Spread of volunteer and feral maize plants in Central Europe: recent data from Austria

Maize (Zea mays subsp. mays) is an annual monoecious crop frequently grown in many countries. In 2014, a total area of 184 Mio hectares was cultivated worldwide (http://faostat3.fao.org/download/Q/QC/E, accessed 24th of July 2016). Currently, around 30% of maize is genetically modified (GM) [1]. In 2014, 143,016 hectares of biotech Bt maize Mon810 have been cultivated in the EU, mainly in Spain. Transgenic maize for commercial production confers either insect resistance or herbicide tolerance or a combination thereof. This crop is mainly used for food and livestock feed, but also for renewable resources. Maize, domesticated by native Indians of Mexico and northern Central America already about 5500 years ago [2], has been introduced to Europe in 1525 owing to the discovery of America by Columbus. Since then, a large number of local varieties have been developed all over Europe. This crop has also been subject to trait improvement via genetic modification since several decades. In case of cultivation of GM maize, the main factors that determine adventitious presence of a genetically modified organism (GMO) in non-GM material are unintended seed impurity, seed planting equipment and practices, cross-pollination between GM and non-GM crops, the presence of GM volunteers, and product mixing during harvest, transport and/or storage processes [3]. Moreover, due to the current focussing in breeding, improvement and use of only a few crop varieties, the diversity of maize landraces could be threatened in future. Cross-pollination is possible in areas with hybridisation partners such as Mexico. However, teosinte—the closest relative of maize—has recently been detected also in Spain where it behaves like an invasive weed of agricultural land (http://www.agpme.es/index.php?option=com_contentview=articleid=181:el-teosintecatid=44:articulosItemid=68, accessed 30th of July 2016). Even though maize is a mainly wind-pollinated crop [4], it has also been observed to function as pollen source for honey bees [5]. So, non-target organisms that collect pollen of maize plants are exposed directly to GM pollen. Additionally, volunteer and feral maize plants contribute to a prolonged GM pollen exposure. Hence, the relevant environmental aspects of volunteer and feral maize include uncontrolled dispersal of GM plants into the environment, prolonged exposure of non-target organisms to GM pollen, increased use of herbicide to remove volunteer and feral maize and an adopted insect resistance management that is mandatory for Bt crops. In the USA, volunteer maize growing in soybean fields above the soybean canopy is known as a highly competitive weed and requires specific herbicide application [6].

It is controversially debated among European scientists, stakeholders and policy makers, whether maize volunteers in subsequent crops may pose a problem also in colder climatic zones of Europe. Moreover, it has been questioned, if maize has the ability at all to become feral outside cultivation in areas with cold winter temperatures and how the likelihood of becoming feral has to be rated under Central European continental climate conditions compared to those in Mexico. Some scientists assume that maize as a highly domesticated crop has very little invasion potential and poses a negligible ecological risk [7]. Maize seeds and seedlings are assumed to survive the winter only in southern European countries, such as Spain, where maize kernels that remain on the soil after harvest can germinate and develop into flowering individuals, which can locally cross-pollinate neighbouring maize plants [8]. However, by a combination of weak growth, asynchronous flowering with the maize crop, low resistance to frost, low competitiveness, absence of a dormancy phase, susceptibility to diseases, herbivory and cold climate conditions survival of the plants is estimated to be unlikely, rendering the risk for outcrossing and establishment of populations limited [3, 9, 10]. So far, there have been no records for survival of volunteer and feral maize plants in the Netherlands [11]. Occasional records for maize growing outside agronomic conditions on the British Island have been made, but are rare [12, 13]. Irish maize varieties, while cold adapted, were observed to be still frost intolerant [14]. However, single plants were registered in two Irish port locations, Limerick and Dublin [15]. In contrast, in an American study, several volunteer maize kernels were found to be winter-hard in northern latitudes and germinated the following spring [16]. Even in Germany, GM volunteer maize plants—containing the Nos-terminator and the CaMV35S-promotor—were recorded for the first time on a field of Monsanto in Nordrhein-Westfalen in 2007 (http://www.proplanta.de/Agrar-Nachrichten/Wissenschaft/GVO-Mais-ueberwintert-erstmals-in-Deutschland_article1185528877.html; http://www.zeitpunkt.ch/news/artikel-einzelansicht/artikel/durchwuchs-gentech-mais-ueberwintert-erstmals-in-deutschland.html; http://www.haerlin.org/Mais_Durchwuchs.pdf, accessed 24th of July 2016). The GM maize had been seeded in 2006 and several seeds obviously survived the mild winter temperatures in 2006/2007. It is stated that climate change could be a driving force for overwintering of maize seeds in future.

