Maize Weevil (Sitophilus zeamais)

The most damaging storage pest of maize in tropical Africa and the Americas. Maize weevil infestations begin in the field (adults oviposit in maturing grain before harvest) and explode during storage, causing 20-40% weight loss within 3-6 months. For smallholder farmers who depend on stored maize for food security, weevil damage is a critical threat to household nutrition and income.

Adults are small (3-4 mm), dark brown to black weevils with a long snout (rostrum) and four reddish-brown spots on the elytra (wing covers). They are strong fliers (unlike grain weevils). Larvae are white, legless grubs that develop entirely INSIDE the grain kernel — invisible until the adult chews its way out, leaving a characteristic round exit hole.

• 1

  Round holes in grain kernels indicate adult weevil emergence

• 2

  Weevils have a distinctive long snout (rostrum) — compare with grain beetles which lack a snout

• 3

  Four reddish-brown spots on wing covers distinguish S. zeamais from the similar rice weevil (S. oryzae, which has four lighter spots)

• 4

  Fine powder (frass) among stored grain is a sign of active infestation

• 5

  Heating grain: place hand into grain bulk — if noticeably warm, heavy infestation is present

• Round exit holes in stored grain kernels (adults emerging)

• Fine powdery flour (frass) in grain storage containers

• Heating of grain bulk from metabolic activity of dense infestations

• Weight loss and reduced germination of stored seed

• Musty smell from heavily infested grain

• Live adult weevils visible on and in stored grain

Maize Weevil (Sitophilus zeamais) can attack 4 crop species.