Resistance mechanisms of Helicoverpa armigera

The cotton bollworm, Helicoverpa armigera, is one of the major agricultural pest species in the Old World and recently also of the New World. This noctuid moth species is highly polyphagous and possesses a huge geographical distribution and the ability to quickly evolve resistance to insecticides from different chemical classes. There are different mechanisms known with which insects combat insecticides. They are ranging from behavioral over morphological to physiological adaptations. These resistance mechanisms can occur alone or in combination and may change in the field according to changing selection pressures. A reduced penetration through the cuticle of H. armigera larvae is known which reduces the concentration at the target site. Also mutations of the target of pyrethroid insecticides, organochlorines, and oxadiazines, voltage-dependent sodium channels, were described that lead to high or lower resistance levels. Furthermore, both carboxylesterases and cytochrome P450 monooxygenases were investigated to determine their role in insecticide resistance. So far, only few enzymes were identified in H. armigera which were proven to metabolize and thus detoxify insecticides. Most studies deal with the resistance against pyrethroids. One important resistance gene is the chimeric P450 CYP337B3 that is present in resistant and absent in susceptible individuals. The corresponding enzyme is capable of metabolizing fenvalerate and cypermethrin and thus confers resistance to H. armigera larvae. This new resistance mechanism by recombination seems to play an important role in H. armigera populations throughout the world

Resistance of Australian Helicoverpa armigera to fenvalerate is due to the chimeric P450 enzyme CYP337B3

Worldwide, increasing numbers of insects have evolved resistance to a wide range of pesticides, which hampers their control in the field and, therefore, threatens agriculture. Members of the carboxylesterase and cytochrome P450 monooxygenase superfamilies are prominent candidates to confer metabolic resistance to pyrethroid insecticides. Both carboxylesterases and P450 enzymes have been shown to be involved in pyrethroid resistance in Australian Helicoverpa armigera, the noctuid species possessing by far the most reported resistance cases worldwide. However, specific enzymes responsible for pyrethroid resistance in field populations of this species have not yet been identified. Here, we show that the resistance toward fenvalerate in an Australian strain of H. armigera is due to a unique P450 enzyme, CYP337B3, which arose from unequal crossing-over between two parental P450 genes, resulting in a chimeric enzyme. CYP337B3 is capable of metabolizing fenvalerate into 4′-hydroxyfenvalerate, which exhibits no toxic effect on susceptible larvae; enzymes from the parental P450 genes showed no detectable fenvalerate metabolism. Furthermore, a polymorphic H. armigera strain could be bred into a susceptible line possessing the parental genes CYP337B1 and CYP337B2 and a resistant line possessing only CYP337B3. The exclusive presence of CYP337B3 in resistant insects of this strain confers a 42-fold resistance to fenvalerate. Thus, in addition to previously documented genetic mechanisms of resistance, recombination can also generate selectively advantageous variants, such as this chimeric P450 enzyme with an altered substrate specificity leading to a potent resistance mechanism