Sitobion miscanthi
(indian grain aphid)

S. miscanthi is an exotic, invasive species in Australia and New Zealand. The alate (winged) forms have the potential for long-distance dispersal. It appears that S. miscanthi first colonized New Zealand as airborne alatae from Australia (Close and Tomlinson, 1975) and population outbreaks have significant plant biosecurity implications (Teulon and Stufkens, 2002).

Once introduced, populations of S. miscanthi have the potential to increase rapidly. Reproduction is by parthenogenesis, so there is no need for females to locate males in order to reproduce. Therefore, when populations are initially low following introduction, the reproduction rate is high. The generation time is short and dispersal via alatae can be rapid. There is also no need for a primary woody host (as occurs for host-alternating aphids) and reproduction can occur all year round on abundant grass species. S. miscanthi colonizes cereal crops from wild grasses, pasture grasses and other cereals.

S. miscanthi populations in Australia and New Zealand have limited gene pools compared to populations in Asia (Turak and Hales, 1994; Wilson et al., 1999). These populations initially arose from a limited number of introduced females. Introduced populations can show founder effects, which lead them to vary in certain characteristics from the species in its native range. The different chromosomal races of S. miscanthi present in Australia have different temperature preferences, growth rates and seasonal abundance (Turak et al., 1998).

S. miscanthi is Asiatic in origin and is widespread in India, China and the Far East and in the Pacific region. It has been introduced into Australia and New Zealand, and possibly into several Pacific islands (Blackman and Eastop, 2000).

The distribution map includes records based on specimens of S. miscanthi from the collection in the Natural History Museum (London, UK): dates of collection are noted in the List of countries (NHM, various dates).

Sitobion akebiae may be a synonym of S. miscanthi, and the taxonomy of the S. miscanthi/akebiae groups requires further clarification. At least some of the records from Poaceae probably apply to S. miscanthi (see CABI/EPPO, 2003).

Around 90% of aphid species present in New Zealand are exotic. They entered via two possible pathways: on infested plant material and produce or by wind dispersal. Although S. miscanthi is capable of being transported on weather systems across water bodies, entry on plant material is the main route for exotic aphids into New Zealand. It is not a post-harvest pest, and leaves and growing inflorescences are not usually traded for cereals. However, this aphid could be present on imported exotic and ornamental grasses. Given that windborne dispersal is likely to predominate, further invasion is difficult to prevent using phytosanitary measures. However, knowledge of weather systems, aerial traps and outbreaks in southern Australia could be used to predict further airborne introductions (Close and Tomlinson, 1975; Teulon and Stufkens, 2002).

S. miscanthi can persist throughout the year in unmanaged grasslands. The aphids can migrate from natural grasslands to infest cereals.

S. miscanthi is polyphagous on many members of Gramineae/Poaceae. It can also sometimes be found on dicotyledons: for example, semi-aquatic species such as Polygonum hydropiper in Australia (Eastop, 1966; Blackman and Eastop, 2000).

Genetics

The chromosome number for S. miscanthi is 2n=18 in India (Kurl and Chauhan, 1988). However, chromosome number varies from 2n=17 to 2n=21 for introduced populations in Australia and New Zealand (Sunnucks et al., 1996). There are four known chromosomal races of S. miscanthi in Australia. Three of these (2n=17, 2n=18 and 2n=20) are believed to have evolved from a recent common ancestor by mutation and chromosomal rearrangement. Significant differences in relative growth rate, developmental time and adult weight were found for laboratory-reared clones of different chromosomal races (Sunnucks et al., 1998). There is a suggestion that hybridization may have occurred between introduced populations of S. miscanthi and Sitobion near fragariae in New Zealand (Hales et al., 1998).

Physiology and Phenology

In eastern Australia, seasonal differences in abundance occur between the chromosomal races of S. miscanthi and the closely related Sitobion near fragariae. Sunnucks et al. (1998) found significant differences in relative growth rate, developmental time and adult weight in laboratory-reared clones of different chromosomal races.

Reproductive Biology

S. miscanthi is anholocyclic throughout its geographic range. Sexual forms have been described on rare occasions: an oviparae (sexual female) collected on Polygonum chinense in India may be S. miscanthi, whereas a pinkish-brown alate male collected from Poa annua in India has been classified as S. miscanthi (David, 1975). In Japan and Korea, Blackman and Eastop (2000) suggested that Sitobion akebiae may be a synonym, although S. akebiae is holocyclic (lays overwintering eggs) and the taxonomy of the miscanthi/akebiae group requires further clarification. The evidence for holocycly remains inconclusive (Blackman and Eastop, 2000).

Reproduction is by parthenogenesis throughout its range, with both apterous (wingless) and alate (winged) females giving birth to live young. The apterae are adapted to exploit the host plants on which they develop, whereas alatae are adapted for dispersal over long distances and are responsible for the initial colonization of cereals and pasture grasses. Rapid reproduction can occur under favourable conditions, leading to population outbreaks. The alates are produced in response to overcrowding or when the plants have become nutritionally unsuitable.

S. miscanthi is monoecious (non host-alternating), having lost the need for a primary woody host. It lives throughout the year on a wide range of grasses and cereals. The apterae usually live on aerial plant parts.


Environmental Requirements

Seasonal differences between chromosomal races of S. miscanthi corresponded to temperature preferences in laboratory studies. S. miscanthi 2n=18 showed evidence of adaptation to warmer conditions, having the highest population growth around 25°C and being the only race able to reproduce above 28°C. S. miscanthi 2n=17 had an intermediate temperature preference (Turak et al., 1998). Sitobion near fragariae showed adaptations to cooler conditions, with the highest population growth between 12 and 20°C and this is consistent with it being more common earlier in the spring.

