June 2015. Tonga has been affected by the EL Nino. There has been a severe prolonged drought since July 2014. There have been outbreaks of several pests, aphids, mites, scales, weevils and fruit flies, as well as powdery mildews. Scale insects on the main island of Tongatapu are particularly troublesome on cassava. The scale, presumably, white peach scale, coats the stems, side branches, side shoots and even sometimes the petioles causing the stems to become thin, weak, and break in the wind. Infested cuttings often do not root, or root development is poor, and they are unpalatable.
Some plantations are approximately 70-85% infested; some only 25-40%, and other less than 10%. In some places the infestation is quite obvious, while elsewhere you need to go inside the plantings to see the infestation that has just started.
The scale on cassava looks like the white peach scale or mulberry scale, Pseudaulacaspis pentagona. The white masses seen at the base of cassava stems, or on chillies, bele (Abelmoschus manihot) and passionfruit, are the males. Some farmers mistake the scales for a fungus infection.
See fact sheet no. 052 in the app Pacific Pests and Pathogens or on line through the PestNet website. Here is the link: http://www.pestnet.org/PacificFactSheets
There is also a useful fact sheet from the University of Florida http://entnemdept.ufl.edu/creatures/orn/scales/white_peach_scale.htm
An Encarsia (and perhaps other parasitoids) was introduced to Samoa to bring the infestation on passionfruit under control. More recently, the department of agriculture, Hawaii, collected the parasitoid for release, and also offered it to Solomon Islands.
A member looked at the problem from the health of the cassava and, in particular, the availability of water. Drip irrigation is becoming important in the dry tropics where good water is scarce. It is being used to reduce the stress on cassava that is otherwise being attacked by pests. In Cambodia, under hot dry conditions, drip irrigation and plastic mulch, often using trellises, is being recommended. It is difficult to justify drip irrigation for a low value, low input crop, such as cassava. But in extreme conditions (especially dry conditions), the use of the higher cost technology such as drip irrigation might be considered. This is what it being done under NZAID projects in SE Asia.
Healthy seeds, drip irrigation and targeted fertilizer input equals healthy plants, minimal stress and minimal pest outbreaks. Also, farmers are being educated about predatory insects, and the need to preserve them by avoiding the use of broad-spectrum chemicals. They are using technologies that save water, resulting in healthy plants, and are using “calendar scouting” in place of “calendar spraying”.