectly responsible for a severe decline in plant recruitment as a result of disrupting the fruit-frugivore mutualism. To assess the impact of frugivore loss on plants, we compare seed dispersal and recruitment of two fleshy-fruited tree species on Guam and three nearby islands with intact disperser communities. We conservatively estimate that the loss of frugivorous birds caused by the brown treesnake may have caused a 61–92% decline in seedling recruitment. This case study highlights the potential for predator invasions to cause indirect, pervasive and easily overlooked interaction cascades.

Efficacy of aerial broadcast baiting in reducing brown treesnake numbers

The brown treesnake (Boiga irregularis) is an invasive predator that was introduced on Guam as a stowaway in cargo after World War II. Since then, the population has exploded, attaining 50 to 100 snakes per ha in some areas. The snake has caused the extirpation of ten of the 12 native forest bird species on Guam. The U.S. Department of Agriculture, Wildlife Services, has a program to deter the spread of snakes from Guam to other islands. Hand capture from fences, trapping, toxic bait stations, and canine inspection of outbound cargo methods are used in the control program in various localized and accessible areas. We investigated aerial delivery of toxic baits as a potential method for a broader landscape control of brown treesnakes. Treated baits were deployed on 6-ha of forest at 37.5 baits per ha. Snake activity was reduced by 80 to 85% by the third application of toxicant. Nontarget bait-take was limited. Of 80 telemetered baits aerially deployed, 30 (38%) baits were taken by snakes, one was taken by a toad (Bufo marinus), and one was taken by a monitor lizard (Varanus indicus). Mortality was observed in all 30 cases of bait-take by the snakes. No evidence of ill effects was observed in the toad or the monitor lizard after bait ingestion. Aerial delivery of toxic baits holds promise as an economical, targeted method to control invasive brown treesnakes over large areas of land.

Experimental landscape reduction of wild rodents increases movements in the invasive brown treesnake (Boiga irregularis)

Experimental landscape reduction of wild rodents increases movements in the invasive brown treesnake (Boiga irregularis)

Invaded Invaders: Infection of Invasive Brown Treesnakes on Guam by an Exotic Larval Cestode with a Life Cycle Comprised of Non-Native Hosts.

BackgroundMultiple host introductions to the same non-native environment have the potential to complete life cycles of parasites incidentally transported with them. Our goal was to identify a recently detected parasitic flatworm in the invasive Brown Treesnake (Boiga irregularis) on the remote Pacific island of Guam. We considered possible factors influencing parasite transmission, and tested for correlations between infection status and potential indicators of host fitness. We used genetic data from the parasite and information about the native ranges of other possible non-native hosts to hypothesize how it arrived on Guam and how its life cycle may be currently supported.MethodsWe identified the parasite by comparing larval morphology and mtDNA sequences with other Pseudophyllid tapeworms. We assessed probability of infection in individual snakes using logistic regression and examined different factors influencing presence of parasites in hosts.ResultsWe identified the parasite as the pseudophyllid cestode Spirometra erinaceieuropaei, with all sampled worms from multiple snakes sharing a single mtDNA haplotype. Infection appears to be limited to the only freshwater watershed on the island, where infection prevalence was high (77.5%). Larger snakes had a higher probability of being infected, consistent with the chronic nature of such infections. While infection status was positively correlated with body condition, infected snakes tended to have lower intra-peritoneal fat body mass, potentially indicating a negative effect on energy stores.ConclusionsWe discovered that B. irregularis inhabiting a small area of forested habitat in a freshwater watershed on Guam are often infected by a novel parasite of Asian origin. While further work is needed, this species of Spirometra, itself a non-native species, likely depends on a suite of recently introduced hosts from different parts of the world to complete the life cycle. This baseline study provides little evidence of any effects on host fitness, but additional data are needed to more thoroughly explore the consequences of infection in this invasive snake population.

Invasive Brown Treesnake Movements at Road Edges Indicate Road-Crossing Avoidance

Abstract Roads have significant impacts on the dispersal of wildlife. Although this poses a threat to the abundance and diversity of desirable flora and fauna, it also affords some opportunity for enhancing control of invasive species. Roads are the most common terrain features that may affect the rate of landscape-scale movements of invasive Brown Treesnakes (Boiga irregularis) throughout Guam. We radio tracked 45 free-ranging Brown Treesnakes in close proximity to two roads in Guam and recorded instances where daily relocations of snakes spanned roads. Then we reconstructed observed movement histories with randomized turning angles, which served as a useful null hypothesis for assessing the effect of roads or road edge habitat on Brown Treesnake movement patterns. Random walk simulations demonstrated that Brown Treesnakes crossed these roads at a rate far lower than would be expected if snake movement was random with respect to roads and road edge habitat. We discuss two alternative hypotheses for these...

