Use of Urophora cardui for suppression of Californian thistle flowering in pea crops

Abstract

Californian thistle (Cirsium arvense L. Scop.) is a pest to the organic pea industry because flower buds may be accidentally harvested and are difficult to remove during processing. The Californian thistle gall fly (Urophora cardui L.), a biological control agent introduced from Europe, is capable of suppressing thistle flowering, but its effectiveness in pea crops has, to date, not been investigated. To prevent flowering, galls need to form in the terminal region of the thistle stem, which requires attack of young stems (less than about 0.2 m tall).

Investigations into the synchronization of adult flies and target height thistles, showed that adult flies emerged too late in the season to effectively attack thistles in pea crops. I therefore investigated factors relating to the success of flies released in patches of young thistles in pea crops. These factors included: manipulation of adult fly emergence, fly yield per gall, oviposition period length, dispersal from target thistle patches, adult fly food requirements, and the release intensity of flies required to give 90-100% control of thistle flowering.

Manipulation of fly emergence is possible and an average of 3.6 flies was found per gall. The main oviposition period is within the first 10-12 days after adult females emerge from the gall and flies do not require supplementary food for reproduction. Flies remain in the crop when released but are not attracted to a yeast-based attractant when applied as a foliar spray. Laboratory testing showed that one pair of flies could produce up to 21 galls and, on average, each pair produced 10.8± 7.8 galls when given access to large numbers of thistle stems, but that an intensity of 0.8 pairs per stem was needed to give 90±5% flower suppression. However, field results showed that field released flies produced fewer galls, and the number of flies required to achieve adequate flower suppression is nearer 2.5 pairs per stem. This equates to the need to collect or rear nearly 1.5 galls to treat each thistle stem. Collecting or rearing galls is more labour intensive than conventional methods of thistle control. Thus, although using U. cardui to suppress Californian thistle flowering in pea crops may be technically feasible, it is neither practical, nor commercially viable.