''Samea multiplicalis'', the salvinia stem-borer moth, is an aquatic moth commonly found in freshwater habitats from the southern United States to

Argentina Argentina, officially the Argentine Republic, is a country in the southern half of South America. It covers an area of , making it the List of South American countries by area, second-largest country in South America after Brazil, the fourt ...

, as well as in

Australia Australia, officially the Commonwealth of Australia, is a country comprising mainland Australia, the mainland of the Australia (continent), Australian continent, the island of Tasmania and list of islands of Australia, numerous smaller isl ...

where it was introduced in 1981. Salvinia stem-borer moths lay their eggs on water plants like '' Azolla caroliniana'' (water velvet), ''

Pistia stratiotes ''Pistia'' is a genus of aquatic plants in the arum family, Araceae. It is the sole genus in the tribe ''Pistieae'' which reflects its systematic isolation within the family. The single species it comprises, ''Pistia stratiotes'', is often call ...

'' (water lettuce), and '' Salvinia rotundifolia'' (water fern). Larval feeding on host plants causes plant death, which makes ''S. multiplicalis'' a good candidate for

biological control Biological control or biocontrol is a method of controlling pests, whether pest animals such as insects and mites, weeds, or pathogens affecting animals or plants by using other organisms. It relies on predation, parasitism, herbivory, or o ...

of weedy water plants like ''

Salvinia molesta ''Salvinia molesta'', commonly known as giant salvinia, or as kariba weed after it infested a large portion of Lake Kariba between Zimbabwe and Zambia, is an aquatic fern, native to south-eastern Brazil. It is a free-floating plant that d ...

'', an invasive water fern in Australia. However, high rates of parasitism in the moth compromise its ability to effectively control water weeds. ''S. multiplicalis'' larvae are a pale yellow to green color, and adults develop tan coloration with darker patterning. The lifespan, from egg to the end of adulthood is typically three to four weeks. The species was first described by

Achille Guenée Achille Guenée (sometimes M.A. Guenée; 1 January 1809 – 30 December 1880) was a French lawyer and entomologist. Biography Achille Guenée was born in Chartres and died in Châteaudun. He was educated in Chartres, where he showed a very ear ...

in 1854.

Geographic range

''Samea multiplicalis'' was first observed in

Brazil Brazil, officially the Federative Republic of Brazil, is the largest country in South America. It is the world's List of countries and dependencies by area, fifth-largest country by area and the List of countries and dependencies by population ...

in 1854, and has since been documented across the southeastern United States, as far west as

Louisiana Louisiana ( ; ; ) is a state in the Deep South and South Central regions of the United States. It borders Texas to the west, Arkansas to the north, and Mississippi to the east. Of the 50 U.S. states, it ranks 31st in area and 25 ...

. This region constitutes its native range. In 1981, the moth was introduced in

as a potential method of biological control of the water fern, '' S. molesta''. It is now common throughout

Queensland Queensland ( , commonly abbreviated as Qld) is a States and territories of Australia, state in northeastern Australia, and is the second-largest and third-most populous state in Australia. It is bordered by the Northern Territory, South Austr ...

and

New South Wales New South Wales (commonly abbreviated as NSW) is a States and territories of Australia, state on the Eastern states of Australia, east coast of :Australia. It borders Queensland to the north, Victoria (state), Victoria to the south, and South ...

. In Australia, it has been found to thrive in tropical climates, with slower growth and dispersal in more temperate climates.

Habitat

''S. multiplicalis'' primarily lives on the water plants, ''Azolla caroliniana'' (water velvet), ''

'' (water lettuce), ''Salvinia rotundifolia'' (water fern) in their native range, and ''

'' (a water fern) in

. These plants grow to form floating mats on the surface of calm or still bodies of water like ponds, lakes, and slow-moving rivers. ''S. multiplicalis'' can survive within a temperature range of 11-36 °C, however it survives best around temperatures of 30 °C. It requires warm conditions to survive and develop, however steady temperatures above the low 30s cause populations to crash. Moth populations can survive year round in habitats where food resources and adequate temperatures are sustained through the winter.

