Pollination by Native Insects

George E. Bohart.

The earliest flowering plants in the fossil record were related to the magnolias, which to this day depend for pollination on the visits of beetles. Beetles, which comprise the order Coleoptera, were the most abundant and adaptable insects during the dawn period of flowering plants and thus, quite naturally, were the first pollinators. The flies and the sawflies and wasps were present but poor in variety and primitively developed. In the ensuing ages, however, their adaptation to the products of flowers became a dominant feature of their structure and habits. The moths and butterflies, which first appeared in the early days of flowers, soon adapted themselves completely to floral offerings. Now nearly all of them are highly developed for taking nectar from flowers.

While the insects were thus becoming specialized to take advantage of flowers, plants were likewise becoming specialized to make more efficient use of insects. Certain flowers developed characteristics limiting them to pollination by certain types of insects, which in turn become highly adapted to these specialized flowers. Today we have many plants so constructed that only a few specially adapted insects can visit them successfully. Figs, orchids, Spanish-bayonet, and monkshood are examples.

The so-called hawk-moth orchids (in the genera Habenaria, Angraecum, and others) exemplify the many intricate modifications possessed by orchids to insure pollination by specific kinds of insects. In these flowers the nectar, lying at the bottom of a long narrow tube, is accessible only to the long-tongued hawk moths. While probing for nectar, the moth brings each eye against a sticky disk to which a mass of pollen is attached, and flies away, carrying the masses on its eyes. The masses (called pollinia) then bend forward on their stalks in such a way that, when the moth inserts its proboscis into the next flower, they fit perfectly against the stigma and adhere to it. From the presence in Africa of an orchid of this type, with a nectar tube 12 inches long, there is inferred the existence in that region of a hawk moth with a tongue equally long.

In most acts of pollination the insect has no interest in the plant beyond its store of nectar or pollen, pollination on its part being an accident. It is the plant which, by its offering of nourishment and by the arrangement of floral parts, insures that such "accidents" will occur.

The yucca moth, which is the sole pollinator of yucca (Spanish-bayonet), is a unique exception and provides a good example of symbiotic relationships between plants and animals. It is no mere nectar sipper. At first, operating somewhat in the manner of the fig wasp, the female stabs the ovary of the yucca flower with her ovipositor and inserts an egg. That is commonplace insect behavior, but her next acts, though instinctive, seem to display careful planning and an uncanny knowledge of botany. She mounts a stamen, scrapes together a wad of pollen, carries it back to the pistil containing her egg, and thrusts it into the funnel-shaped stigma. She takes neither nectar nor pollen for herself but performs the only act that will guarantee the proper food for her offspring, the developing ovules of the plant. The yucca plant in its turn may lose a few seeds to the young worms surely a small price to pay for such perfect pollination service.

SOME YEARS AGO, scientists argued hotly whether insects or flowers became specialized first or whether it was simultaneous. Voluminous papers attempted to explain why and how the process of mutual adaptation developed, but the subject finally became so controversial and unproductive that it was all but dropped. Recently, however, technical advances in agriculture have demanded that progress on problems of pollination keep pace. Knowledge gathered by the early workers in defense of their philosophical arguments is now being put to work in the applied field, but many of the old challenging questions of insect-flower evolution remain unanswered.

Granting the influence of pollinating insects on biological history, what would happen if they should suddenly disappear? It would certainly not mean the end of the flowering plants, because many important plant types have secondarily become adapted to pollination by other agents than insects. The great family of the grasses depends upon cross-pollination by wind or automatic self-pollination within closed flowers. Most of the nut and acorn trees have become adapted to pollen transfer by wind. Even many species within the family of legumes, which is highly specialized for pollination by bees, have come secondarily to depend mainly upon automatic self-pollination within the young blossoms. Peas and beans are familiar examples.

It is likely, therefore, that man could carry on without insects for pollination. The grasses and self-pollinating legumes could form the basis of his agricultural economy. Many of the in- sect-pollinated plants could be maintained by vegetative propagation, although most of them would be barren of fruit. Tomatoes and potatoes he would still have, but he would have difficulty finding substitutes for clover' and alfalfa, and he would have to get along with reduced yields of a variety of crops ranging from cotton to onions. Perhaps the most drastic effects would be in uncultivated areas where a large share of the soil-holding and soil-enriching plants would die out. Furthermore, it would be a bleak springtime if no gay-colored flowers were to grow in the forest glens and open hillsides.

