Ryegrass is an important food for cattle, but it is also a favourite food of competing insects. Can biological control offer a solution?
What's so special about ryegrass?
Ryegrass is an important food for farm animals like cows. Unfortunately, it is also a favourite food of insects, like the Argentine Stem Weevil and the African Black Beetle. These insects compete with cows and other grazers and can have drastic effects on farm paddocks.
Ryegrass: Not only a food
Ryegrass is also amazing because it not only a food source, but sometimes it is the home of a particular fungus called Neotyphodium lolii. This fungus is an endophyte. It lives inside the ryegrass, getting protection from the ryegrass. The fungal endophyte in turn protects the ryegrass by producing chemical toxins, called alkaloids. The alkaloids stop creatures from eating the grass. A large number of different alkaloids are produced.
Some of alkaloids affect ruminant animals, like cows, which eat the top part of the plant. For example, one of the alkaloids causes ryegrass staggers in cows. In this case, eating the ryegrass at certain times of the year (usually summer and early autumn) causes the cows to stagger around as if drunk. A different type of alkaloid can case heat stress in cows, where the cow overheats and stops feeding.
Others alkaloids produced by the endophyte prevent insects from eating the ryegrass.
The relationship between ryegrass and the endophyte is an example of symbiosis. This is when two organisms, usually from two different species, live together in close association with each other. In this case, both of the organisms benefit from the relationship and neither suffers, so we call it mutualism.
A helpful invader
The hunt was on for an endophyte which does not harm cattle, but which does stop insects from eating the ryegrass, and one was found in Europe. After a lot of research by AgResearch scientists, this endophyte (called AR1) was introduced into New Zealand ryegrass.
The endophyte can be inoculated into the ryegrass by taking a small piece of mycelium from a culture of the fungus and placing this in a tiny slit made in a germinating seed. It is a very difficult technique and is not always successful.
Once the endophyte has been introduced into a ryegrass plant, the seeds from that plant will also contains the endophyte. This means that grasses with the endophyte can be produced as be sold to farmers as seed. Care does need to be taken though, because the endophyte can die if the seed is left too long before planting or if the seed is left in unfavorable conditions, for example extreme temperatures or humidity.
AR1: A special endophyte
Ryegrass with the farmer-friendly, cow-friendly endophyte (called AR1) is highly resistant to Argentine Stem Weevils (the main problem insect of ryegrass) but only partially resistant to the Black Beetle (another problem insect of the grasses).
Scientists at AgResearch are working on the Black Beetle problem. One of their experiments is to grow ryegrasses with different endophytes and some without any endophyte (the controls). They then add the Black Beetle to the plants and observe what the beetles eat, and how they are affected. The next step is to determine exactly what it is in the endophyte that affects the insects, how they might make it more effective.
The endophytes contain genes in their DNA which control which specific toxins are produced. Molecular biologists are exploring ways to shut down genes in the endophyte which are involved in producing the alkaloid toxins that they don't want. This would mean that rather than searching for and hoping to find the most useful endophyte in its natural state, endophytes that we already have could be modified to better need farmers' needs - endophytes that produce alkaloids which affect a range of insect pests, but are not harmful to cows or other farm animals.
At this stage the work is only carried out in carefully controlled laboratory studies. The development of all new organisms need to be approved by New Zealand's Environmental Risk Management Authority.
Written by Barbara Ryan, NZ Science Mathematics and Technology Teacher Fellow 2006
- 10 October 2007