Weevils

DESCRIPTION – WEEVILS (SITOPHILUS)

Weevils are dark brown beetles which have the characteristic elongated snout.  They are

2.0 – 3.5 mm long.

LIFE CYCLE / HABITAT

Weevils are pests of whole grain.  Females lay their eggs into holes bored into whole grains, the larvae hatch inside the grain and feed on the inside.  Pupation also occurs inside the grain.  The developed adult feeds on the inside for a few days before emerging.  Adults live for 2 to 3 months.  For this reason they are referred to as Internal Feeders.  Weevils infest whole small grains such as wheat, oats, barley, rice, etc.  Both the larvae and the adult are viewed as pests.  When inspecting closed bags of product, thumping the outside of the bags will cause the adults to appear on the surface.

Distribution

Worldwide. Asia, Africa, North, Central and South America, the Caribbean, Europe, Oceania. It is recorded from Australia, Fiji1 and New Zealand. Because of previous taxonomic difficulties in distinguishing this weevil from Sitophilus zeamais, the reference to Sitophilus oryzae in Fiji needs confirmation.

Hosts

Rice, maize, sorghum, wheat, and cassava. Both greater and lesser grain weevils attack cassava. The weevil also attack processed products, such as pasta. There is evidence (from Indonesia) that Sitophilus zeamais is more common on milled rice, and Sitophilus oryzae is more common on paddy (rice in the husk before processing). Sitiphilus oryzae is also common on wheat.

 

Symptoms & Life Cycle

The adults attack sound grain, and both adults and larvae feed inside them, leaving large cavities and emergence holes (Photo 1). The adults also attack damaged grains (Photo 2). During large infestations, heat and moisture are produced, leading to colonisation by moulds and mites.

Females lay 300-400 eggs, singly, in holes chewed in the grains and covered with a gelatinous substance. On average, four eggs are laid a day for 4-5 months. The eggs hatch into white, legless larvae, which remain inside the grain, and pupate there. Adults are 3-4 mm long, reddish-brown to black (Photos 3&4), with four light-reddish to yellowish spots at the corners of the wing cases (Photo 5). At 30oC and 70% RH, the life cycle takes 25 days. Adults live for several months to a year.

The weevil is not a strong flyer, less so than Sitophilus zeamais, and spread to new locations is by adults and larvae on and inside consignments of grain.

Impact

The weevil is considered a very destructive pest of stored grain, rice, maize, barley and wheat. Attacks from Sitophillus oryzae can start in the field, when the moisture content is about 20% (although Sitophilus zeamais is the stronger flier and more likely to infest crops before harvest). Commonly, loss of weight is up to 5%, but severe infestations increase the losses up to 40%. Secondary damage is caused by moulds, insects and mites.

Detection & Inspection

Look for the reddish to nearly black weevils, and holes in the grain. Look for the faint yellowish or reddish spots on the corners of the wing cases (Photo 4). The snout is long, about 1 mm. Note that identification of Sitophilus oryzae is difficult because of its similarity with Sitophilus zeamais, and needs to be done by a taxonomist. Males are said to produce a pheromone which is attractive to both Sitophilus oryzae and Sitophilus ziamais.

Management

CULTURAL CONTROL
It is most important to dry the grain properly, to keep the storage area clean, and to monitor the grain often and regularly.

Before storage:

  • Harvest maize as soon as it is mature.
  • Dry the grain as soon as possible after harvest, and repeat during storage if necessary. Moisture levels should be about 12% (most stored product pests required moisture levels of 13-18% to reproduce).
  • Keep grain storage rooms, sheds, houses, or “cribs” clean. Remove old kernels, and sweep floors, walls, doors, and vents to collect seeds, grain powder and dust, and burn them before storing the new harvest.
  • Collect and dispose of spilled grain in or around the storage area.
  • If using bins, remove any grains that remain (and, if possible, spray with insecticide). If using sacks, do not reuse those that stored previous harvests.
  • For small amounts, store maize, rice, wheat, etc. in plastic containers

During storage:

  • If there is an infestation, locate it by putting sticky traps around the room or warehouse where it occurred. Usually, the greater the number of weevils the nearer the source. Locate the source and destroy it by wrapping the foods in heavy plastic bags or in sealed containers, and burn or bury deeply in the soil. Note, toys can sometimes be filled with grain and act as a source of infestation; so too can dried flowers.
  • For small infestations, freezing for several days and then heating for 24 hours is affective. Perhaps a method to avoid if seed is for growing.
  • Sieving has been used as a method of removing adult weevils, but it is very labour intensive.

 

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