Unit 10 in Entomology. [1] This is war. Unit 10: insect and plant interactions.

Size: px

Start display at page:

Download "Unit 10 in Entomology. [1] This is war. Unit 10: insect and plant interactions."

Robyn Gregory
6 years ago
Views:

1 Unit 10 in Entomology [1] This is war. Unit 10: insect and plant interactions. [2] What does the Cold War and insect/plant interaction have in common? [3] Believe it or not, plant/insect interaction is very much like the Cold War and the accompanying arms race between the United States and the former Soviet Union. However, plant/insect interaction began much earlier than the beginning of the Cold War in One has to go back approximately 400 million years ago, when plants and animals first appeared on earth. Plants evolved onto land before insects, and when insects followed, they found plants to be a succulent food source. To keep from being eaten, plants had to combat insects with a variety of defenses which will be discussed within this unit. Read on to find out how this war turns out. [4] The objectives of this unit are to explain the connection between the rise of flowering plants and the expansion of the number of insect species. Using examples, explain how insects can protect and help propagate plants. To describe the major ways in which insects gain nutrition from the plant and to describe the ways plants protect themselves from herbivorous insects. [5] Plants and insects have both benefited and harmed one another throughout the ages. Many plants provide sweet nectar as food to bees, wasps, moth, flies, butterflies, etc. In return, these insects can carry pollen caught on their bodies to other plants. This aids their benefactor plants in the reproductive process. On the other hand, harm may come to the plant as insects prey on the plant for food. Those nice tender leaves are just too tempting and provide food for hungry caterpillars, the main consumer of leaves. Plants can be defoliated in short order by these feeding machines. In fact, more than half of all insects are plant feeders. It is not uncommon for insects to consume about 8.8% of the leaves in an entire forest. The major concern happens when there is an outbreak, like the biblical plagues of locusts. There are outbreak conditions in which insects can defoliate entire plants or fields of crops, resulting in major economic loss. Throughout the rest of this unit you will learn about both kinds of plant/insect relationships, beneficial and harmful. [6] Plants and insects have had a long, intimate relationship. Early in the world s history, it is believed that plants evolved onto land first and then insects followed. The insects pressured the plants by feeding on them, so the plants had to develop defenses to overcome the feeding. The insects had to then evolve ways to overcome the defenses, and so forth and so on. This pressure back and forth accelerated evolution of these two groups of organisms. For example, insects may have first eaten plants on the ground. This selected for plants that thrived high up in trees. However, insects that evolved wings could then take advantage of these higher plants for forage. Eventually some insects became more and more selective and specialized regarding the kinds or parts of the plants they ate, such as being specialized to suck the nectar out of flowering plants. [7] Most flowering plants, known as angiosperms, can only reproduce if their pollen is spread to another plant. The pollen must land on a sticky stigma. This is usually the part of the flower that sticks out of the flower head. Once the pollen has landed on the stigma, the pollen will eventually turn into seeds that develop deep inside the flower's ovary. When the seeds are ready, they are released and the plant propagates its species. In the beginning, flowering plants probably relied on the wind to spread their pollen, but this can be inefficient because a lot of pollen must be released in order to insure pollination. Insects became attracted to the sweet smelling nectar in the flower. As they fed, they inadvertently collected pollen on their antennae, their body, their hair or legs. This pollen would fall onto the next plant

