The development of the CrylAc-resistant strains of H. armigera (BX and Is)

provides valuable models for evaluating the risks and consequences of field

populations of this pest becoming resistant to INGARD cotton.

Although the resistance ratio for the BX is strain is low in comparison to data

reported for other species, it is highly significant. H. armigera has a high natural

tolerance for Bt toxins (about 30 times that of HeIiothis virescens) and a low level

of resistance on top of this tolerance is sufficient to allow survival on glasshouse grown

transgenic cotton. Some further evaluation, involving testing of the ability

of heterozygotes to survive on INGARD cotton conducted with higher expressing

varieties and field grown plants, is required to gauge the practical significance of

this level of resistance.

As the BX strain was established from field collected insects that had survived Dr

Neil Forrester's screening assays, it was not possible to determine the frequency

of the resistance al!ele in field populations with a high degree of accuracy.

However, Dr Forrester estimated that the BX strain was established from the

survivors of perhaps 3000 field-collected insects. This suggests that one or more

insects in 3000 was carrying the resistance allele, an estimate that is of the same

order as estimates of CrylA-resistance alleles in Heliothis virescens,

Pectinophora gossypiella and Chilo suppressalis. This relatively high frequency

of resistance alleles in natural populations strongly supports the need for effective

resistance management strategies.

The very strong response of the Is strain to selection indicated that the decline in

resistance ratio for the BX strain after generation 22 was due to inbreeding

lowering the general vigour of the strain. Application of increasing selection

pressure to the Is strain showed that high resistance levels (RR=800) can be

attained by this species. The demonstration by Olsen and Daly (CSE89C) that

heterozygotes of BX and a susceptible strain have some tolerance of INGARD

cotton suggests that heterozygotes of IS and a susceptible strain could be very

damaging.