Whiteflies

Whiteflies are more important as vectors of many virus diseases than as direct pests.  Heavy infestations will reduce plant vigour.

Worldwide.

Both immature and adult whiteflies feed by piercing the undersurface of leaves and sucking the plant sap. 

The nymphs, which feed in one position for 2-4 weeks, may cause yellow spots which are conspicuous on the upper leaf surface.  On older leaves the spots develop a brown centre.  Some sweetpotato cultivars respond to the damage by producing purple pigments, resulting in purple spots with a brown centre.

Feeding by adults usually causes no visible lesion, but high whitefly populations may cause yellowing of infested leaves, and may stunt plant growth. 

Whiteflies produce honeydew, a sugary secretion, making the leaf surface sticky.  In humid conditions, sooty mould may develop on the leaf surface, reducing its capacity to trap light required for the plant's normal growth.

The pest is more important as a transmitter of viruses such as sweetpotato mild mottle virus, sweetpotato yellow dwarf disease, sweetpotato leaf curl, and particularly sweetpotato chlorotic stunt virus, a key component of sweetpotato virus disease.

Egg.  Small, elliptical, smooth surfaced, yellow to tan eggs are laid on leaf surface, usually on undersurface, in irregular, waxy lines,  forming a spiral ( hence the common name). Eggs take about seven days to hatch.

Larva. There are four larval stages that take about 30 days to complete.  The first instar is mobile, crawling a short distance from the egg before settling to feed.  The following 3 instars are immobile, appearing as a small, flattened, oval scale on the leaf surface.  The final instar stops feeding and is referred to as the Pupa. It is colourless or yellowish, nearly oval, flat and about 1 mm long and 0.75 mm wide.  They secrete waxy threads on their back which are about 12-15 mm long and provide a protective cover.

Adult. The adults are white and quite small, about 2-3 mm  in length and coated with a fine dust-like waxy secretion. The eyes are dark reddish-brown, wings are transparent with white powder covering. Females lay eggs throughout their lifetime

Sweetpotato whitefly (B. tabaci)

Egg. The egg is about 0.2 mm long and pear-shaped. It stands upright on the leaf, being anchored at the larger end by a tail-like appendage inserted into a stoma. Eggs are  generally laid on the undersurface of younger leaves. They are white when first laid but later turn brown. There can be as many as 1,200 eggs per square inch.

Nymph. The first nymphal stage is called crawlers, when nymphs only move a very short distance before settling down again and starting to feed. Once settled they do not move again. All the nymphal instars are greenish white, oval in outline, scale-like and somewhat spiny.

Pupa. The last-instar (the so-called “pupa”) is about 0.7 mm long and the red eyes of the adult can be seen through its transparent integument.

Adult. The adult is minute, about 1 mm long and emerges through a slit in the pupal skin and is covered with a white, waxy bloom. it has solid white wings and pale yellow body. The snow-white colour is due to the secretion of wax on its body and wings.

Whiteflies have six life stages - the egg, four nymphal stages, and the adult. The development time of this insect from egg to adult depends upon temperature. Temperature range of 10 to 32^oC favours development.  Low temperatures increase egg mortality. The pest is more common during the dry season.

The female may lay 100 or more eggs on the  back or undersurface of the leaves. They hatch after about seven days. The first instar nymphs emerge and feed. After the first moult they become fixed to one place of the leaf and develop into pale green or creamy flattened scale-like bodies. The third moult gives rise to the “puparium” which is less flat than the previous nymphal stages and the red eyes of the adult can be seen through the integument. Development of one generation takes 2 to 4 weeks depending on temperature. 

In both species, reproduction can occur without mating (parthenogenesis). Unmated females produce only male progeny.

Host range

Whiteflies have a wide host range, including avocado, banana, cabbage, capsicum, cassava, cauliflower, citrus, coconut, cotton,  eggplant, garlic, guava, legumes, mango, mustard, onion, pechay, pepper, radish, squash, soybean, tomato, and tobacco.

Detection and inspection

The insects appear as small white scale-like objects on the undersurface of the leaves. If the plant is shaken, a cloud of tiny moth-like insects flutter out but rapidly resettle.

Cultural control

Field sanitation and use of insect-free planting material.

Movement of clean, healthy germplasm.

