Despite the dominance of oil in the economy of Saudi Arabia, several hundred thousand of the kingdom's inhabitants make their living in agriculture. Others live from raising sheep, camels, goats and cattle for their milk, their meat, their wool and their hide—occupations that for more than 20 centuries were practically the only means of livelihood. Until very recently, the only permanent crop of particular economic significance was the date palm which was carefully developed by Arab farmers in magnificent, extensive oases in several places on the seacoast and in the interior.
The date palm, viewed ecologically, is a logical development. An oasis is very much like an island in that sand is just as hard a barrier for plants and animals to cross as is sea water. Plants in an oasis, therefore, are limited to some endemic species that tolerate the climate, the soil, and the quantity and quality of available water, or can adapt themselves to the environment. In oases, although the number of native and introduced foreign plant species is relatively large, the natural vegetational climax is the date palm.
This climax is the result of a delicate balance in the chain of life, involving the palms and other species that have adapted to the oasis environment, animals and insects that feed on the plants, and birds, lizards, toads and a horde of different insects that feed on the plant-feeders. With time these species usually achieve a more or less stable, proportional population density through a natural regulatory mechanism. However, even on an isolated island, if the balance is disturbed, species not normally a threat could, through unregulated population growth, achieve the status of "pests." Thus, the utmost care must be taken not to upset the balance. In Saudi Arabia, the Arabian American Oil Company (Aramco) demonstrated this some years ago.
In 1956-57, Aramco, as a preliminary to an agricultural development program in the Eastern Province, began to survey the pests, or potential pests that might harm crops already known or likely to be introduced in the area. As a result of the survey about 60 different species of insects and mites were labeled as economically dangerous, or potentially so. Unwise tampering—such as short-term chemical control campaigns—could neutralize the natural regulation by parasites and predators, and change harmless populations into uncontrollable pests.
Actually, of the 60 dangerous or potentially dangerous insects isolated, the seven most common date palm insects cause no serious damage under normal growth conditions. When weather favors the growth of scale insect populations, however, and palm fronds become covered with these sucking pests, the scales inject their poisonous saliva into the green tissues and thus stop photosynthesis. As a result, the palm tree loses much of its vigor. Only after it is weakened beyond a certain point can the tree be attacked by grubs of the long-horned date beetle. The beetle, Pseudophilus testaceus, lays its eggs under the bases of fronds, and in crevices in the bark of the date trunk. They hatch in about two weeks and the ensuing grubs, for 10 months, bore galleries in the trunk. Early in spring, the grub attains its full size, pupates, and produces a beetle in the late spring and early summer months. Another pest of date palms is the rhinoceros beetle, Oryctes elegans. In the adult stage it feeds on the midribs of fronds and on the fruit stalk, thus preventing the fruits from reaching full size. Its grubs, however, feed as borers in the stem of dying palm trees. The rhinoceros beetle usually lays its eggs in trees that have already been weakened by the long-horned beetle. A third beetle, whose grub and adult also attacks weak palm wood, particularly the fronds, is known as Phonapate frontalis . Attacked fronds may break up in a heavy wind. It is not always necessary that attacks by the beetle be preceded by the establishment of high scale populations on the tree; poor management, in the form of no pruning, lack or excess of irrigation will also predispose trees to borer attacks.
Besides borers, the caterpillars of two species of moths feed on the flowers or date fruits. One, known as the greater date moth, Arenipses sabella, lays its eggs on the unopened spathes and other tender parts of the palm. When the spathes open, the caterpillars leave them in the direction of the fruit stalks. They sever the stalks from the strand, but connect them again with silken threads. The fruits then turn grayish-brown and shrivel. Moths of this species appear from February to April, but the second generation in April-May is more serious. Another moth, Batrachedra amydraula , known as the lesser date moth, attacks date fruits in the caterpillar stage, and causes similar damage. The adults of this species appear as of mid-April, but at least another generation is formed before the beginning of summer. In the oases of Qatif and al-Hasa, up to 20 percent of the young fruit on some trees is damaged. This loss occurs only occasionally and in limited areas, thanks to the biological regulating mechanism that reigns in the old, settled date palm climax.
Among the pests that that were singled out in Aramco's survey, however, were threatened, or could threaten or could threaten crops other than the date palm.
