This article evaluates the potential for Bemisia tabaci, commonly called sweet potato whitefly to become invasive in New Zealand. It is not currently known to be extant in New Zealand, but it is on the “100 of the world’s worst invasive alien species” list in the Global Invasive Species Database (GISD) of the Invasive Species Specialist Group (ISSG) (Invasive Species Specialist Group, 2004). Bemisia tabaci, also know as cotton whitefly or tobacco whitefly, is found on all continental landmasses except Antarctica (ISSG, 2005).
Ecology:
The adult female B. tabaci deposits tiny, white, pear-shaped eggs (ISSG, 2005) randomly in singles or scattered groups, or sometimes in semi-circles on smoother-leaved plants (Department of Environment, Food and Rural Affairs, 2006), into the inner tissue layer on the underside of leaves towards the top of the host plant (ISSG, 2005; Mau et al., 2007). Females will deposit between 28 and 300 eggs depending on the species of the host plant and the temperature, in densities of up to 180 per centimetre squared (Mau et al., 2007). The total development period from egg to adult can vary from 15 to 70 days, again depending on the species of the host plant and the temperature, with 27C being the optimal (Mau et al., 2007). The eggs hatch after from 5 to 20+ days depending on temperature (Mau et al., 2007), for example, after 6 to 7 days at 25C (ISSG, 2005).
There are four nymph stages, with the first being known as the “crawler”, as unlike the other stages it is mobile, searching the leaf for a suitable place to imbed its needle-like mouthparts so as to access the sap of the host plant (ISSG, 2005). The other three stages are sessile, with molts occurring between successively larger stages (ISSG, 2005). The adult, about 1mm long with a yellow body and two pairs of white wings, usually emerges from its “pupal” case in the morning and can mate within a few hours (Mau et al., 2007) after elaborate courtship rituals between the normally equally ratio-ed males and females (ISSG, 2005). Oviposition occurs from one to eight days later with the mated females laying eggs that will become female adults and unmated females laying eggs that will become male (ISSG, 2005; Mau et al., 2007). Adult lifespans usually vary from 6 to 55 days dependent on temperature; adult females can live several months through winter in the southern USA (Mau et al., 2007).
Six hundred plant species have been cited as hosts to B. tabaci (ISSG, 2005), including both crop and non-crop or “weed” species (Mau et al., 2007). The insect is also a vector for over 110 plant viruses including tomato yellow leaf curl virus (TYLCV) and cucurbit yellow stunting disorder virus (CYSDV) that are particular concerning for greenhouse growers (DEFRA, 2006). Interestingly, the whiteflies are attracted to the colour yellow although they are not thought to react to odours (Mau et al., 2007). Besides introducing viruses, the adults and nymphs cause direct damage through their feeding and indirect from excreted honeydew, which is excess phloem that passes right through the digestive system of the nymphs, providing a substrate for black, sooty mould (Mau et al., 2007).
Nymphs stay on the leaves where their eggs were laid, but the adults have two modes of flight, controlled around a plant or between neighbouring plants, but when caught by a wind it will passively allow itself to be carried (Mau et al., 2007). Thus dispersal occurs in the direction of the wind (Mau et al., 2007).
Global Status:
B. tabaci is thought to have originated in India, but this is uncertain (ISSG, 2005). It is considered a pest and invasive wherever it’s found (Mau et al., 2007), which is the tropical and sub-tropical regions of all continents, in the glasshouses of much of Europe and within the Pacific Ocean region it is on the Cook Islands; the Federated States of Micronesia; Fiji; Kiribati; New Caledonia; Niue; Palau; Papua New Guinea; Polynesie Francaise; Samoa and Vanuatu (DEFRA, 2006; ISSG, 2005; Mau et al., 2007). It is predominantly found in agricultural and urban areas (ISSG, 2005).
The sweet potato whitefly develops in temperatures from 10 to 32C; eggs do not hatch in temperatures over 36C (Mau et al., 2007). Suitable temperature ranges are virtually the only requirement for this insect because it has such a wide range of host plants available to it for sustenance. This is shown by its extensive spread despite being considered a pest in all human plant growing endeavours.
Threat To New Zealand:
B. tabaci is almost certainly going to arrive in New Zealand if it hasn’t done so many times already in the clothing of travellers from any tropical or sub-tropical region of the planet or through hitchhiking with food imports from infected pacific islands or the increasing supply of fruit and vegetables coming from China (Cheng, 2007). In the UK it is often more likely to arrive on ornamental plants (DEFRA, 2006), but New Zealand’s more vigorous biosecurity controls on importation is likely to stop this. Plant smuggling is sometimes attempted into NZ (Pickmere, 2004) and it is probable that it may sometimes succeed; therefore the whitefly could be accidentally introduced through this process.
If it does arrive, there is a good chance that it would be unable to establish itself here as an alien invasive species because our annual temperature range is likely to be too cold for it. It is possible it could manage to survive through milder winters in Northland and certainly in greenhouses and spread from those reservoirs in the warmer months. But if such an eventuality occurred, eradication methods should be comparatively easy to arrange and implement than in warmer nations.
Based on the above information, it can be concluded that it is likely that B. tabaci will arrive in NZ, but considerably less so that it will be able to establish itself as an alien invasive insect; this would give it a likelihood of “possible”. Based on the impacts on crops being experienced where it is extant, the consequences of it establishing itself here should be considered major. This places Bemisia tabaci in the high-risk category for New Zealand.
References:
Cheng, D. (2007) Food stuff labels to state place of origin. NZ Herald August 11, 2007.
Department for Environment, Food and Rural Affairs (2006) Bemisia tabaci the tobacco whitefly. http://www.defra.gov.uk/planth/pestnote/2006/bemisia.pdf
Invasive Species Specialist Group (2004) 100 of the world’s worst invasive alien species. http://www.issg.org/database/species/search.asp?st=100ss&fr=1&str=
Invasive Species Specialist Group (2005) Bemisia tabaci (insect).
http://www.issg.org/database/species/ecology.asp?si=106&fr=1&sts=&ver=print&prtflag=false
Mau, R., Kessing, J. & Diez, J. (2007) Bemisia tabaci (Gennadius).
http://www.extento.hawaii.edu/kbase/crop/Type/b_tabaci.htm
Pickmere, A. (2004) Obituary: Robin Dare. NZ Herald March 6, 2004.
