Water Lettuce Moth

Scientific name: Samea multiplicalis (Guenée) (Lepidoptera: Pyralidae: Pyraustinae)

Introduction

Water lettuce moth, Samea multiplicalis (Guenée) (Lepidoptera: Pyralidae: Pyraustinae), is a small moth widely distributed in the warmer regions of North and South America (Center et al., 1982). The larvae feed on several floating aquatic plants, including water lettuce, Pistia stratiotes L. (Araceae), mosquito fern, Azolla caroliniana Willd. (Azollaceae), common salvinia Salvinia minima Baker and giant salvinia Salvinia molesta D.S. Mitchell (Salviniaceae) (Center et al., 1982; Walsh and Maestro, 2016). Samea multiplicalis is often found attacking giant salvinia, which is a serious aquatic invasive plant in Texas and Louisiana (Fig. 1). Samea multiplicalis was released as a potential control agent in Queensland, Australia in 1981; however, it has not contributed to the control of giant salvinia (Center et al., 1982).

Description and Life Cycle

The adults measure about 17 mm (0.70 in) in width and are light brown with dark markings on the wings (Fig. 2). Females may live for 4 to 7 days (Center et al., 1982).

The pupae are about 10 mm (0.40 in) in length and are yellow. Pupation occurs within a silken cocoon; on giant salvinia, this cocoon is usually constructed among old leaves (Fig. 3). The duration of the pupa stage has been reported to be 4 to 7 days on water lettuce and 8 to 9 days on giant salvinia (Center et al., 1982).

The larval stage has 5 to 7 instars and is completed in 15 days when fed on water lettuce or common salvinia, and in 21 to 35 days when fed on giant salvinia (Center et al., 1982). The late instars are ca. 11 mm (0.43 in) in length, but the first instars are as small as 1 mm (0.04 in). The color of the larvae varies from white to light yellow (Fig. 4).

Eggs are deposited usually singly among hairs on the upper surface of leaves. One female can lay in average of 93 and up to 291 eggs. The eggs hatch in 5 to 6 days. The life cycle from egg to adult and then egg again is 56 days (Sands and Kassulke, 1984).

Host Plants

Salvinia molesta, S. minima, A. caroliniana and P. stratiotes are four main hosts of water lettuce moth. In a choice experiment, oviposition was higher on P. stratiotes over S. minima and A. caroliniana. The larvae could feed Lemna sp. and E. crassipes, but they could not complete development on these two plants. Although not a preferred host plant, larvae of the water lettuce moth can be found feeding on E. crassipes (Bennett, 1966).

Natural Enemies

Five parasitoids and three pathogens were found attacking the water lettuce moth in Australia (Semple and Forno, 1987). In United States, a report from Florida demonstrated that 52% of S. multiplicalis larvae were parasitized by one Braconid and two Ichneumonid wasps (Knopf and Habeck, 1976). In Trinidad and Argentina larvae are heavily parasitized and preyed on by many natural enemies (Bennett, 1966). Sharkey et al. (2011) reported a new species, Neothlipsis parysae (Hymenoptera: Braconidae), parasitic on the first instar of S. multiplicalis in southern Louisiana.

Effects on Salvinia Weevil Rearing Ponds

Field observations in Louisiana suggest that the water lettuce moth could damage giant salvinia at some locations (Fig. 5). Julien and Bourne (1988) reported that water lettuce moth could destroy giant salvinia even at low densities (0.8 to 1.6 larvae per plant).

This moth is considered a pest in giant salvinia ponds used to rear the salvinia weevil (Cyrtobagous salviniae Calder and Sands, Coleoptera: Curculionidae). Severe damage, by caterpillars, fragments the plant which eventually sinks (Fig. 6).

To control the water lettuce moth in rearing ponds, LSU AgCenter personnel spray Bacillus thuringiensis (Bt, Thuricide ®, 1 tablespoon per gallon of water) in a regular basis. Monitoring rearing ponds for water lettuce moth involves the collection of plant material and extraction using Berlese funnels.

Authors: Yi Wang, Sebe Brown, Rodrigo Diaz, Department of Entomology, LSU AgCenter.