So much for what did not happen and is not likely to happen. Let us examine what has happened or may happen in the future. Probably the insect pollinators will not disappear, and we can go right on eating apples and finding pieces of okra in our vegetable soup.

When the first settlers arrived in America they found no honey bees but there were flowers, fruits, and vegetables in the forests and fields. Furthermore, they were able to produce native American and introduced European crops of many kinds for more than 50 years before honey bees were well established. Native insects were still abundant enough to pollinate the native and introduced insect-pollinated plants. Honey bees were colonized in North America before 1638, but for several decades they were probably more important as honey producers than as pollinators. So long as cultivated areas were composed of small fields surrounded by wild land, native insects were able to handle the pollination job without help from foreign labor. Inevitably, however, as the plow turned under large tracts of sod the native beneficial insects began to disappear. At the same time the available pollinators were spread more thinly over the ever-enlarging orchards and seed fields.

Our native pollinators have suffered the same fate as other forms of wildlife. Certain species have been able to persist and even increase in cultivated areas by taking advantage of road cuts, Outbuildings, eroded areas, and the like for nesting places. But most species have had to retreat into fence rows, stream gullies, wood lots, and waste fields to maintain themselves. In recent years, as clean cultivation and intensive land utilization have become the rule, such havens are fast disappearing within flight range of the crops that need insects for pollination.

The logical question at this point is: How important or necessary are native Pollinator, to our agricultural set-up now that honey bees can be brought in large numbers to any field or orchard? There is no single answer to such a question. An estimated 80 percent of the insect pollination of our commercial crops is performed by honey bees but that figure, which is only an estimate, does not tell us which crops are involved in the 20 percent pollinated by native insects and whether the whole 100 percent is adequate.

Honey bees, unlike most of our native pollinators, collect nectar or pollen or both from a wide assortment of plants. Consequently, there are few crops, whose blossoms are attractive to any insects, that do not hold some attraction for honey bees. Besides, honey bees can be increased and moved about easily. Nevertheless, in the forests and ranges many herbs, shrubs, and trees will always have to depend on native insects for their reproduction. Likewise, many forms of wildlife and range stock depend in whole or in part for food upon the plants or the seeds and fruit that the native pollinators make possible. Bee for bee, various native species are more efficient pollinators of certain crops, such as alfalfa, red clover, and sometimes even fruit, than honey bees. About that, more later; first let us look at the insects themselves.

Thousands of species of insects assist in the pollination of our entire fauna of insect-pollinated plants. They are distributed principally among the bees and wasps, the butterflies and moths, the flies and gnats, and the beetles. Even minute thrips may be important in the self-pollination of certain plants like carrots and some of the composites, which have tiny, closely aggregated flowers. The order Hymenoptera, even without the honey bee, is by far the most important order of insects in the pollination of commercial crops. Flies probably rank next in importance although the moths, which are very abundant, may do more pollinating under cover of darkness than they are given credit for. However, the value of moths and butterflies as pollinators is more often than not offset by the damage they do as larvae. Flies, likewise, are frequently harmful as larvae and many species are carriers of disease as adults.

Among the Hymenoptera, bees, which comprise the superfamily Apoidea, are the most useful pollinators. Some other members of the order, such as the thread-waisted wasps, visit many flowers to partake of nectar, but theirs is a supplementary role on commercial crops and it is difficult to conceive of methods for making better use of them.

THE WILD BEES ( after the honey bee) have rightfully received most of the attention accorded to our insect pollinators.

At least 5,000 species of bees probably exist in North America, many of them still undescribed by the taxonomists. Most of the species are important only to wild plants, but at least several hundred take part in the pollination of cultivated crops. For example, more than 100 species have been reported as visitors to flowers of alfalfa alone.

All but a few of our many species can be grouped in families thus : Colletidae, obtuse-tongued bees; Halictidae, sweat bees and their allies; Andrenidae, mining bees; Megachilidae, thick-jawed bees; Anthophoridae, flower-loving bees; Xylocopidae, carpenter bees, Apidae, honey bees and bumble bees.

The first three families are commonly called short-tongued bees and the last four long-tongued bees, although that is not an invariable distinction.