Samoa guide. 35. Common pests and diseases of local agricultural plants

Banana bunchy top virus (laufeti'iti'i-vaelusi). This is one of the most serious diseases of bananas. The virus can be so devastating that some regions of the world affected by it, like parts of Hawaii, are no longer able to produce bananas commercially. For unknown reasons, places like American Samoa are less severely affected. It may have something to do with the small insect, an aphid (afiti), that carries the disease from plant to plant. Perhaps these aphids are less effective virus carriers or they are not very abundant here. Banana plants infected with the virus produce small, narrow leaves with yellow, tattered edges bunched together, thus the name “bunchy top”. When one plant becomes infected, the virus spreads to the rest of the plants growing from the same “root” mat.

The adult beetle (Oryctes rhinoceros) is amazingly large -- almost two inches long -- black and shiny, with a horn on its head, like a rhinoceros. Although it's hard to visualize, the horn apparently helps them feed by stabilizing the beetle while it moves its jaws up and down, scraping a hole in the tree. The beetles do not eat the wood but live on sap from the damaged plant cells. The female lays one egg on the ground, usually in dead coconut trees, logs, piles of compost or debris. The young larvae are white with a brown head. They grow almost four inches long before they pupate and become adult beetles.

Rhinoceros beetle (manuainiu). These giant beetles cause a distinctive diamond-shaped pattern cut out of coconut leaves (fronds). The adult beetle burrows into the tops of coconut trees, then down toward their center. As it burrows, it cuts through the new, unopened leaves, so when the leaves open later, large wedges of the leaves are missing. If the midrib is cut, the leaf may break on windy days. While this may weaken some leaves and make them look unsightly, we don’t know if the damage is serious enough to reduce the number of coconuts produced by the tree.

The mature larva falls to the ground, forms a pupa which is the resting stage when the caterpillar changes shape and grows wings, and afterwards emerges as an adult moth that flies off to lay more eggs. The complete life cycle, eggs to adult, takes only about 30 days. In American Samoa there are several friendly insects, mainly small wasps, that attack and kill armyworms. But if the armyworms are too numerous, the wasps cannot control them and considerable damage to the plant may result. Unfortunately, spraying the armyworms with pesticides also kills the helpful wasps.

Burrowing nematode (nematota). This pest attacks the roots of banana plants. Nematodes are small, worm-like residents of soil and water. Most of them feed on dead plants and other tiny organisms, but some attack the roots of living plants. The burrowing nematode (Radopholus similis) enters young banana roots and eats the insides. When large numbers of nematodes feed on a plant, the roots are so damaged that the plant can fall over. This is one reason why banana plants topple over during strong winds or rain, or when they bear a heavy bunch of fruit. Damaged banana plants usually produce fewer and smaller fruits than healthy plants. Banana varieties grown in American Samoa have no resistance to this pest. The best way to control the nematodes is to remove their source of food (the banana root mat) for one year or flood the land, making it unsuitable for the nematode.

Gazing up at the beautiful mountains on our islands, we see hillsides of lush green tropical foliage -- a picture of botanical health. That's true for the most part, but if we look closer, we notice that some plants can get sick and die, or they can be injured when insects, animals, strong winds or landslides damage their leaves, stems or roots. Sometimes insect pests or diseases spread fast, injuring or killing many plants over a large area. Then we take notice, particularly if we were growing those plants to eat. Here are five common plant pests and diseases that can cause serious damage to local agricultural plants.

Armyworms (anufe-ailatalo). These insects are more appropriately called cluster caterpillars, because they are not worms at all. They are the juvenile stage of a thick-bodied grayish-brown moth (Spodoptera litura) that flies about at night. It is a serious pest for many crops and may occur throughout the year. The caterpillars are often found on taro leaves in small numbers and if found soon enough, they can be picked off by hand before they eat too much of the leaves. Sometimes there are so many that they can't be removed fast enough. As with taro leaf blight, this has a major impact on the plant, because energy is taken from the corm (i'o ole talo) to replace damaged leaves, so the harvest is reduced. The female moth lays 200 to 300 eggs in a cluster during the night. After the eggs hatch, the young larvae feed together, eating larger and larger holes in the leaves as they grow, until only the thick ribs of the taro leaf remain.

Infected plants stop producing bananas and become shorter and shorter until the plant finally dies. During this time, any aphid feeding on an infected plant can carry the virus to an uninfected banana plant. Another way the disease is spread is by removing suckers from an infected root mat and planting them elsewhere. There is some concern in American Samoa that the disease could begin to spread more widely, but there is a safe way to control bunchy top with a small application of a common herbicide [contact Land Grant for details].

Taro leaf blight disease (lega, laumu). Many people became acutely aware of this plant disease in 1993-94 when it destroyed most of the Samoan taro crop. The disease is caused by a fungus (Phythophthora colocasiae) that has been in Fiji for many years, but we don't know how it came to American Samoa. Perhaps it arrived here on taro tops (tiapula) from another island, or its fungal spores blew in on the winds of Cyclone Val in 1991. Taro leaf blight is most severe when the weather is cool and wet. Rain splashes spores of the fungus into the air, which are then blown by wind from plant to plant. The expanding brown spots made by this fungus cause leaves to die faster than normal, so the plant must use its energy to make more leaves instead of storing the energy in the taro corm (i'o ole talo) that we want to eat. Farmers are now growing taro varieties from Micronesia that are more resistant to the fungus. The Palau taro, for example, still gets the disease, but less severely, so the leaves live longer and corms are of normal size.