Food resources

Host plant

In its native habitat, ''S. multiplicalis'' prefers the water lettuce '' P. stratiotes'' over other aquatic plants for feeding and

oviposition The ovipositor is a tube-like organ used by some animals, especially insects, for the laying of eggs. In insects, an ovipositor consists of a maximum of three pairs of appendages. The details and morphology of the ovipositor vary, but typica ...

, and in its introduced habitat in Australia, it primarily feeds and lays eggs on the water fern ''

''. ''P. stratiotes'' has a rosette of leaves surrounding a short, central stem and a submerged root system. Leaves are covered in short hairs. ''S. molesta'' plants in Australia are all clones, meaning they are genetically identical. However, there is some phenotypic variation due to differing temperature and nitrogen availability across its range, which is important due to ''S. multiplicalis'' larvae's preference for high-nitrogen food sources.

Food preferences

Caterpillar Caterpillars ( ) are the larval stage of members of the order Lepidoptera (the insect order comprising butterflies and moths). As with most common names, the application of the word is arbitrary, since the larvae of sawflies (suborder ...

s of the salvinia stem-borer moth feed on several aquatic plants, primarily ''Azolla caroliniana'' (water velvet), ''Pistia stratiotes'' (water lettuce), ''Salvinia rotundifolia'' (water fern), ''Salvinia molesta'' (a water fern), and occasionally ''

Eichhornia crassipes ''Pontederia crassipes'' (formerly ''Eichhornia crassipes''), commonly known as common water hyacinth, is an aquatic plant native to South America, naturalized throughout the world, and often invasive outside its native range.

Protection while feeding

Often, groups of larvae display cooperative behavior when they feed in groups. They collectively construct a silk canopy over their feeding site while they eat, expanding it as they exhaust their current site and move on to new areas of their host plant. This behavior offers protection from predation, decreasing larval mortality.

Larval nutrition

Nitrogen Nitrogen is a chemical element; it has Symbol (chemistry), symbol N and atomic number 7. Nitrogen is a Nonmetal (chemistry), nonmetal and the lightest member of pnictogen, group 15 of the periodic table, often called the Pnictogen, pnictogens. ...

is very important nutrient for larval development, especially during the first two

instar An instar (, from the Latin '' īnstar'' 'form, likeness') is a developmental stage of arthropods, such as insects, which occurs between each moult (''ecdysis'') until sexual maturity is reached. Arthropods must shed the exoskeleton in order to ...

s, so larvae prefer food plants with higher nitrogen content. Higher nitrogen intake correlates with larger larval biomass and faster development, both of which are favored because they decrease larval vulnerability to predation. Additionally, a lack of sufficient nitrogen intake during early development leads to decreased ability to digest and utilize food throughout the rest of the insect's life.

Compensatory feeding

Water lettuce and other aquatic plants tend to have high water content, which dilutes the nitrogen and other nutrients they contain. To account for this, ''S. muliplicalis'' has a higher feeding rate than many other

Lepidoptera Lepidoptera ( ) or lepidopterans is an order (biology), order of winged insects which includes butterflies and moths. About 180,000 species of the Lepidoptera have been described, representing 10% of the total described species of living organ ...

species. When nitrogen levels are especially low, larvae show compensatory feeding behavior, increasing their already high feeding rate and consuming a larger volume in order to make up for the nutrient deficit. This behavior does not usually mitigate the poor quality food, however, and larvae on nitrogen-depleted host plants still experience slower development and decreased digestive efficiency.

Life history

Larvae

Larvae hatch approximately four days after eggs are laid. Larvae have an off-white or pale yellow color during early instars and develop a yellow-green color as they mature. They typically develop through five instars over the course of about two weeks, and male larvae develop faster than females by about two days. In low-

nitrogen Nitrogen is a chemical element; it has Symbol (chemistry), symbol N and atomic number 7. Nitrogen is a Nonmetal (chemistry), nonmetal and the lightest member of pnictogen, group 15 of the periodic table, often called the Pnictogen, pnictogens. ...

conditions, larvae require a sixth instar and two or three additional days to develop.

