In this article:
|Meet the Salvinia Weevil|
|Feeding and Reproduction|
The salvinia weevil, Cyrtobagous salviniae, was first found on giant salvinia in its native range of southeast Brazil. Since its discovery the weevil has been used in biological control programs worldwide to manage giant salvinia. The population we have in the United States was released in Texas and Louisiana in 2001. Another population of salvinia weevils, known as the Florida ecotype, were found independently on common salvinia (Salvinia minima) in Florida and are still used for controlling common salvinia.
Salvinia weevils are tiny, just two millimeters or eight hundredths of an inch long – about the size and shape of a kiwi seed. They are good at hiding on, in, or under giant salvinia plants, making them doubly hard to find in the field. Mature adult weevils are shiny and black, with the classic elephant’s trunk-like snout that is typical of many weevils. Young, newly matured adults are light brown, getting darker with age after a few days. Under a microscope the weevils have a characteristic pattern of ridges and punctures along the head, thorax, and abdomen with striking golden scales.
Salvinia weevil larvae are white and grub-like, with a small brown or black head. The larvae do not have any legs and possess few microscopic hairs. Since larvae feed internally within the plant’s rhizome they are rarely seen in the field, but are the most important life stage for controlling the plant’s growth.
Adult females lay eggs one at a time in small crevices on the giant salvinia growing tips. Newly hatched larvae feed on external plant parts, then later burrow inside the rhizome to continue their development. Larval feeding activity disrupts the flow of nutrients from the “roots” to the newly growing tips, causing plants to turn brown and die. Adults feed preferentially on nitrogen-rich buds, slowing new growth as they attack growing tips.
Development time from egg to adult can vary with temperature
and plant quality. At 78°F (26°C)
development takes about 45 days, and multiple generations can occur in a
growing season. The ideal temperature
range for reproduction is 73° to 81°F (23° to 27°C), but adults can feed from 55°
to 91°F (13° to 33°C).
At high densities (greater than 18 weevils per pound of salvinia), weevil feeding causes giant salvinia mats to turn brown and eventually die. The plant mat sinks as it decomposes, releasing nitrogen and other nutrients back into the water column. Sinking of plant mats restores sunlight penetration through the water surface and allows native flora and fauna to return.
Salvinia weevil populations can move with plant mats along
water currents or in floodwaters. In
dense plant mats, adult weevils walk from plant to plant to find mates and
fresh plants. Adults fly at night, and
are most active during warmer parts of the year. It is not yet known how far or frequent flight
paths are. Aquatic plant managers help
weevils reach new salvinia infestations by releasing mass-reared populations.
Salvinia weevil populations are impacted by severe winters in northern parts of Louisiana and Texas. As such, releases made in these areas sometimes fail to establish. Current research by the LSU AgCenter aims to explore how weevil populations adapt to local conditions in temperate climates and what management techniques can be used to increase overwinter survival.
For more information, see the Bug Biz publication on biology and ecology of the salvinia weevil.