Phenacoccus solenopsis
(cotton mealybug)

Phenacoccus solenopsis is a mealybug native to North America that is now widespread. It has the ability to multiply rapidly, but is inefficient at dispersal unless assisted by wind, animals or machinery passing through the crop. When its dispersal is assisted, it can spread to vast areas where its host plants occur, in a relatively short period of time. The species is polyphagous and has been reported from host-plants in over 200 genera belonging to 64 families. After its original description from Boerhavia spicata and Kallstroemia californica in New Mexico, USA in 1898, P. solenopsis was not reported again until 1967. Later, in 1988, small, sporadic populations were reported on cotton in Runnels County, Texas, USA that spread 75 to 200 miles from the original site with contiguous populations reported by 1990. With the increase in international trade in live plant material over the last few decades this mealybug has been accidentally spread, and has been collected and identified on host material at international ports and in greenhouses outside its native range. P. solenopsis has become established as an invasive pest in the Afrotropical, Australasian, Nearctic, Neotropical, and Oriental regions. As an introduced species, P. solenopsis has the capability to cause direct economic and ecological damage to native fauna and flora, with heavy infestations reducing plant vigour and causing plant death

Phenacoccus solenopsis is widespread and has become established in a variety of regions ranging from arid to tropical. Due to the difficulty of identifying the species, it is likely to be more widespread than the countries listed below.

Based on the range of climates and large number of host species on which P. solenopsis can survive, and the damage inflicted on the host plants, this species poses a serious threat of expanding its range (Wang et al., 2010). The hydrophobic waxy test, cryptic habits, small size, ability to feed on all parts of the plant, multiple overlapping generations and high reproductive rates, allow P. solenopsis the opportunity to disperse over extended areas. Its cryptic colouration and small size mean that it can be overlooked on infested vegetables, fruits and other crops, facilitating transport to other regions.

As an introduced species, P. solenopsis has the capability to cause direct economic and ecological damage to native fauna and flora, with heavy infestations reducing plant vigour and causing plant death. Once the species has established within a region, it has the capability of rapid multiplication resulting in significant damage to the crop. Sharma (2007) documented a seasonal outbreak of P. solenopsis on okra (Abelmoschus esculentus) in 2007, which developed into a heavy infestation by the end of the growing season and resulted in a 90% loss of seeds. Wang et al. (2009) recorded 17 provinces and 11 regions in China where this invasive species could spread and cause significant economic and environmental damage. Based on an international pest risk analysis for P. solenopsis, Wang et al. (2009) rated this mealybug as a high-risk invasive threat to China, with risk score of 0.856.

This species has been documented infesting 204 plant species representing 64 families with a distribution in Africa, Asia, North America and South America and Oceanic regions including the Caribbean nations (García Morales et al., 2016). Specimens from different geographical regions exhibit similar morphological characteristics (Hodgson et al., 2008). Hodgson et al. (2008) noted a significant difference in the ecology of P. solenopsis from the hot, dry climate of southwestern USA; here the mealybug occurs primarily on the roots and underside of the foliage and stems, compared to the more humid regions of India and Pakistan, where it is found almost entirely on the upper portions of the foliage, well above the soil line. Dhawan et al. (2009a) inferred that meteorological parameters influenced the presence and population size of the mealybug, with high humidity and rainfall having a negative effect.

The solenopsis mealybug has been recorded on 204 host plant species that include field crops, ornamentals, trees, vegetables and weeds (García Morales et al., 2016). In Pakistan, P. solenopsis obtained the status of a serious pest on a wide host range. In a field survey, Arif et al. (2009) identified the mealybug from 154 plant species, the majority of which belong to the families Malvaceae, Solanaceae, Asteraceae, Euphorbiaceae, Amaranthaceae and Cucurbitaceae. Significant economic damage occurs on cotton (Gossypium spp.), brinjal (Solanum melongena), okra (Abelmoschus esculentus), tomato (Solanum lycopersicum), sesame (Sesamum indicum), sunflower (Helianthus annuus) and China rose (Hibiscus rosa-sinensis; Sharma, 2007; Arif et al., 2009; Jagadish et al., 2009b).

