October 22, 2017

The effects of perennial irrigation on pest populations

Published as Chapter 6 of The Social and Environmental Effects of Large Dams: Volume 1. Overview. Wadebridge Ecological Centre, Worthyvale Manor Camelford, Cornwall PL32 9TT, UK, 1984. By Edward Goldsmith and Nicholas Hildyard.

To introduce perennial irrigation into an arid area is to change its micro-climate. Among other things, the moisture level of the atmosphere increases as a result of evaporation from the newly constructed reservoir, the irrigation canals and flooded fields. The moisture level of the soil, too, increases drastically: in the sandy soils of Egypt, for instance, by something approaching 25 times, from 0.7 percent to between 15 and 22 percent. [1]

As Professor Uvarov points out, the introduction of perennial irrigation into an arid area is like creating an oasis in the desert. [2] What was previously a hot dry ecosystem (in which moisture was only introduced temporarily, with the flooding of the river or with the seasonal rains) is transformed into one that is permanently moist.

The new moist ecosystem will attract all sorts of micro-organisms, insects and other forms of animal life which are particularly adapted to the new conditions. Inevitably, their populations build up at the expense of those species which previously lived there and which cannot adapt to the new moisture levels.

Some of the new species may, of course, be beneficial to agriculture. Thus earthworms attracted to a moist environment are useful, among other things, because their excrement contains a great deal of nitrogen. In experimental conditions, according to Professor Ghabbour of the University of Cairo, the common earthworm (allolobophora caliginosa ) produces 6.5 percent of its fresh weight in the form of urine every day. [3]

On the other hand, many of the forms of life favoured by the new moist conditions are highly undesirable. Moreover, where land is irrigated by overhead sprinklers, those undesirable species are provided with a uniformly moist ecosystem in which they may become permanently established. Indeed, to make that point, Professor Ezekiel Rivnay, one-time Head of the Plant Protection Department of Israel’s Volcani Institute of Agricultural Research, actually entitled an article which he wrote on the problem, “How to provide a nice wet place where insects you don’t want thrive”. [4]

It is not, of course, just the undesirability of the newcomers, but their sheer numbers which poses the problem. We all know just how much more likely living things are to be attracted by an oasis environment than by a desert. Moreover, many of those living things are likely to be adapted to living off the very crops which are cultivated in the irrigated fields of the new ‘oasis’.

Thus it is not surprising that the building of the Aswan Dam should have led to an increase in the number of agriculturally harmful insect pests such as the great moth (polychrosis botrana ) and the cotton-leaf worm (spodoptera littoralis ) – neither of which were previously known in Upper Egypt and which have now become common – and also the Corn stalk borer (Chilo agamemnon ). [5]

Nor is it surprising that the introduction of perennial – and in particular sprinkler irrigation in Israel – caused, as Rivnay points out, “invasions and an increase in many pests of agricultural importance” which has led to “the destruction of entire fields of crops.” [6]

Rivnay provides a number of examples of such pest invasions. Thus, until three decades ago, the onion fly (Hylemyia antiqua ) Meigen) was considered by Israelis to be a pest of little importance: no insecticides were used against it “as its damage was hardly felt.” [7] Under modern irrigated conditions, however, it has become “conspicuous, thereby necessitating measures of control.”

Nor was the seed corn maggot (Hylemyia cilicrura Rondani) any problem in Israel until the early 1950s. It now thrives under irrigated conditions. Likewise, the red pumpkin beetle (Rhaphidopalpa foreicollis Lucchese), which eats the leaves of cucurbit seedlings, now causes so much damage that entire fields must often be re-seeded. The reason is clear enough: under dry irrigation, there was not always enough moisture for the beetles’ eggs to hatch out but, as Rivnay points out, “with overhead irrigation, the entire area is uniformly watered, offering ideal conditions for the eggs to hatch.” [8]

The cultivation of vast stretches of the same crop (monoculture) under perennially irrigated conditions must of course attract pests that are specific to single crops (monophagous). Thus the weevil (hypera variabbilis ) is a pest of alfalfa. It “was hardly noticeable in Israel in the early thirties.” Today it has become very common and has to be controlled by spraying. [9]

The same is true of another weevil which is specific to beet (lixus yunci ). Its population expanded considerably once sugar and fodder beets came to be grown under perennial irrigation over large areas.

Unfortunately, insects also appear quick to modify their feeding habits in order to take advantage of the vast new niches created when new crops are introduced as a result of perennial irrigation.

Thus the cotton-leaf worm (spodoptera littoralis ) has become a serious pest in Egypt since the building of the Aswan Dam, although it appears to have been present in the region since the 1870s. With the extension of irrigation in Egypt and with the introduction of new crops such as peanuts, cotton and sugar beet, which do particularly well under perennial irrigation, its population has radically increased, as has the period during which it does damage and also the number of crops which it now infests . These now include grape vines and apples, for instance, which it apparently never attacked before.

