Plant growth parameters of musk thistle Carduus nutans and egg distribution patterns of Rhinocyllus conicus on their blooms

Plant growth phenology of musk thistle, Carduus nutans, L., in Oklahoma was three weeks ahead of patterns previously reported from Virginia. For example, plant dormancy ended in late February to early March, bolting initiated in early April, anthesis began in middle to late May and plant senescence occurred in early July. The musk thistle head weevil, Rhinocyllus conicus Froelich, was present when bolting occurred and was active for five and one-half weeks, ovipositing for up to four weeks. In a two-year study, preference sites for oviposition by musk thistle head weevils were heads on the outside and top of plants. In 2001, surveyed locations had been infested with weevils for six or more years with larger weevil populations, and greater numbers of eggs present. Checking the top, outside heads for egg numbers will allow growers a focused, more decisive method in determining levels of weevil infestation. Irrespective of ovipositional preference, redistribution of adult weevils can occur when 50% of thistle heads contain at least six eggs.