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Biological Control

Water lettuce weeviljpg

Water lettuce weevil (Neohydronomus affinis). Image taken by Able Chow (LSU AgCenter).


Summary

The water lettuce weevil (Neohydronomus affinis) is native to Argentina and Brazil [1, 3, 9]. This weevil is a natural enemy of water lettuce and feeds nearly exclusively on this plant [2, 6, 7]. It was first introduced into the US in 1987 [1].


Identification

Overhead view of water lettuce weeviljpg

Water lettuce weevil (Neohydronomus affinis). Note the distinct patterns on the abdomen. Image taken by Able Chow (LSU AgCenter).

This insect is small, at about 1.8-3 mm in length, and is brown to greyish-blue in coloration [1, 9]. Its back is intermittently covered in dense, white scales, creating distinct chevron-like and banded patterns (See figure above.) [1, 9]. Like other weevils, they have long elephant trunk-like mouthparts called rostrums. The rostrum of this weevil is straight instead of curved like many other weevils [1, 9]. Water lettuce weevil larvae are grub-like with creamy white to yellow bodies, orange to black heads, and without legs [1].


Life cycle

Female weevils chew holes in the surface of the leaf in which they deposit their eggs [1, 9]. They then close this hole with a black substance to protect the egg. Eggs will typically hatch within four days [1]. The small, newly-hatched larvae will then bore into the leaf, where they feed and pupate [1, 9]. The larval stage typically lasts about 11-14 days and pupation occurs over the course of roughly 4-5 days [1]. The complete life cycle from egg to sexually mature adult lasts about 4-6 weeks [1]. Up to 3 generations can occur in a season [9].


Feeding

Feeding damage done by weeviljpg

Adult water lettuce weevil feeding damage.

Larval water lettuce weevil feeding damagejpg

Larval water lettuce weevil feeding damage.

Larvae bore into and feed on the spongey tissue inside the leaves of water lettuce [1, 2, 9]. Larval feeding damage results in telltale scars on the outer portions of the leaf [1]. Adults feed on both the inner and outer portions of the leaves, creating circular feeding scars [1].


Impact on Water lettuce

The water lettuce weevil has been shown to be very effective at controlling water lettuce throughout the world [1, 8, 9]. In tropical areas such as South Africa and northern Australia, the weevil has been extremely successful, eliminating 80% to 100% of water lettuce in heavy infestations [2, 3, 9]. In subtropical regions, such as Louisiana and other parts of the Gulf Coast, weevil populations fluctuate seasonally due to our colder winters, but have been very successful biological control agents none the less [1, 7, 8,]. Feeding damage caused by the adults and larvae can reduce the growth rate of the plants, resulting in smaller plants with fewer leaves [2, 3, 9]. Large populations of water lettuce weevils can substantially reduce the area and plant density of water lettuce mats [2, 3, 9].

Author: Seth Spinner

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The LSU AgCenter and the LSU College of Agriculture

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