Diamondback moth: biocontrol and a preference for peas

Diamondback moth, a common and widespread pest of crucifer crops worldwide, has astounded entomologists by developing a taste for peas. Reports from Kenyan vegetable export growers that the pest was attaching sugar-snap pea crops during the 1999 season were at first dismissed. However, field observations of the damage and the presence of the pest in fields near Lake Naivasha by the International Centre for Insect Physiology and Ecology (ICIPE) confirmed that the pest was indeed Diamondback moth - DBM (Plutella xylostella).

DBM larvae feeding on cabbages cannot normally survive on peas. But closer analysis of the larvae attacking peas has revealed that is a new strain, which can survive equally well on both hosts. More recently, DBM has also been reported attacking mange-tout peas which, at the time of initial reports, had been free of the pest. The reasons for the change in host plant are not yet understood, although currently being investigated.

The adaptability of Diamondback Moth has already been well demonstrated in its remarkable ability to thrive in tropical, subtropical and temperate climates. In hot conditions, the pest is able to develop rapidly throughout the year with a new generation emerging every two to four weeks. Consequently, pest numbers increase significantly within a very short time. Not surprisingly, over a billion US dollars are spent on pesticides each year to control this voracious pest. However, DBM has developed resistance to all commonly used pesticides and, as a result, farmers are increasingly using a cocktail of chemicals and spraying more frequently. Intercropping with repellent plants, such as tomato, has been reported to be effective in Kenya and the use of neem has been shown to achieve slow but effective control. However, the focus of an ICIPE-led biocontrol project is on the assessment of indigenous parasitoids and introduction, where necessary, of more efficient parasitoids from other countries. These parasitoids are highly specific small wasps that lay their eggs inside DBM larvae.

Credit: Bernhard Löhr, ICIPE

Using parasitoid
Use of parasitoids for the control of DBM is not new. A well known species, Diadegma semiclausum, originated in the UK and during the early part of last century it was taken to New Zealand, introduced then to Malaysia and Taiwan, and is now to be found throughout most of south-east Asia. The species has been found to perform particularly well in temperate highland conditions but, until 2001, it had not been introduced to East Africa. To determine key indigenous parasitoid species of DBM in East Africa, and their level of parasitation, surveys have recently been carried out in co-operation with national research institutions in Ethiopia, Kenya, Tanzania, and Uganda. Oomyzus sokolowskii, another parasitoid found worldwide, was found to be one of the important indigenous natural enemies as well as D. mollipa, which until recently was believed to be identical to D. semiclausum. However, overall level of parasitism of indigenous parasitoids was only recorded as 10-15% - much lower than was found in similar situations in SE Asia and South Africa.

Another parasitoid, Cotesia plutellae, was also recorded in the four countries but only in very low numbers. Elsewhere, this parasitoid is known to be very effective and, in recent years, has been used to control DBM in St Helena. The South African strain is also known to parasitise at effective levels in the lowveld so an agreement has been signed with the Plant Protection Research Institute of South Africa to work on the biology of C. plutellae in South Africa. It is hoped that this particular species may be introduced into the semi-arid regions of East Africa.

During the last year, populations of DBM at the four selected pilot sites in Kenya and Tanzania have been assessed every two weeks and samples collected for estimation of parasitism along with climatic data over a period of a year. These records will allow impact assessment of releases of D. semiclausum imported from Taiwan . Releases in all major production areas are planned for 2003 and impact monitoring will be conducted for at least one year. Additional pilot sites will then be established in lowland areas in preparation for introduction of parasitoid species adapted to higher temperatures such as C. plutellae from South Africa.

It is unlikely that one parasitoid will prove totally effective even within one pilot site. However, as different species parasitise the various larval and pupal stages of DBM, it is hoped that a combination of parasitoids will achieve the greatest level of control in crucifer crops as well as decreasing the pest numbers attacking peas.

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