One such pest, apparently unique in Saudi Arabia, is a moth named Aproerema alfalfella , whose caterpillar feeds on the top growth of alfalfa and does particular damage to its flower buds. The white-banded, light violet caterpillar ties together a number of growing points of the plant, making a sort of a nest out of them. It feeds on the tender buds and foliage inside the nest. Much damage to alfalfa grown for seed can result since this species can produce one full generation in about 20 to 30 days—a considerable threat if you recall that alfalfa still ranks as one of Saudi Arabia's major crops, topping even the date palm. The average annual yield of alfalfa is about 2 million metric tons, that of dates about 257,000 metric tons.
A number of endemic insects seem to feed on different plants of the cucumber family. One is a brownish fly, about the size of a housefly, known as Daculus frontalis . Like all its relatives, the fertilized female inserts a needle-sharp, egg-laying apparatus into a young cucumber fruit and lays about 7 to 12 eggs at a time, about 2 mm. deep. In about a week these hatch into white, legless maggots. Together with the eggs, the fly introduces a rot-causing bacterium that decays the fruit and helps the maggots eat and digest the pulp.
Another species, Eudioptes indica , prefers watermelons. It lays its eggs on the underside of leaves, and the beautiful green caterpillars that hatch in about 3 to 4 days tie the leaves together and feed by scraping their tissues; later they may enter the stem of the plant as borers. Newly-set fruit is often attacked as well. In May-June the caterpillars attach a leaf to the surface of the fruit and hide underneath it. They feed by scraping the rind fairly deeply, leaving an ugly brown callus which impairs quality.
In contrast to all the pests that limit feeding to one plant species in Arabia, there are also many species that feed on numbers of different plants. This group is rightly called the "opportunists," because they switch without hesitation from one plant species to another. To this group belongs the beautiful green moth Earias insulana . Its caterpillar naturally feeds in the fruits of different wild malvaceous plants, but when cultivated malvacae such as okra or cotton are available, the moth prefers to lay its eggs on them, not because they taste better but because the progeny will find much more food there. Under such conditions of abundance, a bigger progeny is produced, and moths of the opposite sexes have a much better chance of meeting for adequate egg fertilization. The caterpillar of this moth, known as the spiny bollworm, opens large galleries in cotton bolls and okra fruits and completely destroys them. Because of the fertility of the female and the large number of generations produced per year, this insect is a very serious pest, particularly to cotton. It succumbs to a number of phosphoric acid esters and chlorinated hydrocarbons, however.
Other opportunists are Trichoplusia ni and Sesamia cretica. The caterpillars of the first attack many totally unrelated plants such as lettuce and watermelon. In the Eastern Province of Saudi Arabia lettuce has been totally ruined in three days, and fine watermelon crops rendered useless in less than a week. Caterpillars of this moth burrow deep in the green fruit rind. This insect produces a number of generations per year but the moths usually migrate to far away places after devouring a crop; otherwise their progeny will find nothing to eat. The caterpillars of the second species, Sesamia cretica , bore into the stalks of different graminaceous plants, including maize, sweet corn, sorghum, sugar cane and some reeds. The damage is particularly serious when the whorl of sorghum is killed. Often the caterpillars cause a great deal of damage to corn and maize cobs. Young plants are eaten up entirely.
The most serious potential pest, however, is the one known as the Egyptian cotton leafworm, Prodenia litura. This is the opportunist par excellence . The food plants of its caterpillar comprise a horde of wild species, but among the cultivated ones are cotton, tobacco, alfalfa, eggplant, melons and peanuts. The moth lays up to 1,000 eggs. The caterpillars literally graze a field in about a week; even hot pepper fruit are totally consumed, as seen on more than one occasion. The insect is favored by warm, humid weather and thrives particularly well in the coastal oases.
Obviously, then, care must be taken as the techniques of modern agriculture—including wider use of chemicals—spread throughout Saudi Arabia and change the settled relationships of crop, insect and animal. Unwise upsets in their relationships could easily turn what is now a minor threat into a force able to severely damage the food supply of the entire kingdom.
Abdul Mon'im S. Talhouk, who has extensive research experience on the insects of the Middle East, is Professor of Economic Entomology at the American University of Beirut.