GRASS LEAFTIER: Marasmia trapezalis (Guenee), Pyralidae
ADULT: These narrow, 3/8 in. long moths have a light brown body. The light brown, triangular-shaped front and hind wings are similarly marked with dark brown lines. Leading edges and ends of wings are also marked with a dark brown pattern. The lower mouth parts (labial palps) extend 1/16" in front of head.
EGG: The flat, translucent, oval-shaped eggs are deposited singly on the oldest leaves within whorls of pretassel-stage plants. Larvae emerge in 3 to 5 d.
LARVA: The shiny, non-descript, medium green to medium brown caterpillars have a medium brown, flattened head and grow to 1 1/8 in. Larvae have three pairs of true legs and five pairs of prolegs. Larvae are usually found feeding between tied leaves in the whorl. Leaves are strongly tied with silken bands spaced 3/8 to 3/4 in. apart. Larvae complete development in 14 to 17 d.
PUPA: Narrow, medium brown pupae are found within loose silken cocoon among tied leaves or plant trash. Adults emerge in 6 to 8 d.
GENERATION TIME: 23 to 30 d.
DAMAGE: M. trapezalis was originally described from malojilla or carib grass (Eriochloa polystachya) in Cuba. This grass was introduced for livestock forage throughout the states and countries bordering the Gulf of Mexico. The insect has similarly been collected from Florida through Texas and south through Mexico into the rest of Central America. Besides corn, the insect also attacks rice and sugar cane. In south Florida, the insect is most common in Fall-planted corn. Larvae web together and feed upon leaves emerging from the whorl. Webbing is so tight that it can cause distortions of emerging leaves as they try to expand out of the whorl. Leaf feeding results in small holes and large areas of leaves with window paning. Feeding damage can lead to leaf necrosis distal of feeding damage. Plants less than 2 wk old can be stunted and their development severely slowed by this insect. Adults prefer to deposit eggs on young corn, but will chose plants up until at least 2 wk prior to tassel push.
CONTROL: Although killed by most pesticides that are effective against armyworms, penetration of the tied leaf mass can be difficult. There are no pesticides specifically labeled for control of this insect.