Testing for multiple invasion routes and source populations for the invasive brown treesnake (Boiga irregularis) on Guam: implications for pest management

The brown treesnake (Boiga irregularis) population on the Pacific island of Guam has reached iconic status as one of the most destructive invasive species of modern times, yet no published works have used genetic data to identify a source population. We used DNA sequence data from multiple genetic markers and coalescent-based phylogenetic methods to place the Guam population within the broader phylogeographic context of B. irregularis across its native range and tested whether patterns of genetic variation on the island are consistent with one or multiple introductions from different source populations. We also modeled a series of demographic scenarios that differed in the effective size and duration of a population bottleneck immediately following the invasion on Guam, and measured the fit of these simulations to the observed data using approximate Bayesian computation. Our results exclude the possibility of serial introductions from different source populations, and instead verify a single origin from the Admiralty Archipelago off the north coast of Papua New Guinea. This finding is consistent with the hypothesis that B. irregularis was accidentally transported to Guam during military relocation efforts at the end of World War II. Demographic model comparisons suggest that multiple snakes were transported to Guam from the source locality, but that fewer than 10 individuals could be responsible for establishing the population. Our results also provide evidence that low genetic diversity stemming from the founder event has not been a hindrance to the ecological success of B. irregularis on Guam, and at the same time offers a unique ‘genetic opening’ to manage snake density using classical biological approaches.

Island of the Snakes

The invasive brown tree snake has devastated Guam's ecosystems. Can it be eradicated?

Effectiveness of bait tubes for brown treesnake control on guam

In 2008, we studied simulated toxicant efficacy to control invasive brown treesnakes (Boiga irregularis) using bait tubes (elongate bait stations that reduce non-target bait take) in a 5-ha enclosure in Guam (U.S. Territory) with a known population of snakes. Instead of toxicants, we implanted radiotransmitters in small (6.6 ± 1.4 g) and large (21.8 ± 2.9 g) bait-mouse carcasses, offered from 2 types of bait tubes over a 3-month period. The known snake population allowed us to characterize not only the snakes taking bait, but also those evading our mock control effort. Tube design had no effect on take rate, but snout–vent length was a strong predictor of bait take: none of the 30 snakes <843 mm in length took any bait, whereas 77 of the 126 snakes ≥843 mm in length took 164 baits. While medium-sized snakes preferentially ingested small bait (and the largest snakes tended to take large mice more frequently), some of the smallest snakes that took bait ingested large mice. Snake body condition was positively correlated to take rate, but snake sex had no discernible effect. Our data show that there is a relatively narrow size (and, thus, time) gap between the size at which the snakes become susceptible to bait-mouse take and the size at which they become sexually mature. This has implications for the timing of repeated baiting efforts, if the goal is eradication rather than suppression. Published 2013. This article is a U.S. Government work and is in the public domain in the USA.

Natural Experiment Demonstrates That Bird Loss Leads to Cessation of Dispersal of Native Seeds from Intact to Degraded Forests

In healthy forests, vertebrate frugivores move seeds from intact to degraded forests, aiding in the passive regeneration of degraded forests. Yet vertebrate frugivores are declining around the world, and little is known about the impact of this loss on regeneration of degraded areas. Here, we use a unique natural experiment to assess how complete vertebrate frugivore loss affects native seed rain in degraded forest. All native vertebrate frugivores (which were primarily avian frugivores) have been functionally extirpated from the island of Guam by the invasive brown tree snake (Boiga irregularis), whereas the nearby island of Saipan has a relatively intact vertebrate frugivore community. We captured seed rain along transects extending from intact into degraded forest and compared the species richness, density and condition of the seed rain from native bird-dispersed tree species between the two islands. Considering seeds from native bird-dispersed species, approximately 1.66 seeds landed per 26 days in each square meter of degraded forest on Saipan, whereas zero seeds landed per 26 days per square meter in degraded forest on Guam. Additionally, on Saipan, 69% of native bird-dispersed seeds in intact forest and 77% of seeds in degraded forest lacked fleshy fruit pulp, suggesting ingestion by birds, compared to 0% of all seeds on Guam. Our results show an absence of seed rain in degraded forests on Guam, correlated with the absence of birds, whereas on Saipan, frugivorous birds regularly disperse seeds into degraded forests, providing a mechanism for re-colonization by native plants. These results suggest that loss of frugivores will slow regeneration of degraded forests on Guam.