Several cultivated plants, as well as weeds, have been used as trap crops to suppress the population numbers in an area. In surveys of the insect fauna in the southwestern USA, this pseudococcid was recorded on several plant hosts ranging from silver nightshade (Solanum elaeagnifolium; Goeden, 1971) to jojoba (Simmondsia chinensis; Pinto and Frommer, 1980).

Reproductive Biology

P. solenopsis reproduces sexually and has multiple generations annually, the number depending on temperature and host quality. Each adult female lays 150 to 600 eggs, protected within a waxy ovisac (Lu et al., 2008). Females undergo three immature stages prior to reaching adulthood, whereas males undergo first-instar nymph, second-instar nymph, prepupal and pupal stages prior to adulthood. The period of development from crawler to adult is approximately 25-30 days, depending upon temperature and host quality (García Morales et al., 2016).

Nutrition

The mealybugs feed by extracting sap from phloem cells in leaves or stems. The sap contains water, soluble sugars, phenols, proteins and other potential nutrients. Mealybug-infested leaves produce high quantities of sugars and proteins compared to the amounts produced in uninfested leaves (Jagadish et al., 2009a). Conversely, Jagadish et al. (2009a) reported a decline in the phenol content in infested sunflower plants.

Environmental Requirements

Maximum temperature and sunshine hours have a positive influence on P. solenopsis, whereas high relative humidity and rainfall have a negative influence (Suresh and Kavitha, 2008b). The mealybugs are capable of surviving temperatures ranging from 0-45°C (Sharma, 2007). The location of the feeding site on the plant appears to be influenced by humidity, as Hodgson et al. (2008) concluded that P. solenopsis occurred more commonly on the roots, stems and foliage close to the soil line in very dry climates compared to settling on the upper foliage in more humid areas (Gerson, 2020).

The solenopsis mealybug possesses a number of defence capabilities to protect itself from natural enemies. The waxy secretions that cover the body often protects it from attack by parasitoids and predators. This waxy covering may be the reason for the rare occurrence by pathogens and nematodes infesting the mealybug (Franco et al., 2009). Also, ants often attend the mealybugs for the honeydew they excrete and in return, often defend the mealybugs from natural enemies. The solenopsis mealybug was initially discovered in a nest of Solenopsis geminata (Tinsley, 1898a). More recently, P. solenopsis has been recorded in the nests of the southern fire ant (Taber, 2000) and associated with several other species of ants including Myrmicaria brunnea, Oecopylla smaragdina, Solenopsis invicta, and Tapinoma melanocephalum (Helms and Vinson, 2002; Jagadish et al., 2008, 2009b; Zhou et al., 2012).

Several parasitoids and predators have been documented attacking P. solenopsis. Three parasitic wasps (Chalcaspis arizonensis, Cheiloneurus sp., and Aprostocetus minutus) were discovered attacking the mealybug on cotton in Texas, USA (Fuchs et al., 1991). In India, an unidentified species of the solitary endoparasitoid genus Aenasius was reported to attack P. solenopsis (Sharma, 2007; Tanwar et al., 2008). Hayat (2009) described a new species of parasitoid from India, Aenasius bambawalei, associated with P. solenopsis, which has been documented as a very effective biological control agent. Parasitized mealybugs cease feeding and their exoskeletons turn into reddish-brown mummies (Pala Ram et al., 2009). This parasitoid has been reported to parasitize up to 72% of P. solenopsis on cotton in some districts in India (Muniappan, 2009; Pala Ram et al., 2009). Within one growing season, on okra (Abelmoschus esculentus) parasitism of P. solenopsis by A. bambawalei reached 89% parasitism at harvest time (Sharma, 2007). In addition, Paranathrix tachikawai was recorded parasitizing 30-39% of the mealybugs on cotton in India (Bambawale, 2008). However, a hyperparasitoid, Promuscidea unfasciativentris, attacks A. bambawalei, which potentially may reduce the efficiency of this parasitoid as a biological control agent.