Pupae

Caterpillars construct a silk cocoon inside of a leaf petiole on their host plant in order to pupate. Pupation lasts between four and ten days, with females developing faster than males. Time spent in this stage of development is not dependent on larval nutrition.

Adults

After they emerge from their cocoons, adult salvinia stem-borer moths spend the rest of their three- or four-week life span around their host plants, mating and laying eggs. Female moths lay an average of 150 eggs over the course of several days on the surfaces of host plant leaves or among hairs or leaflet structures. Moths prefer the host plant ''P. stratiotes'' for laying eggs due to its structure which provides abundant oviposition surfaces. Moths are not known to oviposit on ''E. crassipes''. Adults are tan with darker markings on both sets of wings and a wingspan of about 20 mm. Adult size is not significantly affected by larval nutrition levels, however insufficient nitrogen during larval development does lead to reduced egg production in female moths.

Enemies

Parasites

''S. multiplicalis'' is affected by a number of

parasites Parasitism is a close relationship between species, where one organism, the parasite, lives (at least some of the time) on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. The en ...

, most commonly by wasp species of the order

Hymenoptera Hymenoptera is a large order of insects, comprising the sawflies, wasps, bees, and ants. Over 150,000 living species of Hymenoptera have been described, in addition to over 2,000 extinct ones. Many of the species are parasitic. Females typi ...

and parasitic flies of the order

Diptera Flies are insects of the order Diptera, the name being derived from the Greek δι- ''di-'' "two", and πτερόν ''pteron'' "wing". Insects of this order use only a single pair of wings to fly, the hindwings having evolved into advance ...

, as well as miscrosporidia. Parasitic wasps attack ''S. multiplicalis'' larvae during their first instar and pre-pupal wasps emerge during the caterpillars' last instar, killing their hosts. Parasitism rates are fairly high in some populations, which diminishes their effectiveness at controlling weedy host plants. ''S. multiplicalis'' larvae are parasitized in both their native and introduced ranges by similar species. Parasitism rates remain steady through spring, summer, and fall and decrease slightly during winter months.

Interactions with humans

Biological control agent

''S. molesta'' is an aquatic plant native to Brazil that is invasive in Australia due to a lack of sufficient

herbivory A herbivore is an animal anatomically and physiologically evolved to feed on plants, especially upon vascular tissues such as foliage, fruits or seeds, as the main component of its diet. These more broadly also encompass animals that eat n ...

to control the growth of the plant. It has become a common weed in many bodies of water, especially in eastern Australia in Queensland and New South Wales. The water fern grows in expansive mats, potentially crowding out other native aquatic plants and preventing light penetration in water bodies where it grows. Efforts to control ''S. molesta'' began in 1980 with the introduction of the salvinia weevil '' Cyrtobagous salviniae'', which is also native to Brazil. ''S. multiplicalis'' was introduced the following year in 1981 to aid in the biological control of the weed.

Comparison with ''C. salviniae''

Since their introduction, ''C. salviniae'' and ''S. multiplicalis'' have had different levels of effectiveness; the

weevil Weevils are beetles belonging to the superfamily Curculionoidea, known for their elongated snouts. They are usually small – less than in length – and herbivorous. Approximately 97,000 species of weevils are known. They belong to several fa ...

has proved more useful than the moth at keeping salvinia populations down through intensive feeding. While ''C. salviniae'' and ''S. multiplicalis'' both have similar habitat needs, including common host plants, similar

requirements, and an optimal temperature of 30 °C, ''S. multiplicalis'' populations are highly susceptible to

and

parasitoid In evolutionary ecology, a parasitoid is an organism that lives in close association with its host (biology), host at the host's expense, eventually resulting in the death of the host. Parasitoidism is one of six major evolutionarily stable str ...

s, which prevent the colony growth and dispersal that would be required for effective weed control. ''S. multiplicalis'' actually has a higher rate of reproduction and dispersal than ''C. salviniae'', but infection prevalence still prevents sufficient levels of feeding to significantly affect ''S. molesta'' numbers.

References

{{Taxonbar, from=Q7409221 Moths described in 1854 Spilomelinae