The scale and design of the study are what set it apart from previous work. Most rodenticide surveys rely on animals that are found sick, dying, or dead, which means the samples skew toward the worst cases and overestimate exposure in the broader population. This new research took a fundamentally different approach. The owls in the dataset were removed as part of an ongoing regional conservation program targeting Barred Owls, an invasive species that competes with the threatened Northern Spotted Owl, rather than recovered as carcasses. As a result, the sample represents apparently healthy birds going about their normal lives, providing a far more accurate cross-section of what is actually circulating through the population. Dr. Vitek Jirinec, lead author of the study and research scientist at IERC, emphasized that this distinction gives researchers much greater confidence that the observed trends are real and population-wide rather than artifacts of sampling bias.

The headline result concerns second-generation anticoagulant rodenticides, or SGARs, an extremely potent class of compounds that prevent blood clotting and can kill predators through a single feeding on a poisoned rodent. Because these chemicals persist in the tissues of poisoned prey, they bioaccumulate up the food chain, devastating owls, fishers, bobcats, mountain lions, and other carnivores that never touched the original bait. Among young owls sampled in the study area, exposure to SGARs fell from roughly 50 percent of birds in 2019 to about 20 percent in 2024. That steep decline aligns closely with California’s phased statewide restrictions, most notably the California Ecosystems Protection Act, known as AB 1788, which was passed in 2020 and banned most second-generation anticoagulant use, along with a 2024 law, AB 2552, that extended restrictions to older-generation anticoagulant compounds as well.

For Dr. Greta M. Wengert, co-founder and co-director of IERC and principal investigator on the study, the results represent the culmination of more than a decade of work connecting contamination data to policy action. She noted that illegal use and unregulated products had driven rodenticide exposure in these forests for years, and that seeing anticoagulant exposure actually decline within a few years of the regulations taking effect demonstrates that policy grounded in good data from effective biomonitoring efforts can improve outcomes for wildlife. The study embodies what researchers describe as a feedback loop central to IERC’s mission: supplying the science that informs legislative decisions, and then documenting the real-world benefit that those decisions deliver on the ground.

But the study is not simply a victory lap. For the first time in this species, the researchers screened for cholecalciferol, commonly known as vitamin D3, a rodenticide that works through an entirely different mechanism than anticoagulants. Rather than disrupting blood clotting, cholecalciferol in toxic doses causes life-threatening hypercalcemia, raising calcium levels in the blood until organ failure occurs. The compound turned up in nearly one in five owls, at 18.5 percent of the sample, and detections rose over precisely the same period that anticoagulant exposure was falling. The researchers interpret this inverse pattern as evidence of product substitution: as older, regulated compounds became harder to access, users appear to have shifted toward cholecalciferol-based products, which currently face fewer restrictions in California.

The overall contamination picture remains sobering. When all rodenticide compounds were counted together, as much as 46 percent of the owls sampled carried at least one rodenticide in their system. This means that even as the most lethal anticoagulants recede, nearly half of the owls in the study area are still being exposed to some form of poison through the prey they consume. The data also revealed telling patterns within the population: female Barred Owls and older adults carried the highest exposure levels and the widest mix of compounds, a result consistent with pesticides accumulating in tissues over a lifetime of feeding on contaminated rodents.

That demographic pattern carries particular weight for conservation, because the individuals with the highest burdens, breeding-age females and established territory holders, are exactly the animals a population depends on most to sustain itself. The implications extend beyond Barred Owls to the species they share the forest with. Northern Spotted Owls, a closely related and federally threatened species, have a higher proportion of rodents in their diets, which may place them at even greater risk of accumulating these compounds. The new findings reinforce and extend earlier IERC research: a 2012 study first documented widespread anticoagulant exposure in Pacific fishers linked to trespass cannabis cultivation on public lands, and a 2018 study found rodenticides in 70 percent of sampled Northern Spotted Owls in the same region. Barred Owls, Spotted Owls, fishers, and other sensitive wildlife all feed from the same contaminated food web, and this much larger dataset adds substantial weight to that earlier evidence.

The authors are careful to caution that the exact sources of ongoing exposure are not fully resolved, and this uncertainty points to significant gaps in regulatory oversight. State pesticide-use records capture only licensed commercial applicators, leaving household purchases and illegal use entirely invisible to regulators. Trespass cannabis cultivation, which IERC has long documented as a major source of rodenticide contamination on public and tribal lands, remains untracked by any regulatory database. Legacy contamination from past illicit grow sites may also continue cycling through these forest ecosystems long after active cultivation has stopped, as persistent compounds move from soil and bait stations into rodents and then into the predators that hunt them. Researchers say these hidden and historical sources likely explain at least part of the exposure that persists even as legal, regulated use declines.

