Drugs present in wastewater used to irrigate farmland can affect the growth and development of insects feeding on plants, new research has shown.

Prescription rates for pharmaceuticals have tripled in the last fourteen years, and currently more than 6000 tonnes of antibiotics are fed to farm animals annually to boost meat and milk yields. Increasingly, these chemicals are finding their way into the environment, often passing unchanged through wastewater plants. Now researchers have a growing body of evidence that they may be affecting insects and other species that come into contact with them.

University of California, Riverside, scientist Marcus Pennington and his colleagues fed cabbage looper caterpillars on a diet spiked with a range of drugs, including hormones, paracetamol, antibiotics and caffeine at levels similar to those found in discharged wastewater. They also reared caterpillars on tomato plants growing hydroponically in water containing levels of antibiotics similar to those found in treated sewage or leaching out of farm manure.

The caterpillars fed the drug-laced diets had higher mortality rates compared with control larvae for all of the drugs tested, and the differences were statistically significant for antibiotics and hormones. Individually these two chemical classes increased mortality rates by about 3-fold. But fed a mixture of all of the agents, as would perhaps occur in nature, mortaility was 6-8 fold greater. The rate of growth and attainment of adulthood was also significantly slower for all of the drugs tested.

Caterpillars fed on the tomato plants were also affected, although only antibiotics produced a statistically significant effect. Tests on the plants confirmed that the roots and leaves of the plants contained detectable levels of ciprofloxacin, one of the antibiotics added to the water used to grow the plants.

The reason for the increased mortality isn't known, but the researchers speculate that impacts upon both the insect's physiology, and the bacteria they carry on- and inside themselves are the likely reasons.

Exposure to the drugs, either directly in the diet or indirectly by eating tomato plants grown with antibiotic-laced water, produced changes in the relative abundances of different microbial species that could be recovered from the insects.

The researchers, who have published their work this week in PNAS, do nevertheless urge caution in interpreting their findings. Different soil types and local bacterial communities, they say, will dramatically affect the levels and persistence of drug molecules in the environment.

That said, because these drugs are likely to be delivered constantly through irrigation, they can be regarded as persistent all the same. Also, the soil isn't the only route of exposure: insects can also encounter liquids sprayed onto plants by overhead irrigation.

Pennington and his colleagues point out though that the cabbage loopers they studied are crop pests worldwide and have a track record of developing resistance to chemical treatments. A more vulnerable species might therefore be more adversely affected; and what about the human consumers? The downstream effects of these molecules on the nutritional value of the plants we are eating, and the effects of these drugs re-entering the food chain via this route, also needs to be considered...