Insect Pests of Vegetable, Ornamental and Spice Crops (2+1)

Locusts

1. Locusts

• Locusts are the swarming phase of short-horned grasshoppers.
• They can breed rapidly under suitable conditions and subsequently become gregarious and migratory.
• Nymphs form bands and adults swarms.
• They can travel great distances, rapidly stripping fields and greatly damaging crops.
• The origin and apparent extinction of certain species of locust (some of which were 150 mm in length) is unclear

Important species

• Desert locust (Schistocerca gregaria), probably the best known owing to its very wide distribution
• Migratory locust (Locusta migratoria)
• Red locust (Nomadracis septemfasciata)
• Australian plague locust (Chortoicetes terminifera)
• American desert locust (Schistocerca americana)
• Brown locust (Locustana pardalina)
• Moroccan locust (Dociostaurus maroccanus)
• Rocky Mountain locust (Melanoplus spretus) in North America had some of the largest recorded swarms, but died out in the late 19th century.
• Bombay locust, Patanga succincta (Acrididae : Orthoptera)
• Sporadic grasshopper, Oxya nitidula
• Surface grosshopper Chrotogonus trachypterus

Identification:

• Though the female and the male look alike, they can be distinguished by looking at the end of their abdomens.
• The male has a boat-shaped tip, while the female has two serrated valves that can be either apart or kept together.
• These valves aid in the digging of the hole in which an egg pod is deposited.
• Desert locusts can measure roughly 75 millimetres (3.0 in) in length.
• Some grasshopper species such as the Senegalese grasshopper Oedaleus senegalensis, and the rice grasshopper Hieroglyphus daganensis also display locust-like behaviour and change morphologically on crowding.

Swarming behaviour and extinction:

• There is no taxonomic difference between locust and grasshopper species.
• The term "locust" is used for grasshopper species that change morphologically and behaviourally on crowding, to form swarms or hopper bands (of immature stages).
• These changes, or phase polymorphism, were first identified by Sir Boris Petrovich Uvarov, who studied the desert locust, whose solitary and gregarious phases had previously been thought of as separate species.
• Charles Valentine Riley and Norman Criddle were also involved in the understanding and destructive control of locusts.
• Research at Oxford University has identified that swarming behaviour is a response to overcrowding.
• Increased tactile stimulation of the hind legs causes an increase in levels of serotonin which causes the locust to change colour, eat more, and breed more easily.
• The transformation of the locust to the swarming variety is induced by several contacts per minute over a four-hour period.
• It is estimated that the largest swarms have covered hundreds of kilometers and consisted of many billions of locusts.
• Plagues of locusts appear in both the Bible and the Koran, including one of the biblical plagues, where locusts ate all the crops of Egypt.
• Six stages of development, from newly hatched nymph to fully winged adult.
• The extinction of the Rocky Mountain Locust has been a source of puzzlement.
• Recent research suggests that the breeding grounds of this insect in the valleys of the Rocky Mountains came under sustained agricultural development during the large influx of gold miners destroying the underground eggs of the locust.
• In a paper in the 30 January 2009 edition of the AAAS magazine Science, Anstey & Rogers et al. showed that when desert locusts meet up, their nervous systems release serotonin, which causes them to become mutually attracted, a prerequisite for swarming.

Locust swarms

• Swarming grasshoppers have short antennae and hearing organs on the abdomen.
• As winged adults, flying in swarms, locusts may be carried by the wind hundreds of miles from their breeding grounds; on landing they devour all vegetation.
• Locusts occur in nearly every continent.
• The migratory locust (Locusta migratoria) ranges from Europe to China, and even small swarms may cover several square miles, and weigh thousands of tons.
• They eat the equivalent of their own weight in a day
• Flying at night with the wind, may cover 500 kilometres
• The largest known swarm covered 1,036 square kilometres comprising approximately 40 billion insects.

Damage

• Adult and nymphs cause the damage
• Gregarious and voracious feeders
• Eat up the entire vegetation
• In spite of some expensive control measures, the damage caused to crops by locusts during 1926-31 cycle was estimated to 100 million.
• They also climb on walls, invade kitchens, store rooms thus causing nuisance

Life cycle

• Incubation period depends upon the soil conditions, temperature and moisture.
• Eggs laid during Feb-March hatch in 3-4 weeks and those laid during May-June hatch in 12-15 days.
• In very dry soils the eggs may remain unhatched until rain shower occurs.
• Nymphal stage lasts for 6-8 weeks during spring and 3-4 weeks during summer.
• There are two broods in a year.

Salient features

• Eggs are laid in soil in egg pods.
• Each female can lay up to 11 egg pods, each pod containing up to 120 eggs.
• Before egg laying each female with the help of its ovipositor, bores a 5-10 cm deep hole into the loose soil.
• Having laid a pod, female secretes a frothy material over the eggs, which hardens on drying and makes the pod water proof.

Management

• Spreading poisoned food among the bands is very effective
• It is cheaper to spray spray lindane (2%) from aircraft over the insects or the vegetation on which they feed.
• Lindane dust (2.0%) on crops be dusted
• Adults can be beaten to death with thorny stick, broom and swept to gather and burry.
• Eggs are laid in a well defined area, a trench around it may be dug to restrict the movement of young hoppers.
• Dig trenches in front of the moving army to stop their further advancement
• During night locusts rest on bushes where they can be burnt with flame throwers
• Number of birds also eats locusts, Indian mynah is the common
• Dried fungal spores sprayed in breeding areas pierce the locust exoskeleton on germination and invade the body cavity, causing death.
• The fungus is passed from insect to insect and persists in the area, making repeated treatments unnecessary.
• The International LUBILOSA Programme was set up to find methods of nonchemical control of locusts. Not only did it successfully develop the mycoinsecticide 'Green Muscle', but over its 12-year period, programme staff also contributed a large number of scientific papers on subjects as diverse as fungal production, (bio)pesticide application, socio-economics and thermal ecology.