Locusts

What are locusts

Locusts are a collection of certain species of short-horned grasshoppers in the family Acrididae that have a swarming phase. These insects are usually solitary, but under certain circumstances they become more abundant and change their behaviour and habits, becoming gregarious. No taxonomic distinction is made between locust and grasshopper species; the basis for the definition is whether a species forms swarms under intermittently suitable conditions.

These grasshoppers are normally harmless , their numbers are low, and they do not pose a major economic threat to agriculture. However, under suitable conditions of drought followed by rapid vegetation growth, physiological changes lead to a dramatic set of changes: they start to breed abundantly, becoming gregarious and nomadic (loosely described as migratory) when their populations become dense enough. They form bands of wingless nymphs which later become swarms of winged adults. Both the bands and the swarms move around and rapidly strip fields and cause damage to crops. The adults are powerful fliers; they can travel great distances, consuming most of the green vegetation wherever the swarm settles.[1]

Locusts have formed plagues since prehistory. The ancient Egyptians carved them on their tombs and the insects are mentioned in the Iliad, the Mahabharata, the Bible and the Quran.[2] Swarms have devastated crops and been a contributory cause of famines and human migrations. More recently, changes in agricultural practices and better surveillance of locations where swarms tend to originate, have meant that control measures can be used at an early stage. The traditional means of control are based on the use of insecticides from the ground or the air, but other methods using biological control are proving effective.

Swarming behaviour decreased in the 20th century, but despite modern surveillance and control methods, the potential for swarms to form is still present, and when suitable climatic conditions occur and vigilance lapses, plagues can still occur.[3][4] Locusts are large insects and convenient for use in research and the study of zoology in the classroom. They are also edible insects; they have been eaten throughout history and are considered a delicacy in many countries.

The word "locust" is derived from the Vulgar Latin locusta, meaning grasshopper.[5]




Overview

content below from FAO http://www.fao.org/3/i6152en/i6152en.pdf

Locusts are members of the grasshopper family Acrididae, which includes most short-horned grasshoppers. Locusts differ from grasshoppers because they have the ability to change their behaviour and physiology, in particular their morphology (colour and shape), in response to changes in density, when meteorological conditions are favourable. Adult locusts can form swarms that may contain millions or billions of individuals that behave as a coherent unit (Figure 1). The non- ying hopper (or nymphal) stage can form cohesive masses that are called hopper bands.

Desert Locusts (Schistocerca gregaria) are always present somewhere in the deserts between Mauritania and India. When numbers are low, they behave as individuals (solitarious phase); when high, they behave as a single mass (gregarious phase). Colour and shape are an indication of how they been behaving but may not be a reliable guide as to how they will behave in the future.

When plentiful rain falls and annual green vegetation develops, Desert Locusts can increase rapidly in number and, within a month or two, start to concentrate and become gregarious. Unless checked, this can lead to the formation of small groups or bands of wingless hoppers and small groups or swarms of winged adults. This is called an outbreak and usually occurs within an area of about 5 000 km2 (100 km by 50 km).

An outbreak or contemporaneous outbreaks that are not controlled can evolve into an upsurge if widespread or unusually heavy rain falls in adjacent areas, creating favourable breeding conditions. An upsurge generally affects an entire region and occurs after several successive seasons of breeding and further hopper-band and adult-swarm formation takes place.

 

If an upsurge is not controlled and ecological conditions remain favourable for breeding, locust populations continue to increase in number and size, and the majority of locusts behave as gregarious bands or swarms, then a plague can develop. A major plague exists when two or more regions are affected simultaneously.

Although outbreaks are common, only a few lead to upsurges. Similarly, few upsurges lead to plagues. The last major plague was in 1986–1989 and the last major upsurge, or regional plague, was in 2003–2005. Upsurges and plagues do not occur overnight; they take many months
to develop. During plagues, Desert Locusts may spread over an area of some 29 million km2, extending over or into parts of some 60 countries.

The Desert Locust has the potential to damage the livelihoods of one tenth of the world’s population. Recent increases in cultivated areas on the edges of deserts in northern Africa, the Near East and South-West Asia make the Desert Locust a threat to the livelihood, income and food source of local populations. Swarms are often tens of square kilometres in size. A swarm of 1 km2 eats the same amount of food in one day as 35 000 people. A swarm the size of Bamako (Mali) or Niamey (Niger) can consume what half the population of either country would eat in a single day.