Because insects can travel great distances, it is not unusual for newly irrigated areas to be infested by pests which were not previously known in the area. We have seen, for example, that this was the case in Egypt after the building of the Aswan Dam. It has also proved a considerable problem in parts of Israel. An attempt to grow rice under perennial irrigation in the Hula area, for example, proved disastrous because the rice was quickly infested with corn borers – even though the pest had never been seen in the area before. The loss of yields in the first year was 10 to 15 percent; by the second year, that figure had risen to 33 percent; and, eventually, losses were so high that rice cultivation had to be abandoned. [12]

Again, at Nitzana, an isolated desert area 40 kilometres from the nearest agricultural land, a tract of land was converted to a truck crop farm. This led to an invasion of aphids which frequently reduced the yield of such crops as vetch and beans by as much as 40 percent. [13] Aphids have wings and can be transported by air currents at high altitudes and dropped long distances away. They have been known to infect an area as far as 130 kilometres [14] distance from where they previously thrived.

At Avdat, in the middle of the Negev desert, a small tract of land was irrigated and put under cotton. It was 50 kilometres from the nearest cotton field. This did not prevent it from being attacked by the spiny boll-worm (Earias insulana Boisduval), which has been known to reduce yields by as much as 80 percent.

The problem is further compounded by the need to take full advantage of perennial irrigation facilities by growing several annual crops where, under conditions of basin-irrigated or rain-fed agriculture, only one would have been grown. As Rivnay points out, this radically prolongs the vegetative period of a crop, provides a permanent rather than a temporary niche for pests, and may even increase the number of pests that can live off the crop.

Thus, the oriental corn borer (Chilo agamemnon ) has become a major pest in Israel and, as we have seen, in Egypt too after the building of the Aswan Dam. Under conditions of dry farming, it could only raise two to three generations because by August the crop had become dry and provided an unsuitable habitat for the development of its larvae; but once perennial irrigation and multi-cropping were introduced, food was provided for the borer all summer, which enabled it to raise six generations between May and early September – with predictably disastrous consequences.

The same thing occurred in Israel with the Spodoptera moth (Spodoptera littoralis ). Under dry farming conditions it only tended to become active in June by which time crops such as wheat, chick peas and lentils, on which it might have lived, had already matured and dried, while those which were still growing, like sesame, had already been watered and did not attract moths. Once perennial irrigation was introduced, new crops were cultivated, in particular peanuts, cotton and sugar beets, which meant “offering food to the polyphagous Spodoptera continuously from May until late in November; when one kind of crop dried out, another was still fresh a short distance away.” [15] As a result, the cotton-leaf worm became a serious pest.

Dr. Taghi Farvar complains that in Algeria most scientists “have not even thought about the possibility that such changes (as the introduction of perennial irrigation) can have a profound impact on the ecology of insects or insect pests.” [16]

Rivnay also notes that the tendency of perennial irrigation – and in particular overhead sprinkler irrigation – to increase pest infestations seems to have attracted very little attention on the part of those concerned with the building of dams and large scale irrigation works. [17] If the paucity of material on the subject in the currently available literature is anything to go by, then he is clearly right.

References

1. S. T. Ghabbour, ‘Effect of Irrigation on Soil Fauna’, in E. Barton Worthington, Arid Land Irrigation in Developing Countries: Environ-mental Problems and Effects, Pergamon, Oxford, 1977, p.329.

2. B. P. Uvarov, ‘Problems of Insect Ecology in Developing Countries’, Journal of Applied Ecology, 1 (1964), pp.159-68. Quoted by E. Rivnay in T. Farvar and J. Milton (Eds), The Careless Technology, Tom Stacey, London, 1973, p.352.

3. S. T. Ghabbour, op.cit. 1977, p.330.

4. E. Rivnay, ‘How to provide a nice wet place where insects you don’t want thrive’, Natural History, 1969.

5. A. Mahir Ali, ‘Impact of Changing Irrigation on Agricultural Pests on Wildlife in Egypt’, in E. Barton Worthington, (Ed), op.cit. 1977, p.331.

6. E. Rivnay, ‘On Irrigation-induced Changes in Insect Populations in Israel’, in T. Farvar and J. Milton (Eds), op.cit. 1973, p.350.

7. Ibid, p.357.

8. Ibid, p.359.

9. Ibid, p.354.

10. Ibid, p.355.

11. Ibid, p.352.

12. Ibid, pp.354-355.

13. Ibid, p.354.

14. Taghi Farvar in John Milton and Taghi Farvar (Eds), op.cit. 1973, p.365.

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