Several predator species are associated with P. solenopsis. Most predators require high populations of prey to complete their development. Cryptolaemus montrouzieri has been used to control several mealybug pest species. In India, two coccinellids (Brumoides suturalis and Hyperaspis maindroni) were identified to be associated with P. solenopsis (Patel et al., 2009). Most predators feed on the eggs or crawlers within the mealybug’s ovisac and reduce the number of mealybugs available to extract sap and weaken the plant. Other predators like the larvae of the lacewing, Chrysoperla carnea, were found to consume 30 mealybug eggs daily in developmental laboratory tests (Rabinder Kaur et al., 2008).

In life, body of female 2-5 mm long, oval, dusted with powdery white wax but often with 2 submedian lines of dark grey bare patches, with 3 conspicuous pairs of patches on anterior abdominal segments and often 1 or 2 less obvious pairs on thorax. Marginal wax filaments very short but longest at posterior end; a short, fluffy white ovisac secreted at maturity. Venter of abdmen with a dark circulus visible on midline. Adult male Phenacoccus mealybugs have 1 pair of wings and 2 pairs of long white wax tail filaments. Microscopic examination of the adult female is required to verify the species identity, or molecular analysis.

On a microscope slide, body of adult female oval, with 18 pairs of cerarii on margins, each cerarius containing 2 lanceolate setae. Quinquelocular pores absent, and antennae usually 9 segmented. Multilocular pores absent from dorsum, present ventrally at least around vulva and on posterior abdominal segments, medial area of abdominal segment VII with pores scattered across entire depth of segment from anterior edge to posterior edge. Translucent pores present on hind tibia; claw with a denticle. Dorsal setae short and lanceolate. Circulus usually oval and small, often slightly sclerotized, present between abdominal segments III and IV. Specimens living in hot conditions may have a few translucent pores near apex of hind femur, and small clusters of ventral multilocular disc pores on submargins of some or most abdominal segments.

Molecular characterisation of P. solenopsis was carried out by Tian et al. (2013) and its gene sequences are available on GenBank.

Initiate field surveys to detect potential infestations early in the growing season, to provide maximum opportunities for spot treatments (either by hand picking or pesticidal sprays) to prevent spread and limit damage to commercial crops. Closely examine stems, leaves and flowers for small white oval insects without wings, each often with 2 broken grey submedian longitudinal stripes. In addition, check for infestation of the base of the plant several millimetres below the soil surface. The presence of sticky honeydew deposits and traces of sooty mould on the leaves may also indicate presence of an infestation. Sticky traps set out in the fields and around their borders can be used to detect the presence of the winged male mealybugs.

P. solenopsis is closely related to P. solani (of which P. defectus is a junior synonym (Chatzidimitriou et al., 2016)). However, adult female P. solenopsis have (contrasting condition in P. solani): antenna usually 9 segmented (usually 8 segmented); ventral multilocular disc pores present as far forward as abdominal segment VI (as far forward as abdominal segment IV); multilocular disc pores on middle of segment VII present across entire depth of segment (present in distinct rows along anterior and/or posterior edges of segment VII, the pores sometimes very few); submargins of abdominal segments sometimes with small groups of multilocular pores (lacking submarginal multilocular pores); and distal end of hind femur sometimes with a few translucent pores (hind femur never with translucent pores; Zhao et al., 2014).

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.

Prevention

SPS measures

Several countries require inspection of plant material imported from countries where P. solenopsis is known to occur. China has initiated a notice of inspection and quarantine for P. solenopsis (Ministry of Agriculture, 2009).

Early warning systems

Use of sticky traps placed throughout the field is an effective means to monitor for the presence and population density of P. solenopsis, by catching the winged males.

Control

Cultural control and sanitary measures

It is important to cut infested stems or branches from plants and destroy the infested plant material by burning or deep burial; infested prunings must not be left lying in the field, as the mealybugs will walk onto nearby plants. After harvest, crop residue and weeds left in infested fields should be gathered up and buried or burned, as mealybugs on plant material left in the field can survive to infest the next crop. Field borders should be inspected for alternative host plants for the mealybug; these should be removed to prevent the mealybugs from overwintering and infesting the next crop. Trap plants like Hibiscus rosa-sinensis may be planted, to initially attract the mealybugs; these serve to alert farmers to the presence of P. solenopsis in the field, and can then be treated to protect the primary crop.

