Biocontrol for Mexican Daisy?
Mexican daisy. Photo credit: Auckland Council.
Mexican daisy (Erigeron karvinskianis), an emerging environmental weed in New Zealand, was introduced as an ornamental plant in the 1940s. Since then it has escaped from gardens to spread around both the North Island and South Island. Mexican daisy has even found its way to the Chatham Islands, so it isn’t fussy about where it grows and its range is still expanding. The plant has ‘unwanted’ status, making it illegal to propagate, sell or knowingly spread the plant around.
In its native range (Mexico and South America) Mexican daisy is found from sea level to as high as 3200 m, so it doesn’t mind cold temperatures. An invader also in Hawai‘i, this plant forms dense mats there that displace native vegetation, and it seems likely that, given the chance, Mexican daisy could do the same in warmer parts of New Zealand. In suburban areas of Auckland Mexican daisy is now considered a major roadside weed. It may look innocent enough, but this weed has the potential to invade important ecological areas such as the Waitakere ranges and other areas of conservation value.
Although Mexican daisy responds well to herbicide, it is now so widespread and expensive to control that Auckland Council are looking at biological control as a possible solution. Mexican daisy has established in some regional parks and is competing for habitat with threatened native plants. “It produces vast amounts of small windborne seed and can spread vegetatively,” said Paul Peterson, who has recently looked into the feasibility of biocontrol for this potential target on behalf of Auckland Council. The Department of Conservation (DOC) also consider the weed a high priority and have reported it to be difficult to control, especially in coastal areas.
“Biocontrol of Mexican daisy has not been attempted anywhere before, but it could provide a cost effective solution to the problem,” said Paul. “Unfortunately, being a novel project does escalate the likely overall cost,” he added. Host-range testing could be extensive given that in New Zealand we have eight native genera also in the Astereae tribe, although in different sub-tribes to Mexican daisy, with 68 species considered of conservation value here in New Zealand. However, we may only need to test a much smaller list of species if a candidate agent were found that did not attack the more closely related exotic species also present here.
In terms of natural enemies, Mexican daisy is known to host at least two pathogens. One is a leaf spot (Didymella glomerata), which is relatively common in New Zealand but has only been recorded once from Mexican daisy. Searching for a more virulent strain to attack Mexican daisy would be precluded by its wide host range. The other, a powdery mildew (Podosphaera fusca), is also unsuitable because it is too damaging to valued plants in the gourd family. However, very little research been done on the natural enemies of this plant, so other potentially useful pathogens may be out there. As far as insects go, there are several associated with Mexican daisy in its native range, but none are considered to be specific to the plant. “Proper surveys would be expected to have a reasonably good chance of finding potential agents worthy of further study,” said Paul.
In 2012 Quentin Paynter developed a framework to help make decisions about the likelihood of success of biocontrol programmes given the traits of a weed. “Information about the life history and breeding strategy of the weed are compared to other weeds that have been targeted with biocontrol agents,” said Paul. This results in an overall score reflecting the mean proportional reduction of the weed expected due to biocontrol, and for Mexican daisy we can anticipate a moderate to good outcome. “But this does depend a little on what we can learn about the way Mexican daisy reproduces here, which is currently unknown. If it reproduces sexually rather than producing clonal seed (apomictic), then there is less chance of successful biocontrol,” Paul cautioned.
As well as surveys in the native range, a key first step in a biocontrol programme would be to survey Mexican daisy growing in New Zealand to see what is living on it here that might be detrimental to any potential biocontrol agents. “We also need to study the DNA profile of Mexican daisy plants growing here and compare them with plants from overseas, to try and pinpoint exactly where they originated from. This would give a better picture of where we might find the most suitable agents and the climate match with New Zealand,” explained Paul.
This study was funded by Auckland Council.