As Zygogramma bicolorata will not lay eggs, and larvae cannot survive, on plants other than parthenium and ragweed, and in the native range there is never attack on sunflower in the field, it seemed clear that this beetle was safe to introduce into countries where biological control of parthenium and ragweed is required, even if sunflower is a major crop in the same area. Zygogramma bicolorata was therefore released in both India and Australia for the biological control of parthenium.

In Australia, the beetle immediately established and became abundant on annual ragweed in the coastal areas, where no sunflower is grown. In the parthenium area where sunflower is also grown, the beetle did not do well initially and has only become abundant since 1990. Since then, beetle numbers have been enormous in some localities and seasons, but because of drought conditions, little sunflower has been grown in the area and there are no reports of Z. bicolorata damaging sunflower. When beetle numbers are very large, related plants such as Verbesina encelioides (tribe Heliantheae, subtribe Verbesiniinae) and Xanthium occidentale (tribe Heliantheae, subtribe Ambrosiinae) have been extensively damaged in the field but these are both weeds and the damage has not caused any problems.

In India the beetle spread and increased rapidly, building up very large populations on parthenium in the Bangalore area and spreading from there. After a few years, there were reports of serious damage to sunflower crops in the area. In southern India, sunflower is grown in very small fields, only 0.5 hectare in extent, often surrounded by fallow fields or rough pasture infested with dense parthenium. When there were no green leaves on the parthenium, either because it had dried up or because the beetles had eaten all the leaves, adult beetles fed on adjacent sunflower crops causing near-total defoliation of the outer rows. Feeding usually occurred over a 3 to 4 week period, after which the beetles moved away in search of parthenium or to diapause in the soil. Some eggs were laid on the sunflower, but as the larvae did not survive, this was not significant (Jayanth et al. 1993).

In both Australia and India, feeding on non-host plants only occurred when enormous populations of the beetle, usually of newly-emerged adults, were present in the field, with 30 to 40 adult beetles per parthenium plant and several hundred per m². All green parthenium had been eaten and the beetles were starving. These are the conditions prevailing in starvation or no-choice tests, and the beetles reacted in the same way, feeding on and causing heavy damage to normally rejected plants. Field results from Mexico or the USA were not relevant, because these enormous beetle populations have never occurred there. This demonstrates the value of starvation tests on mobile stages of the life cycle, and the problems that may result from too great a reliance on field results in the insect's country of origin.

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