2 one of these insects fed on. It's much more efficient to produce only a few pollen grains that an insect will carry to the next plant than to produce a lot of pollen, hoping the wind will carry at least one or two of them to a mate. Today certain kinds of flowers are only attractive to specific pollinators. Bees, for example, are generally attracted to flowers that are bright purple or blue. Moths are attracted to flowers that open at dusk and are a pale color so they are easier to see in dim twilight. Flowers that attract butterflies often have a landing platform for these insects to stand as they feed on the nectar. Be sure to read your textbook readings and answer your study guide questions regarding flowering plants. [8] People tend to view insects more of pests that destroy agricultural crops. However, many insects are beneficial and help protect and propagate plants. Interestingly, as the insect helps the plant, the insect is benefited as well. This is called a mutualistic relationship, which means that both organisms involved in the relationship will benefit. For example, bees get nectar from a plant, but carry pollen to the next plant they land on. The bee gets fed, and then the plant is helped in the reproductive process. Besides helping plants spread pollen, there are many other mutualistic relationships between plants and insects. Some insects help protect the plant from predators or spread the plant's seeds. Some of these relationships have become so interwoven that the plant can actually not survive without the insect and the insect cannot survive without the plant. An example of this is the Yucca plant and the Yucca moth. You will read about that a little bit. First we'll discuss some examples of mutualism. How many plant/insect mutualistic relationships can you think of? [9] There are three types of insect/plant mutualistic relationships we will discuss: seed dispersal, protection, and reproduction/pollination. Seed dispersal: ants pick up seeds off the forest floor to carry back to their nest. Once inside the nest, the ants chew the ends of the seeds for food. This action promotes germination in the seeds at a later date. This is how we get many of our wildflowers. Protection: the cecropia plant provides shelter and food for the ant, and the ant protects the plant from herbivory. And lastly, reproduction/pollination: in order for most plants to make seeds, pollen from one plant must be transferred to another plant. Butterflies, bees and other insects often suck sweet nectar from flowers for food and pick up pollen as they do so. So you can see these relationships in action, take a few minutes to watch the videos Ants in North America: The Wildflower Story, Cecropia Plants and Azteca Ants, Butterfly Landing Platforms and Hover Fly. [10] (Ants in North America video) Up to half the wildflowers in America woodlands are planted by ants. Spring beauties, violets, trillion and many others encourage ants, not birds or mammals to sow their seed. Some produce pods which fall and burst open, while others lay their seeds directly on the ground for the ants to distribute. But this is no free service, the ants are enticed by the white flesh on the seed, whose taste they find irresistible. Unselfish as ever, the ants carry these treats back home to share. Their nests is just below the ground where the ants tend their hordes of hungry young. These tasty prizes attract workers chained to underground duties, where their flesh is a delicious kind of fruit. The strawberries, apples, and melons of the ants. Chewed seeds are dumped in compost heaps inside the nest. They look damaged, but they are perfectly healthy and are ideally placed to germinate. Ants are now known to disburse thousands of different plants around the world for such prize. [11] (Cecropia Plants and Azteca Ants video) The South American plant cercropia has one of the closest relationships of all with ants. Its hollow stem gives them a home and it provides a daily crop of little white nodules to feed them. The ants harvest this crop rapidly and carry it back into the nest. The starchy nodules are produced exclusively for the ants, who eat very little else. Inside the stem, a huge queen lives in safety, surrounded by piles of grubs. No home could be better. But why should the plant be so hospitable? When it s threatened, the reason becomes clear. These tiny ants which are pouring off the tree onto my hand are the cercropia plant s private army. Scientists have very appropriately given them the name Azteca, after the Indian warrior nation. Even though they re only a millionth my size, they re able to drive me off pretty quickly with their sharp little jaws, but their real importance is the protection they give against the plant s natural enemies. Many insects, like this beetle, feast on cercropia leaves, unless they meet resistance. But a plant with an

3 army of ants to defend it, is safe. Buying protection with ant food pays off in a big way. Cercropia, with a lot of guards, thrive. [12] (Butterfly Landing Platforms video) And this is it a red orchid. It s got a vessel at the back here, which is full of nectar, and the butterfly knows exactly where it is. Orchids have a special way of rationing their pollen. They parcel it up into a pair of packets called pollinia. Here, they re hidden at the back, behind those two white knobs at the top. The white lump at the bottom is the stigma. Like other orchids, this one provides its visitors with a landing platform. There s only one position in which a butterfly can stand in order to reach the nectar. As the mountain pride pushes forward to get the last drop, a pad from one of those white knobs is glued onto its back legs, and as the butterfly moves again, it pulls out the yellow pollinia. When it visits its next flower, it must, once again, alight on the landing platform in precisely the same position and that now causes the pollinia to be dragged across the stigma. That, too, is sticky. The orchid has been fertilized. [13] (Hover Fly video) This small iris grows in South Africa. Each flower has a hugely elongated tubular base. The tiny entrance to it is indicated with exemplary clarity and absolute accuracy by these white arrows on the petals. And here is the only tongue that can reach that nectar. It belongs to a hover fly. It s not so much a tongue as a fine tube, but in proportion to body length, it is one of the longest feeding implements in the animal kingdom. The clear markings are particularly important, because the tongue is so long that even in the lightest wind, the end blows about. An extra thrust is needed to get the last drop. [14] For a few more examples, let's have some textbook readings. Some fig trees can only be pollinated by the fig wasp. Be sure to read about this in your text. There's also a really great article about bees that are sometimes commercially used to pollinate plants. You can read this on the web at the USDA website. [15] An example I mentioned before was the Yucca plant and the moth. The state flower of Mexico owes its existence to insects. The Yucca plant, which Native Americans used to make baskets, has a mutualistic relationship with the Yucca moth. These moths are specific pollinators for the plant and the plant cannot survive without them. The male and female Yucca moth emerge from their cocoons under the soil in the early spring and congregate on flowering plants where they mate. The female Yucca moth uses her maxillary palps to place a small bundle of pollen into the bowl shaped stigma of the plant s flower. This pollination is important because the moth only lays eggs during this time so the larva can feed on the newly formed seeds. The males do not have these prehensile appendages, therefore only the female Yucca moth can pollinate the delicate flower. This relationship is absolute for the survival of both the moth and the plant. In fact, Yucca plants in the old world will only produce seeds by hand pollination because the moth has virtually gone extinct. You can read more about this in your text. [16] Another interesting plant/insect relationship is the formation of a gall. Galls occur on a wide variety of plants. These growths may be a result of fungi, bacteria, nematodes or mites, but insects are the prime cause. Gall forming insects include aphids, phylloxerans, psyllids, midges and cynipid wasps. Of the more than 2000 gall producing insects in the United States, 1500 are either gall gnats or gall wasps. About 80% of the gall wasps produce galls specifically on oak trees. In fact, 60% of all known insect galls occur in the oak family, and 30% occur in the daisy, rose and willow families. These growths are called galls because they contain large amounts of tannin, which has a very bitter taste. Long ago they were known as gall nuts because they tasted bitter as gall. [17] Plant galls are abnormal growths of plant cells formed as a response to a stimulus. Insect stimuli include egg laying, or larval or nymphal feeding. In the spring, before the leaves are fully developed on a plant, insect eggs are laid in the leaf or stem. Gall production is believed to result from the reaction of the cambium and other meristematic tissues to stimuli produced by the larva to cause the abnormal growths.