Pheromonal control of the invasive brown treesnake: potency of female sexual attractiveness pheromone varies with ovarian state

The female sex pheromone has potential a s a control tool for the Brown Treesnake, Boiga irregularis Merrem, a pest introduced into Guam. We conducted male-guided bioassays to determine whether potency of the female pheromone varied according to reproductive state and during vitellogenesis. In weekly simultaneous choice tests, we presented males with skin lipid samples from vitellogenic females, non-vitellogenic females, conspecific males, and carrier controls. The total time each sample was tongue-flicked was recorded: mean total tongue-flick time was highest for samples from vitellogenic females followed by samples from non-vitellogenic females. Pair-wise comparisons of male tongue-flick time for samples of non-vitellogenic females versus males and vitellogenic females versus non-vitellogenic females revealed differences in some weeks, but the greatest and most frequent differences were between vitellogenic females versus males. We observed no trend in the potency of the pheromone during vitellogenesis. Pheromone potency was greatest during vitellogenesis, with lesser potency retained while non-reproductive. Efforts at chemical identification of the pheromone and development of control tools should focus on material obtained from vitellogenic females.

‘Natural experiment’ Demonstrates Top-Down Control of Spiders by Birds on a Landscape Level

The combination of small-scale manipulative experiments and large-scale natural experiments provides a powerful approach for demonstrating the importance of top-down trophic control on the ecosystem scale. The most compelling natural experiments have come from studies examining the landscape-scale loss of apex predators like sea otters, wolves, fish and land crabs. Birds are dominant apex predators in terrestrial systems around the world, yet all studies on their role as predators have come from small-scale experiments; the top-down impact of bird loss on their arthropod prey has yet to be examined at a landscape scale. Here, we use a unique natural experiment, the extirpation of insectivorous birds from nearly all forests on the island of Guam by the invasive brown tree snake, to produce the first assessment of the impacts of bird loss on their prey. We focused on spiders because experimental studies showed a consistent top-down effect of birds on spiders. We conducted spider web surveys in native forest on Guam and three nearby islands with healthy bird populations. Spider web densities on the island of Guam were 40 times greater than densities on islands with birds during the wet season, and 2.3 times greater during the dry season. These results confirm the general trend from manipulative experiments conducted in other systems however, the effect size was much greater in this natural experiment than in most manipulative experiments. In addition, bird loss appears to have removed the seasonality of spider webs and led to larger webs in at least one spider species in the forests of Guam than on nearby islands with birds. We discuss several possible mechanisms for the observed changes. Overall, our results suggest that effect sizes from smaller-scale experimental studies may significantly underestimate the impact of bird loss on spider density as demonstrated by this large-scale natural experiment.

The bioinvasion of Guam: inferring geographic origin, pace, pattern and process of an invasive lizard (Carlia) in the Pacific using multi-locus genomic data

Invasive species often have dramatic negative effects that lead to the deterioration and loss of biodiversity frequently coupled with the burden of expensive biocontrol programs and subversion of socioeconomic stability. The fauna and flora of oceanic islands are particularly susceptible to invasive species and the increase of global movements of humans and their products since WW II has caused numerous anthropogenic translocations and increased the ills of human-mediated invasions. We use a multi-locus genomic dataset to identify geographic origin, pace, pattern and historical process of an invasive scincid lizard (Carlia) that has been inadvertently introduced to Guam, the Northern Marianas, and Palau. This lizard is of major importance as its introduction is thought to have assisted in the establishment of the invasive brown treesnake (Boiga irregularis) on Guam by providing a food resource. Our findings demonstrate multiple waves of introductions that appear to be concordant with movements of Allied and Imperial Japanese forces in the Pacific during World War II.

Scented Guide Ropes as a Method to Enhance Brown Treesnake (Boiga irregularis) Trap Capture Success on Guam

Current methods for controlling the invasive Brown Treesnake (Boiga irregularis) on Guam include a modified minnow trap with a live mouse lure. We investigated the effects on capture success of augmenting these traps with scented guide ropes leading to trap entrances. Initial screening of scent preferences was based on time spent in scented and unscented arms of a Y-maze. Preferences of large and small snakes were scored for six different prey scents (live and carrion gecko, skink, and mouse). Large snakes spent more time in the maze arm scented with live gecko and carrion gecko, whereas small snakes spent more time in the arm scented with carrion mouse and carrion gecko. After the laboratory study, a pilot trapping session was conducted in the field using three treatments (live mouse-scented ropes, carrion gecko-scented ropes, and carrion mouse-scented ropes) and two controls (traps with unscented guide ropes and those with no ropes attached). Contrary to laboratory results, live mouse-scented ropes were most effective. We conducted a second trapping session using live mouse-scented ropes as well as the two controls used in the pilot study. For snakes of below-average to average condition, the number of captures for traps with live mouse-scented ropes was higher than for traps with no ropes. However, for snakes of above-average condition, there were no differences in capture rates between trap treatments. Overall, treatment effects were weaker than latent individual heterogeneity and the influence of snake body size, with large snakes trapped more readily.