The Desert Locust plague of 1986–1989, the subsequent upsurges in the 1990s and the regional plague in 2003–2005, drew the world’s attention to the threat they pose to the food security of the affected countries, especially in the developing world. This situation calls for an integrated approach to understanding the conditions that lead to the locust build-up and their migration so that effective solutions can be developed for controlling damage.




Lifecycle

Life cycle

A Desert Locust lives about three to ve months, although this is extremely variable and depends mostly on weather and ecological conditions. The life cycle comprises three stages: egg, hopper (nymph) and adult (Figure 2). Eggs hatch in about two weeks, depending on temperature (the range is 10–65 days). Hoppers shed their skins ve or six times, each time growing in size. This process is called moulting and the stage between moults is referred to as an instar. Hoppers develop over a period of about 30-40 days; adults mature in about three weeks to nine months but more frequently from two to four months, depending on environmental conditions, mainly temperature. If conditions are dry and cool, adults may remain immature for six months.

Adults do not moult and therefore do not grow in size but gradually increase in weight. An adult locust can eat its own body weight every day, about 2.5 g. Adults that can y are initially sexually immature, but eventually become sexually mature and can copulate and lay eggs. Solitary individuals always remain somewhere in the desert, ready to mate when conditions are favourable.

Eggs

Laying

Eggs are usually laid in areas of bare sandy soil and require previous rainfall. Generally, the female will not lay unless the soil is moist at about 5–10 cm below the surface. In soft sandy soils, females have been known to lay when moisture is found only at depths below 12 cm. Before laying, the female will often probe the soil, inserting the tip of her abdomen to determine if there is enough moisture.

The female lays eggs in batches called pods. The eggs look like rice grains and are arranged
like a miniature hand of bananas. The pods contain fewer than 80 eggs in the gregarious phase and typically between 90 and 160 in the solitarious phase. Swarms often lay egg pods in dense groups, with tens and even hundreds of pods per square metre. Laying occurs in only a small number of the apparently suitable sites. This behaviour, as well as an agent added to the egg pod foam when adult females are crowded, will help induce gregarization of the next generation.

The number of egg pods a female lays depends on how long it takes for her to develop a pod and how long she lives. An average of two pods per female is the norm. Because of natural mortality, not all the eggs hatch and, of those that do, not all reach the adult stage. In optimal temperature and habitat conditions, a single female can produce up to 16–20 viable locusts in a single generation.

Development and incubation

The Desert Locust nearly always lays her eggs in soil that is wet enough to allow the eggs to absorb suffcient moisture to complete their development. If eggs are laid in dry soil, they desiccate (dry out) unless rain falls soon afterwards. The rate of development is exclusively a function of the soil temperature at pod depth (Figure 3). There is a reasonably good relationship between soil temperature and screen (air) temperature so rates of egg development can be predicted satisfactorily from air temperatures and even from long-term mean values since temperatures do not vary greatly between years for a given place and time of year in most of the breeding areas. However, there can be exceptions to this, notably during the winter, when the weather may be unusually warm, allowing development to continue.

Mortality

The proportion of eggs that survive to hatching varies widely according to habitat conditions and the presence of egg parasites and predators. While eggs can dry up, especially if exposed
by wind, and can also be destroyed by persistent ooding, such events are uncommon. High mortality may occur if soil temperatures are above 35°C. Estimates of total losses vary from about 5% to 65%.

Hoppers

Hoppers immediately moult until the rst instar. They then pass through ve instars (sometimes six in the solitary phase), shedding a skin (moulting) between each. The development from eggs into hoppers (wingless larvae or nymphs) is a function of temperature. The hopper development period decreases with increasing daily air temperature from 24°C to 32°C (Figure 4). The correlation with air temperature is less clear than with eggs because the hoppers can control their body temperature to a considerable extent by basking or seeking shade.

When solitarious hoppers increase in number, their behaviour changes, they become concentrated and can form groups. Grouping often occurs in open, less uniform, habitats, where there are patches of relatively dense vegetation separated by large areas of bare soil.

Bands

As hoppers continue to concentrate, they become more gregarious and the groups fuse to form bands. During warm and sunny days, hopper bands follow a pattern of behaviour alternating between roosting and marching throughout the day. On overcast days, bands usually do not move very far. For example, measurements for predominantly fourth instar bands range from about 200 m to 1 700 m in a day. If the vegetation is very dry, bands may continue moving at night in search of green vegetation. The band usually maintains a constant direction during a day; even a major obstruction is not always su cient to change its path. The heading is often, but not always, downwind.