Published as Forest food web contamination by rodenticides is pervasive but responsive to regulation: Insights from Barred Owl biomonitoring, the study by Jirinec, Gabriel, Peery, Higley, Hofstadter, Varian, Rex, Franklin, Poppenga, Woods, Volker, and Wengert arrives at a pivotal moment for pesticide policy. It demonstrates that science-based regulation can produce measurable improvements for wildlife within a few years, a finding that should embolden policymakers elsewhere considering similar restrictions. At the same time, the rising tide of cholecalciferol detections serves as a warning that substitution effects can undermine hard-won gains when regulatory frameworks address only one class of compounds at a time. For the old-growth forests of northwestern California and the threatened species that depend on them, the message from more than a decade of IERC research is twofold: the same underlying toxic threat has persisted across multiple species, but for the first time, it is demonstrably responding to policy, and the next chapter of that policy response will determine whether the progress holds.

Subject of Research: Rodenticide contamination in Barred Owls and the effect of California pesticide regulations on wildlife exposure

Article Title: Largest-ever study of its kind shows California’s rodenticide restrictions are working, and flags new rodenticides rising in their place

Article References: Largest-ever study of its kind shows California’s rodenticide restrictions are working, and flags new rodenticides rising in their place. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: rodenticides, Barred Owls, California Ecosystems Protection Act, second-generation anticoagulants, cholecalciferol, Northern Spotted Owl, biomonitoring, wildlife toxicology, Integral Ecology Research Center, Environmental Pollution, pesticide regulation, forest food web

Tanzania’s economy is deeply rooted in agriculture, which contributes roughly a quarter of gross domestic product and employs about two-thirds of the national workforce. Smallholder farmers tilling plots of 0.5 to 3 hectares make up more than 90 percent of the agricultural labor force. Over the review period, several structural forces converged to drive pesticide demand upward: the liberalization of pesticide trade policies, which produced an approximately 80-fold increase in private retailer involvement from the late 1980s onward; the rapid commercialization of horticulture, the fastest-growing agricultural subsector with annual growth rates of 6 to 10 percent; expansion of irrigated farming that allows up to six cropping cycles per year and sustains continuous pest pressure; and the Southern Agricultural Growth Corridor of Tanzania initiative, an ambitious intensification program in the country’s south. FAO statistics corroborate the trend, documenting steadily rising imports of insecticides, fungicides, and other hazardous pesticide classes from the mid-1990s through the 2010s, while surveys of farmers across Iringa, Arusha, Manyara, and Kilimanjaro found that 58.4 percent reported increasing use on their own farms.

The chemistry of this expansion has shifted over time. Organophosphates such as profenofos, chlorpyrifos, and dimethoate now dominate, appearing in 96.6 percent of pesticide classes used in smallholder vegetable production, followed by carbamates and pyrethroids, with neonicotinoids such as imidacloprid rising steadily since 2015. More troubling is what persists beneath this modern portfolio. Organochlorine compounds, including DDT, endosulfan, aldrin, dieldrin, and lindane, remain officially banned yet continue to be detected in soils, water, fish, vegetables, and even chicken eggs, signaling either ongoing illegal use or stubborn environmental persistence. A national soil survey found hexachlorocyclohexane in 88 percent of agricultural samples, with DDT most abundant in the productive Southern Highlands. Sugarcane plantation soils in Kilimanjaro still contained DDT and metabolites more than two decades after their official use ceased.

The quantities being applied frequently exceed international norms by wide margins. One assessment reported fungicide application rates of 17.18 kilograms of active ingredient per hectare in tomato and 13.05 in onion cultivation, alongside insecticide rates of nearly 6 kilograms per hectare, all far above world averages. More than half of vegetable farmers in northern Tanzania spray five or more times per cropping season, and mixing pesticides is the norm rather than the exception: 93.7 percent of surveyed farmers in the Ulanga and Kilombero districts combined products, often near water sources and without label guidance. WHO Class II moderately hazardous compounds account for 68.9 percent of what smallholders use, but extremely and highly hazardous Class Ia and Ib products remain in circulation, reflecting weak enforcement and the influence of unscrupulous agro-dealers.

The review found that unsafe handling is a structural, sector-wide problem rather than a localized one. More than 90 percent of farm workers use no personal protective equipment, farmers in one Arusha study ate, drank, and smoked while spraying, and repacked agrochemicals sold in unlabeled containers are common, with some containers later reused domestically. Fewer than half of farmers have received any formal safety training, most rely on pesticide dealers for instructions, and over half discard empty containers into rivers or bushes. Knowledge gaps extend into the clinic: a survey of 104 physicians, clinical officers, and nurses in agricultural areas found that only 1 percent could identify the pesticide groups predominantly used in their communities, compounding chronic underdiagnosis and underreporting of poisoning cases.