S. miscanthi favours warmer conditions than other cereal aphids. It replaces Sitobion avenae in importance on cereals in tropical regions. S. miscanthi is adversely affected by cold winters, an effect compounded by its anholocyclic life cycle, i.e. overwintering is done as adult apterae rather than through a more cold-tolerant egg stage (Dixon, 1987). Aphid populations in northern India decline in response to heavy winter rainfall and hail. Simulated rainfall (1-2 cm) in wheat led to 95% mortality of S. miscanthi. Mortality was greater for nymphs rather than adults, and being greater before earing rather than after. However, sudden high temperatures also cause mortality (Sandhu and Deol, 1975). High temperatures, acting in conjunction with natural enemies and grain maturity, for instance, led to a population decline on wheat in the Punjab (Grewal and Bain, 1975).

Associations

S. miscanthi often occurs within a cereal aphid species complex. In India, it is often found with Rhopalosiphum padi and Rhopalosiphum maidis on wheat and barley (Grewal and Bain, 1975). These three species often have distinctive feeding sites. R. padi infests all above-ground plant parts, R. maidis is confined to leaf whorls and S. miscanthi infests leaves and earheads (Sekhar and Singh, 1999). In Australia, S. miscanthi is associated with R. padi, R. maidis and also S. near fragariae on wheat (Turak et al., 1998). S. near fragariae, in common with S. miscanthi, is an introduced cereal aphid that reproduces parthenogenetically in Australia and has populations that derive from a few initial colonizations (Sunnucks et al., 1996).

Natural enemies contribute to the biological control of S. miscanthi in cereal crops. Ahmad and Singh (1995, 1997) and Das and Chakrabarti (1989) described parasitoids of S. miscanthi in India, with the braconid wasp, Aphidius uzbekistanicus being the most important.

A range of generalist aphid predators also attack S. miscanthi. In northern India, coccinellids, syrphids and Leucopis sp. contribute to population declines in wheat (Grewal and Bain, 1975; Sandhu and Deol, 1975; Sandhu and Kaushal, 1975). Similarly, the coccinellid beetle Hippodamia variegata controls S. miscanthi in barley in Himachal Pradesh, India (Hameed et al., 1975b).

Two entomophagous fungal pathogens, Entomophthora aphidis and Entomophthora planchoniana, were recorded on S. miscanthi from wheat in New Zealand (Lowe and Hall, 1978).

The presence of winged migrants (alatae) of S. miscanthi can be detected on a regional scale using suction traps and on a local scale using yellow sticky traps within crops. The detection of migrants in spring can be used to formulate aphid control strategies in cereals.

Within crops, small to moderate colonies of aphids can be found on upper leaves and earheads during routine inspections of cereals.

A number of aphids cause similar damage to cereals and grasses as S. miscanthi. S. miscanthi can be distinguished from Sitobion avenae by its longer siphunculi (more than 1.4 times longer than the cauda cf. less than 1.4 times longer in S. avenae) and shorter hind tarsi (the hind tarsus segment II is less than 1.3 times longer than the last segment of the rostrum cf. more than 1.3 times longer in S. avenae). S. miscanthi differs from Sitobion fragariae by having shorter siphunculi (1.4-1.9 times longer than the cauda cf. 1.8-2.3 times longer in S. fragariae) and a cauda that is more pointed at the apex. S. miscanthi can be distinguished from Sitobion africanum, on maize, millet, sorghum and tropical pasture grasses, by its variable and ill-defined abdominal pigmentation (cf. well-defined black bars in S. africanum) and longer hairs (over 20 µm) on antennal segment III (Blackman and Eastop, 2000).

Turak and Hales (1994) described an allozyme method for separating the morphologically similar Sitobion species complex on grasses in Australia. S. miscanthi occasionally colonizes bamboo in northern India, where it can be distinguished from Sitobion bambusicola due to its shorter siphunculi (1.4-1.9 times the length of the cauda) compared to S. bambusicola (2.0-2.1 times the length of the cauda) (Blackman and Eastop, 1994).

Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.

Host-plant Resistance

The use of resistant varieties of wheat and other cereals can reduce aphid infestations and yield loss. Most of the work on host-plant resistance in cereals has concentrated on other aphid species, such as Schizaphis graminum and Sitobion avenae, rather than S. miscanthi. However, resistant varieties are often effective against cereal aphid species complexes, which may include S. miscanthi. Studies in India have evaluated the impact of S. miscanthi on different wheat varieties (Saikia et al., 1998; Sekhar et al., 2001).

Biological Control

Native parasitoids and predators play a role in controlling outbreaks of S. miscanthi in cereals. The parasitoid Aphidius uzbekistanicus has potential as a biological control agent (Das and Chakrabarti, 1989).

Chemical Control

A range of insecticides have been used against cereal aphids where S. miscanthi is one of the species present, including bifenthrin, cypermethrin, deltamethrin, dimethoate, and pirimicarb. A number of insecticides have been evaluated specifically for use against S. miscanthi on wheat in India and Pakistan (Sekhar and Singh, 2001a; Khan and Maqbool, 2002). Out of 10 insecticides tested in India, fenvalerate was the most effective against S. miscanthi and Rhopalosiphum padi (Sekhar and Singh, 2001a).

Early Warning Systems

The use of suction traps to monitor migrants (alatae) provides a warning of potential outbreaks.

Field Monitoring/Economic Threshold Levels

Monitoring for migrant aphids in suction traps and counting apterous aphids on plants can aid in the forecasting of pest incidence and in determining the necessity and timing of insecticide applications.

Integrated Pest Management

Checking for the presence of aphids on tillers can aid in spraying decisions. The use of aphid-specific insecticides is usually advocated because natural enemies (e.g. parasitoids, ladybirds and syrphid larvae) are helpful in preventing secondary outbreaks of aphids.