The term “to become feral” in the context of a crop refers to the crop’s occurrence outside cultivation. The invasiveness potential of a crop is the likelihood that it will persist and spread in non-agricultural habitats [7]. Ecological harm in connection with a GMO includes that the transgenic crop produces seeds, which then disperse to non-agricultural habitats, that the crop establishes in the non-agricultural habitat and forms a self-sustaining population. If feral plants spread and thereby influence the abundance of native species, they will cause ecological harm [17–19]. It is often argued—because no visible ecological harm has been identified during the long history of cultivation of the conventional crop-type—that there would be no negative effect originating from the GM crop. It is assumed that the chain of the above listed events from cultivation to ecological harm is obviously broken at one or more links [7]. An Irish Study [14] says:

“Evidence for this can be seen in the lack of anecdotal evidence supporting the existence of feral maize populations. It is safe to conclude therefore that under current climatic conditions and in the absence of selection pressure there is no likelihood of GMHT maize persisting over adjacent flora and hence there would be no detrimental impact on the Irish landscape should GMHT maize seed be lost pre–sowing”.

Also the Organisation for Economic Cooperation and Development, OECD, is very sceptical towards a potential invasiveness of the crop maize [20]:

“Volunteers are common in many agronomic systems, but they are easily controlled; however, maize is incapable of sustained reproduction outside of domestic cultivation”.

The Netherlands Commission on Genetic Modification, COGEM [11] states:

“During its long domestication process, maize has lost its ability to survive in the wild. In the Netherlands, the appearance of maize volunteers is rare and establishment of volunteers in the wild has never been reported. There are no reasons to assume that the introduced trait will increase the potential of maize to establish feral populations”.

Contrastingly, other scientists consider volunteerism and ferality of maize as at least in principle possible [3, 21, 22]. Even the European Food Safety Authority (EFSA) gives the following statement concerning the occurrence of volunteer maize:

“Maize is highly domesticated and generally unable to survive in the environment without management intervention. Maize plants are not winter hardy in many regions of Europe; furthermore, they have lost their ability to release seeds from the cob and they do not occur outside cultivated land or disturbed habitats in agricultural landscapes of Europe, despite cultivation for many years. In cultivation, maize volunteers may arise under some environmental conditions (mild winters). Observations made on cobs, cob fragments or isolated grains shed in the field during harvesting, indicate that grains may survive and overwinter in some regions, resulting in volunteers in subsequent crops. The occurrence of maize volunteers has been reported in Spain and other European regions” [8, 23].

The present article will contribute to the debate whether maize is able to become feral and to exist as a volunteer plant in Central Europe, exemplarily shown with Austrian data. In Austria, a temperate Central European transition climate is predominant with a continental climate in the east of the country and influences of the oceanic climate in the west. Large climate differences exist between the moderate climate in the Alpine north and the Mediterranean influences in the Alpine south. Austria is rich in diversity of landscapes and of animal and plant species [24, 25]. A release of GM crops has been performed neither for field experiments nor for cultivation in this EU member state. Several proofs (photographs taken during fieldwork) of the occurrence of volunteer and feral maize plants in Austria will be presented here.