Isotima javensis (Rohwer)

Isotima javensis is an important parasitoid of sugarcane top borer Scirpophaga excerptalis (Walker). It shot into prominence when a strain of this parasitoid was introduced from North India to South India. Between 1955 and 1958, 78% of the top borer population was parasitised by this parasitoid in Pugalur. The parasitoid can be multiplied and released in endemic areas for the suppression of top borer.

The larvae which are about to pupate are exposed to female parasitoids to obtain eggs and these eggs inoculated or transferred on individual larva or the larvae are exposed directly either in paper straw or infested tops of sugarcane for 24 hrs and the emerging adults collected from the cages where the parasitized material is kept. But a method developed at Indian Institute of Sugarcane Research, Lucknow with some local modifications is now widely accepted and is detailed below.

Rectangular cages fitted with wire mesh on upper and two sides and sleeves and acrylic shutters on the remaining two sides i.e,. front and back are used. Four five cm wide wooden planks with 2.5 cm diameter holes 15 cm apart are fixed on the top of the cage. The bottom is provided with five iron rods (0.6 cm in diameter) to the upper planks. For each cage 20 wooden rods (70 cm length), each having 20 holes are used. The rods are inserted from the top holes to rest on the bottom iron rods. In this assembly, holes in wooden rods are available at every five cm distance. The bottom of the cage is lined with 2.5 cm thick sponge sheet which is moistened daily or on alternate days to provide 50-60% humidity.

For introducing larvae into cage, paper straws are cut into 2.5 cm long pieces and one side is plugged with cotton. From the open end top borer larva is inserted in every straw piece. The larva spins a silken disc at the open end.

Before introducing parasitoid adults split raisins and saturated cotton swabs of honeywater in ratio of 1:1 are provided for adult feeding. Two to three day old males and freshly emerged female wasps are introduced into the cage. The female parasitoid on finding the silk disc inserts its ovipositor to sting and paralyze the mature larva or pre-pupa of top borer and deposits its egg. The straw pieces containing parasitoid larvae are removed every 24 to 48 hrs and fresh straw pieces containing top borer larvae are kept again in the same manner as described above. For further development of parasitoids, the straws containing parasitized larvae are kept in adult emergence cages. The process is continued. The temperature inside the cage could vary from 26-30oC. Seventy to eighty per cent of the parasitized top borer larvae give rise to adults in 17-39 days (eggs hatch in 2.1 days, larvae and pupae complete development in 4.5-9 days and 6-9 days, respectively). Each female can lay 6 to 10 eggs every day during its active oviposition period.