The human toll is substantial. In cotton-growing communities, 48 percent of 1,074 surveyed farmers experienced symptoms of unintentional acute pesticide poisoning within a year, yet only 6 percent sought formal medical care. Hospital surveillance documented 108 poisoning cases among women in a single year, with suicide the leading circumstance and organophosphates, zinc phosphide, paraquat, and endosulfan the principal agents. Biomonitoring tells a parallel story: exposed horticultural farmers showed significantly depressed acetylcholinesterase levels, a marker of organophosphate neurological impact, and long-term professional sprayers faced a 7.7-fold higher risk of peripheral neuropathy. A case-control study at Tanzania’s Ocean Road Cancer Institute found that agricultural workers had 2.6 times the odds of head and neck cancers, rising to 5.1-fold among those with more than a decade of farm work. Women, who make up 65 to 70 percent of the horticultural labor force and increasingly handle spraying and contaminated laundry, face added risks of endocrine disruption and reproductive harm.

Food safety surveillance reveals contamination reaching consumers directly. In the largest multi-regional study, 613 vegetable samples were tested and 74.2 percent of those with detectable residues exceeded Codex maximum residue limits, with organophosphorus compounds dominating. Tomatoes from Dar es Salaam markets carried chlorpyrifos at concentrations up to 3,810 nanograms per gram, while Makambako tomatoes showed residues in 78.5 percent of samples. Recent testing of Dar es Salaam cucumbers and watermelons found imidacloprid at 15.71 milligrams per kilogram, far above the 1.0 milligram per kilogram limit. Nile perch and tilapia from Lake Victoria carried DDT and endosulfan at levels indicating recent exposure, and poultry eggs from Ilala and Kibaha contained DDT at 8.14 milligrams per kilogram, multiples of international standards. Irrigation water in Arusha contained lindane and endosulfan sulphate at concentrations that violated WHO drinking water guidelines.

The review also documents a striking cross-sector consequence: agricultural pesticide use is undermining malaria control. Mosquitoes from villages with heavy agrochemical use showed resistance to pyrethroids and other public health insecticides, while those from low-use villages remained susceptible. National monitoring data show pyrethroid resistance in Anopheles mosquitoes rising from zero percent of sentinel sites in 2004 to more than 80 percent by 2020. Because lambda-cyhalothrin and cypermethrin are both top agricultural insecticides and mainstays of insecticide-treated nets and indoor spraying, farming practices now threaten to erode decades of progress against one of the country’s deadliest diseases.

Regulatory capacity has not kept pace with the market. Tanzania has 1,182 registered pesticide products, yet unregistered and counterfeit formulations abound, inspections are inconsistent, and porous borders allow banned products to enter. The Tanzania Plant Health and Pesticides Authority, successor to the Tropical Pesticides Research Institute, faces chronic staffing and laboratory constraints, and the Vikuge obsolete pesticide stockpile site remains a cautionary tale of incomplete remediation decades later. Meanwhile, proven alternatives struggle for traction: farmer field schools trained in integrated pest management cut use of the most hazardous pesticide classes by 85 percent, and a best-practice training program reduced synthetic pesticide spending by 51 percent while boosting vegetable profits by 72 percent, yet most farmers surveyed in Morogoro and Iringa had never heard of integrated pest management at all.

The authors of the review frame their conclusions with appropriate caution, noting that the evidence base skews toward the better-studied northern horticultural zones and relies heavily on cross-sectional, convenience-sampled studies. Even so, they argue the convergent findings demand urgent action: expanded inspector capacity and market surveillance, a national acute poisoning surveillance system, mandatory point-of-sale training for pesticide vendors, accelerated phase-out of WHO Class Ia and Ib compounds, mainstreaming of integrated pest management into national policy, gender-responsive interventions, and routine residue monitoring linked to enforcement. As Tanzania pursues agricultural intensification under SAGCOT and expands irrigation nationwide, the review warns that without such reforms the pesticide burden on public health, food safety, and the environment, already severe, is likely to worsen before it improves.

Subject of Research: A systematic review of pesticide use trends, exposure pathways, health risks, environmental contamination, and policy challenges in Tanzanian agriculture from 2000 to 2026.

Article Title: Pesticide use trends (2000–2026), exposure pathways, human health risks, environmental contamination, and policy challenges in Tanzania

Article References: Pesticide use trends (2000–2026), exposure pathways, human health risks, environmental contamination, and policy challenges in Tanzania. (n.d.). https://doi.org/10.1016/j.envc.2026.101666

Image Credits: AI Generated

DOI: 10.1016/j.envc.2026.101666

Keywords: Tanzania, pesticides, organophosphates, DDT, acute pesticide poisoning, pesticide residues, food safety, integrated pest management, malaria vector resistance, environmental contamination, smallholder farmers, pesticide regulation