4 The immature insects can often be found in a cell or cells within the developing gall. After a brief period of cell growth, all development stops. The insect becomes enclosed by the gall and feeds only on the gall tissue during its development. Small holes on the outside of the gall indicate that the adult insects have emerged. [18] Galls are found most commonly on the stems and leaves but also occur on trunks, flowers, fruit, leaf shoot terminals, petioles and roots. Each gall forming insect produces a gall that is characteristic of that particular insect. Some galls may be 2 inches in diameter while others are so small they are barely noticed. They occur in almost every conceivable form and color and their shapes range from spheres to tubes. The surface may be smooth, hairy or covered with spines. Gall susceptibility varies greatly between species in the same location. This is probably due to the general condition of the particular plant and its natural resistance. Galls seem to cause a lot of concern to the general public. They usually don't cause serious harm to the plant. Most ornamental plants and trees are not apparently injured even by relatively large numbers of galls. Some people actually look for plants which are susceptible to gall formation because of their unusual and aesthetic appearance. After formation, it is impossible to eliminate the galls or the pests, for that matter, with insecticides because the pests are enclosed and well protected inside of the gall. Those that occur on the leaves will drop off with the leaves, but those occurring on the trunk, roots and stems may persist for several years. If the tree or ornamental plant is unhealthy or under stress, it will be damaged to a greater extent than healthy ones. Fertilize, irrigate, prune and carry out other approved practices to keep them healthy. [19] Using insecticides to control insects that form galls is usually not practical because damage is not significant most of the time. Parasites normally build up populations and suppress the populations of gall makers before serious injury can occur to the plant. If treatment is required, correct timing of the application is essential to control the adult pests before the gall is ever formed. Environmental contamination and expense are also involved, especially on larger trees. Some galls that occur on the stems and leaves may be hand-picked and destroyed. Take a look at some of the photos from the gall gallery to see how interesting galls can actually be. For more information, please read your textbook reading about plant galls. [20] During the previous sections you have learned about the beneficial insect/plant relationships. But we re all too familiar with insect pests that are not beneficial and plague farmers. Grasshoppers, caterpillars, locusts and many others, in outbreak conditions, can defoliate a plant or an entire field crop causing severe financial loss to the farmer. About 20% of crops are lost annually to insects. During this section you will learn about how plants defend themselves against these insect predators. Some plants have evolved tough bark, slippery leaves, or noxious or distasteful chemicals contained within the plant body. These methods of defense can be grouped under two categories: physical and chemical. Tough bark and slippery leaves are examples of physical defenses which relate to structures on the plant that deter the insects. Noxious or distasteful chemicals are the chemical defenses which plants produce. To complete this section, be sure to read your textbook readings and answer the questions in your study guide. [21] Review Quiz [22] In conclusion, you've seen how insects and plants have evolved together through time. Plants colonized first and insects followed. It didn't take long for the insects to begin eating on the plants and the plants had to find ways to adapt to this. They developed physical and chemical defenses to ward off insect predators. But insects then developed ways to get around these defenses. Bark beetles can bore through bark, for example. Some plants contain chemicals called protease inhibitors that mess up an insect s digestive system once ingested. And the war between insects and plants continue. However,

5 there are several beneficial insect/plant relationships. Ants eat fleshy portions of wildflower seeds and this prepares the seeds to germinate. Azteca ants eat starchy nodules secreted by the Cecropia plant, and in return, the ants fight off any of the plant's predators. The fig wasp uses the fig fruit as an incubator for its larval stage and the fig is therefore pollinated. Bees, beetles, hover flies, and other insects are also pollinators of various plants. Pollination is a more efficient way to spread pollen rather than using the wind. It's quite fascinating to learn about these specialized relationships. Orchids even have adapted evolved platforms for butterflies to use when sucking the nectar out of their flowers. [23] Now when you hear about insect/plant relationships, hopefully you won't just think about all the many agricultural pests that are devastating to crops. Now you can think about all the other beneficial insects that play a vital role in the ecosystem. This concludes unit ten.

How Does Pollination Work?

How Does Pollination Work? What is pollination? What is pollination? Pollination the transfer of pollen from the anther of one flower to the stigma of another flower Fertilization occurs when the male