Zygogramma bicolorata was known to feed on parthenium and on the closely related plant annual ragweed,
Ambrosia artemisiifolia. Ragweed and parthenium are in the subtribe
Ambrosiinae, and sunflower
Helianthus annuus is in another subtribe of the same tribe
Heliantheae. Other species of
Zygogramma are pests of sunflower in the USA, but Z. bicolorata
has never been recorded feeding on sunflower in Mexico or the USA, where it is native. Two other species,
Z. disrupta and Z. suturalis, have been introduced into Russia and the Ukraine for the biological control of annual ragweed, resulting in at least partial control of the ragweed and without any reports of damage to sunflower
(Julien 1992).
Prior to introduction into Australia and India, detailed host
specificity testing of
Z. bicolorata was carried out. Both choice and non-choice tests were undertaken with adults and first instar larvae, on a wide variety of plants in the Asteraceae
(McFadyen and McClay
1981). The results were quite clear; when given a choice, adults would neither feed nor lay on any plants except parthenium and ragweed. However, when given no choice, after 1 to 2 days adults will feed on most plants in the tribe
Heliantheae, and on some there was extensive feeding and considerable damage. Few eggs were laid on these plants, and larvae did not feed or survive on them.
