January 2012. A plea for help from a cassava grower who has a 20 ha farm in Battambong Province, Cambodia.

A member wrote: The underside of the cassava leaflet shows a mealybug and one immature aleurodicine whitefly on the middle leaf (photo, top left). The mealybug body colour is yellow, which suggests that it could be papaya mealybug (Hemiptera: Sternorrhyncha: Coccoidea: Pseudococcidae: Paracoccus marginatus Williams & Granara de Willink). If you put the mealybug in 80% ethanol at room temperature and the body contents turn black in 24 hours or less, that will help confirm this identification. If the body contents do not turn black in alcohol then the mealybug is likely to be Phenacoccus manihoti Matile-Ferrero, a serious pest of cassava. Both Paracoccus marginatus and Phenacoccus manihoti are species of New World origin that have been accidentally introduced to southern Asia in recent years. Ph. manihoti is fairly host-specific, but Pa. marginatus is quite polyphagous and will attack a wide variety of crops including cassava, vegetables, cotton and weeds in addition to seriously damaging papaya.

The second picture (photo, top right) shows immature and adult spiralling whitefly (Hemiptera: Sternorrhyncha: Aleyrodoidea: Aleyrodidae: Aleurodicinae: almost certainly Aleurodicus dispersus Russell).

For authoritative identification of mealybugs and whiteflies to species, they require preparation on slides and high power microscopic study. The largest immature stage of the whitefly is needed.

Note, an authoritative identification is needed before thoughts can be given to biological control as eacvh has host-specific biological control agents.

Summaries of past Pestnet discussions

Following on from the above preliminary identifications, there is information on the insects mentioned on the PestNet website. They are summaries of past discussions.

For Paracoccus marginatus, see:

www.pestnet.org/SummariesofMessages/Crops/Rootstubers/Cassava/ParasitoidscassavamealybugThailand

www.pestnet.org/SummariesofMessages/Pests/PestsEntities/Insects/ScalesMealybugs

For spiralling whitefly, see:

www.pestnet.org/SummariesofMessages/Pests/PestsEntities/Insects/Whiteflies.aspx

Control measures

Form sprays using natural products use soap or a horticultural (petroleum) oil. The recipes for these can be found in the Pest Fact Sheets on the PestNet website (www.pestnet.org). Choose either the Farmer or Extension version and go to number 56, especially 56d, of the flipping sheets.

Be careful when using either soap or horticultural oil, and make sure that you have the correct type. This website is useful:

http://gardening.about.com/od/naturalorganiccontrol/a/Hort_Oil.htm

If you buy a horticultural oil, follow the instructions. Also, always try the soap or oil on a few plants first to check that they are not going to damage the foliage.