Physical/mechanical control

Small populations of P. solenopsis can be controlled by regular inspection of plants, removing loose bark where mealybugs might be difficult to observe and hand-picking or crushing specimens from newly-infested plants.  

Movement control

When working in fields known to be infested with P. solenopsis, it is necessary to sanitize farm equipment and check clothing items before movement to uninfested fields, to prevent the transfer of the pest to new locations. Infested prunings should be destroyed on site, not transported elsewhere for disposal, as crawlers falling off transported material will infest plants along the route.

Biological control

The use of biological control agents provides a cost-effective, non-toxic, self-perpetuating means of suppressing invasive pest populations of exotic mealybugs. Several parasitoids and predators have been identified that attack P. solenopsis and can control pest populations at low densities. Established parasitoids have been recorded parasitizing over 70% of the cotton mealybug population (Pala Ram et al., 2009). Other tactics suggested to suppress populations of P. solenopsis include allowing the parasitoids to build up their numbers prior to applying chemical insecticides, and to use pesticides that minimise parasitoid fatalities; to attach plant parts bearing parasitoid-infested mealybugs (mummies) onto the host plant to be protected; mass-rearing and releasing parasitoids into mealybug-infested areas; and using a combination of several natural enemies to control the exotic pest (Pala Ram et al., 2009). Predators are used to control P. solenopsis on cotton in several countries. The commercially available predatory ladybeetle, Cryptolaemus montrouzieri (Coleoptera: Coccinellidaae), has been imported into India and released in cotton-infested fields to help to control P. solenopsis.

Several species of ants are often associated with honeydew-producing mealybugs, and protect them from attack by their natural enemies. To increase the chances of success in using biological control agents against P. solenopsis, attendant ants need to be eradicated if at all possible (Helms and Vinson, 2002; Tanwar et al., 2007).

Chemical control

It is often effective to spot treat small populations of the mealybug, detected during early monitoring, with soap solution. Before Bt cotton was introduced to Pakistan and India, P. solenopsis was considered to be a secondary pest of cotton, maintained at low population levels by chemical applications to control the primary pest, Helicoverpa armigera. With the cultivation of transgenic cotton with a reduced need for chemical applications, the mealybug emerged as a major pest requiring chemical applications to manage it. The use of insecticides is most effective against the mealybug when applications are timed to coincide with emergence of the vulnerable crawler stage. The use of ant baits to eliminate mealybug-attendant ants from the field is also beneficial. Dhawan et al. (2008; 2009a) evaluated the effectiveness of a variety of insecticides against P. solenopsis.

IPM

In India, a management strategem to control P.solenopsis that incorporates cultural, mechanical, biological and chemical control factors has been developed (Tanwar et al., 2007). This involves a pre-planting survey for P. solenopsis; targeting and spot treating small populations chemically; removal of alternate host plants and ant colonies; using recommended insecticides for optimal effectiveness on the plants and around their root systems; providing an attractive habitat for native and exotic natural enemies; and using a variety of sanitation methods to prevent spread of the pest to new fields.

Monitoring and surveillance

The placing of sticky traps in the fields and around their borders, to catch P. solenopsis males, is useful in early detection of the mealybug in the area. Observations of plant stems, leaves and flowers aid in detecting the white, waxy masses produced by P. solenopsis.

The biology, development and control of P. solenopsis need to be researched further to determine the biotic and abiotic factors that may impact its development and survival. Because the mealybug has a varied host range, studies on the preferred hosts could aid in developing and implementing effective control methods. Comprehensive studies need to be conducted on the parasitoids and predators associated with P. solenopsis, including their life histories, and a determination of the combinations of natural enemies that would be most effective in suppressing the pest populations in areas with different climatic conditions and altitudes. It is important that information is obtained and distributed regarding those risk factors that may be necessary to address in order to implement a successful control programme. Evaluation of the impact of crop damage or loss in reference to the use of chemical applications, compared to the time needed to develop established natural enemy populations to suppress P. solenopsis, need further study.

11/12/09 Original text by:

Paris Lambdin, Department of Entomology, Agricultural Experimental Station, University of Tennessee, PO Box 1071, Knoxville, TN 37901, USA

13/01/20 Updated by:

Gillian Watson, Consultant, UK