Modelling detection probabilities to evaluate management and control tools for an invasive species

Summary 1. For most ecologists, detection probability (p) is a nuisance variable that must be modelled to estimate the state variable of interest (i.e. survival, abundance, or occupancy). However, in the realm of invasive species control, the rate of detection and removal is the rate‐limiting step for management of this pervasive environmental problem. 2. For strategic planning of an eradication (removal of every individual), one must identify the least likely individual to be removed, and determine the probability of removing it. 3. To evaluate visual searching as a control tool for populations of the invasive brown treesnake Boiga irregularis, we designed a mark–recapture study to evaluate detection probability as a function of time, gender, size, body condition, recent detection history, residency status, searcher team and environmental covariates. 4. We evaluated these factors using 654 captures resulting from visual detections of 117 snakes residing in a 5‐ha semi‐forested enclosure on Guam, fenced to prevent immigration and emigration of snakes but not their prey. Visual detection probability was low overall ( = 0·07 per occasion) but reached 0·18 under optimal circumstances. 5. Our results supported sex‐specific differences in detectability that were a quadratic function of size, with both small and large females having lower detection probabilities than males of those sizes. There was strong evidence for individual periodic changes in detectability of a few days duration, roughly doubling detection probability (comparing peak to non‐elevated detections). Snakes in poor body condition had estimated mean detection probabilities greater than snakes with high body condition. Search teams with high average detection rates exhibited detection probabilities about twice that of search teams with low average detection rates. Surveys conducted with bright moonlight and strong wind gusts exhibited moderately decreased probabilities of detecting snakes. 6. Synthesis and applications. By emphasizing and modelling detection probabilities, we now know: (i) that eradication of this species by searching is possible, (ii) how much searching effort would be required, (iii) under what environmental conditions searching would be most efficient, and (iv) several factors that are likely to modulate this quantification when searching is applied to new areas. The same approach can be use for evaluation of any control technology or population monitoring programme.

Acetaminophen as an oral toxicant for Nile monitor lizards (Varanus niloticus) and Burmese pythons (Python molurus bivittatus)

Context. Invasive species are a growing global problem. Biological invasions can result in numerous harmful impacts on local ecologies, and non-native herpetofauna are frequently ignored. Nile monitor lizards (Varanus niloticus) and Burmese pythons (Python molurus bivittatus, recently reassessed as Python bivittatus bivittatus), have become established in southern Florida. Both are large, semi-aquatic predators that pose serious threats to a variety of threatened and endangered species, as well as to the unique ecology of the area. Aims. Acetaminophen (CAS#103-90-2), a lethal oral toxicant for the invasive brown treesnake (Boiga irregularis) on Guam, was investigated as a possible toxicant in juvenile Burmese pythons and Nile monitors. Methods. Dead neonatal mouse (DNM) baits containing 0, 10, 20, or 40 mg acetaminophen were force-fed to Nile monitors, whereas DNM containing doses of 0, 20, 40, or 80 mg were freely consumed by Burmese pythons. Subjects were frequently observed post-treatment for general condition and position, with special attention paid to activity (if any), behaviour, respiration, bleeding, emesis, ataxia, and mortality. Key results. In Nile monitors, acetaminophen doses of 10, 20, or 40 mg resulted in 0, 50 and 100% mortality, respectively. In Burmese pythons, doses of 20, 40, or 80 mg resulted in 14.3, 85.7 and 100% mortality, respectively. No mortality was observed in control individuals of either species. A negative correlation between dosage (mg kg–1) and time-to-death was observed in both species. Dosages ranging from 522 to 2438 mg kg–1 and 263 to 703 mg kg–1 were uniformly lethal to monitors and pythons, respectively. Neither species exhibited signs of pain or discomfort following acetaminophen treatment. Conclusions. Acetaminophen is an effective toxicant in juvenile Nile monitors and Burmese pythons. Further investigation into acetaminophen toxicity in adults of these species is merited. Implications. Although further investigation into adult lethal dosages and strategies to optimise bait deployment while minimising secondary hazards is required, acetaminophen may have a role to play in the control of these invasive species in Florida.