|Mexican Rice Borer: Eoreuma loftini (Dyar)|
|Description and Biology: The
Mexican rice borer is a stalk-boring insect that recently invaded the United
States from Mexico. The creamy white, 12- to 19-mm-long moth has enlarged
mouthparts that protrude in front of the head. This nocturnal moth has a
dark spot in the center of each front wing and no other wing markings. The
cream-colored Mexican rice borer larva has an orange-brown head and two
broken, purple-red lines on each side along the length of the body. Unlike
many other borer species, the Mexican rice borer has no conspicuous hairy
plates on the dorsal surface of the body.
Masses of five to 100 cream-colored, globular eggs are cemented between layers of dry leaf tissue. Moths prefer to lay eggs hidden on dry leaf material close to the ground but can lay eggs on upper green parts of the plant. The mature larva, 19 to 25 mm long, chews a pupal chamber within the stalk toward the outer edge. The pupa is 19 to 22 mm long, orange-brown, with small projections (tubercles) toward the posterior of the abdomen. The moth emerges through the "window" of one to two layers of leaf tissue that cover the pupal chamber within the stalk. Each generation is completed in 45 to 50 days under warm conditions. Moth emergence, oviposition, and larval infestation also occur during the winter, but at a slower rate than during the summer.
| Symptoms and Damage: Newly hatched larvae
move to a feeding site on leaf blades or sheaths where they chew into soft
tissue inside the sheath that wraps around the stalk. Tunneling within the
leaf sheath produces a blotch mine, with small perforations and dramatic
discoloration of tissue. Larvae feed in leaf sheaths for several weeks before
boring into the plant stem. They enter the stem usually by chewing directly
into the internode adjacent to the leaf sheath on which they were feeding.
Most tunneling occurs near the outer surface of the stalk adjacent to the
rind. Tunnels may be vertical, typical of most borers, but also may be made
horizontally or diagonally within the stalk. Mexican rice borer tunnels
are filled with frass and, thus, differ from tunnels of other stalk borers
that periodically remove frass from tunnels and deposit it between the leaf
sheath and stalk. Larval feeding near the growing point of the plant early
in the season can, when the plant's growing point is killed, cause "deadheart,"
a green shoot having a dead whorl center. Many young plants can be affected,
but this thinning usually does not reduce final plant stand. Moths lay eggs
on deadhearts when little other dry leaf material is available.
Leaf sheath feeding disrupts normal leaf function. Leaf sheath feeding causes dark purple discoloration, often the first evidence of infestation. This symptom should be monitored during sampling. Damaged leaves often die, reducing plant productivity.
Larval tunneling within the stalk damages the vascular system that moves water and nutrients within the plant. Stem damage can result in incomplete panicle filling or aborted development. Tunnel openings allow fungal organisms to invade the plant. Reddish discoloration caused by a fungus infection usually is seen in the pith of the damaged internode. Decomposition and structural damage to the stem allow plant lodging. Plants with stem damage often break or fall during harvesting. Tillers and post-harvest regrowth are susceptible throughout the year to attack by Mexican rice borer larvae.
Management: Plowing under debris before
planting and maintaining healthy plants early in the season reduce the
amount of dry leaf material available for oviposition and thus postpone
infestation by Mexican rice borers. Damage is minimized when sorghum is
planted and matures at a normal time.