Planting non-host crops after sweetpotato.

Biological control

Natural enemies were introduced to control whiteflies. The parasitic wasp Encarsia formosa is an effective biological control agent against both A. dispersus and B. tabaci while some species of coccinellid beetles, true bugs (Hemiptera: especially Anthocoridae and predatory Miridae), lacewings (Neuroptera: Chrysopidae, Hemerobiidae, Coniopterygidae), flies (Diptera: Dolichopodidae, Syrphidae, Anthomyoodae), ants (Hymenoptera: Formicidae), spiders (Araneida) and mites (Acarina: Phytoseiidae, Stigmaeidae) also prey on them.

Chemical control

Although control measures are not usually needed, insecticides may be warranted during high infestation - consult your local supplier or extension service for appropriate products. Sugar esters from Petunia spp. are also found effective against sweetpotato whitefly. However, controlling whiteflies is not usually an effective means of limiting the incidence of the viruses they transmit.

References

Amalin, D.M. and Vasquez, E. A. 1993. A handbook on Philippine sweetpotato pests and their natural enemies. International Potato Center (CIP), Los Baños, Philippines. 82 p.

Ames, T., Smit, N.E.J.M., Braun, A.R., O’Sullivan, J.N., and Skoglund, L.G. 1996. Sweetpotato: Major pests diseases, and nutritional disorders. International Potato Center (CIP). Lima, Perú. 152 p.

Denholm, I., M. Cahill, T.J.Dannehy and A.A. Horowitz. 1998. Challenges with managing insecticide resistance in agricultural pests, exemplified by the whitefly Bemesia tabaci. In: Insecticide Resistance From Mechanism to Management. I. Denhollm, J.A. Picket and A.L. Devonshire. The Royal Society CABI Publishing. UK. pp. 81-91.

Hill, D.S., 1983. Agricultural Insect Pests of the Tropics and their Control. Second Edition. Cambridge University Press, Cambridge, UK, 198 p.

Kays, S.J., Severson. R.F., Nottingham. Ss.F., Chalfant, R.B., and Chortyk, O. 1997. Possible biopesticides from Petunia for the control of the sweetpotato whitefly (Bemesia tabaci) on vegetable crops. Proc. Fla. State Hort. Soc. 107:163-167.

Martin Kessing, J.L. and Mau, R.F.L. 1993. Aleurodicus dispersus (Russell). http://www.extento.hawaii.edu/kbase/crop/Type/a_disper.htm. 16 July 2002.

Martin Kessing, J.L. and Mau, R.F.L. 1992. Bemisia tabaci (Gennadius).  http://www.extento.hawaii.edu/. 16 July 2002.

PANS. 1978. Pest Control in Tropical Root Crop. Manual No. 4. Center for Overseas Pest Research. London 235 p.

Shepard, B.M., Carner, G.R., Barrion, A.T., Ooi, P.A.C. and van de Berg, H. 1999. Insects and their natural enemies associated with vegetables and soybean in Southeast Asia. 108 p.

United States Department of Agriculture, WHITEFLY KNOWLEDGEBASE. http://whiteflies.ifas.ufl.edu/wfly000d.htm

Vasquez, E.A. and Sajise, C.E. 1990. Pests of sweetpotato: Insects, mites and diseases. Philippine Root Crop Information Service, Philippine Root Crop Research & Training Center. 65 p.

Contributed by: Erlinda Vasquez, Nicole E.J.M. Smit and Jane O'Sullivan

Taxonomy

Economic importance

Geographical distribution

Damage

Morphology

Biology and ecology

Detection and inspection

Host range

Management

References

Yellow leaf spots, some of which develop dark centres, due to feeding by whitefly nymphs (J. O'Sullivan).

Purple spots induced on some cultivars (J. O'Sullivan).

An egg mass of spiralling whitefly, and irregular yellow trail, becoming nectrotic in parts, thought to be a result of feeding by the scale-like whitefly

nymph (J. O'Sullivan).

Egg masses of spiralling whitefly on leaf (E. Vasquez).

Sweetpotato whitefly adults  on leaves (E. Vasquez).

Spiralling whitefly pupae with wax threads (E. Vasquez).

Spiralling whitefly adult (E. Vasquez).

Sweetpotato whitefly adult (A. Braun).