Saving plants from pests too efficiently could make it difficult for the plants themselves, says S.Ananthanarayanan.
The natural world is so interconnected that how things may impact the environment often comes as a surprise. Drs Richard J. Gill, Oscar Ramos-Rodriguez & Nigel E. Raine of the School of Biological Science, University of Cambridge, report in the journal,
Nature, their findings of the effects of pesticides on the health of the agents of pollination.
Pollination
Agriculture depends on having the correct strain of plant or tree, the right conditions of soil, rainfall or water supply, temperature and so on. But for all the inputs, what it vital is that the pollen be transferred from flower to flower, for the production
of seeds and the reproduction of plants. The pollen consists of part genetic information of the plant, which combines with a corresponding other half to result in cells that again divide to produce more cells with half genetic information. The pollen is the
part that moves to bring about the combining of genetic parts, usually between plant and plant, and it consists of grains that have a hardy cover to protect their sensitive payload.
Although plants can reproduce by fertilization of the female part-cells by the pollen grains, which are the male part, from the same plant or flower, it is by cross-pollination, or by pollen of other plants, that there is genetic diversity, healthy offspring
and resilient and stable plant population in an area. And this process of transfer of pollen from one plant to another has never involved any human effort, in contrast of other phases of the growth of vegetation which have been channeled and have been the
most important human activity through history. The job of distribution of pollen has been done without cost or effort, in some plants by the agencies of wind or water and by a variety of insects in the case of most plants.
A most important class of pollinators are bees, which plants have evolved to attract with the incentive of nectar. There are flies and other insects, of course, but the category of bees is the leading pollinator of most fruit trees and vegetable plants.
But because bees and insects do their good work silently and without effort of the human cultivators, and they find their food from the plants themselves, they have not merited as much attention as other factors which act against the plant population. With
the spread of agriculture and specialized strains of plants, insects and organisms, including other plants, that attack plants or negatively affect their growth, have also grown and a large part of the farmers’ effort is to safeguard the plants and the crop
from damage by pests.
Pesticides
To this end has arisen the industry of pesticides. These are chemical or biological preparations that are sprayed or otherwise distributed over plants and orchards to prevent, destroy, repel or lessen different kinds of pests. With growing human populations
and growing agriculture, the use of pesticides has burgeoned and the world now uses more than 3 million tones. Over decades, many pesticides were found to persist in the crop, in water and in the soil with adverse effects on consumers and the environment.
The use of these have been banned, modified, etc, to contain the damage but there is always a compromise on how much damage to the crop and consumers to permit and how effectively to control pests.
Important among pesticides in use are the categories of synthetic compounds called neonicotinoids and pyrethroids. Neonicotinoids affect the nerve system of insects and have become very popular as they have less toxicity than insecticides used earlier
and they are known to be less harmful to mammals, compared to insects. Pyrethroids are often used as insect repellents and are common in use both with plants and with domestic pets.
Bee populations
But another effect of pesticides, which has now captured interest,s is their effect, not on the plants or human consumers, but on the insect population that is involved in pollination. For years there have been reports of falling bee populations and breakdown
of bee colonies, to the disquiet, both of beekeepers and of farmers. Bees contribute 80% of the work of pollination by insects and any sustained reduction in their numbers can spell disaster for the food economy, apart from the vegetation of the world. Just
how different pesticides affect the population and foraging of insects, bees in particular, has become a subject of research and more than 100 papers and reports have been published on this topic so far in this year itself.
The group in Cambridge note that although chemical pesticides have been associated with changes in bee behaviour and reductions in colony queen bee production, the key link between changes in individual behaviour and the consequent impact at the colony
level has not been shown. As bees work collectively in colonies, it is possible that the effect of field level pesticides at the individual bee level could be buffered at the colony level to bring about cumulative effects. Another possibility is that exposure
to different pesticides, as occurs in present day conditions, could have different effects on bees than when encountered separately, as in laboratories.
Dr Gill and associates investigated the effects of two pesticides, one a neonicotinoid and the other a pyrethroid, on the development and growth of bumblebee colonies and on the foraging activity of individual bees, by tagging the bees with microchips.
They placed feeders of sugar syrup that had been spiked with the first pesticide and/or filter paper treated with the other, in the path of bumble¬bees leaving their nest boxes. It is significant that the bees could freely bypass the filter paper and the feeder
and forage in the surrounding landscape for pollen and nectar.
The result of the trials were that exposure to the neurotoxin resulted in fewer adult workers emerging from pupae. While this was similar to the results of other studies, Gill et al also found that these workers tended to forage over wider areas and many
did not return to the hive. Bees that were exposed to the other pesticide showed greater mortality in the nest. And when exposed to both the insecticides, the effects appeared to add to the reduction in numbers. An important aspect of the study is the emphasis
of effect both on individual bees as well as on the colony. While a lethal dose of toxic pesticide would just eliminate the worker bee, a sub-lethal dose would be carried back to the hive. Further study, of course, is needed to understand how the size of the
hive, as honey hives have much greater numbers than bumble-bee hives, affects acute and chronic mortality.
The other aspect of interest in the study is the effect of combining insecticides. There are regulatory norms now in place, which control the use of individual pesticides in an area. But the use of pesticides in combination may enhance the toxicity of
any one or more than one. The study of Gill and Co is a first step in detailed study of this area that is necessary, to ensure that pest control does not result in birth control in the sector being protected!
[the writer can be contacted at simplescience@gmail.com]
(Photographs Susan Sharma and Ashwin Baindur)