Biotic potential can be used to control grasshoppers – Organic Matters

We are depressingly aware of the damage that grasshoppers cause in field crops. We are much less aware of the damage that field crops cause grasshoppers. This change in perspective offers hope that long-term cultural controls for grasshoppers are possible.

The number of grasshoppers in a given year is determined in large part by how many there were last year, and by the weather. In general, grasshoppers increase during warm dry years and decrease in cool wet years. So far, at least, this part of the grasshopper puzzle is out of our hands. But we can influence diet.

Grasshoppers that eat certain crops grow more quickly and are more likely to survive and lay more eggs than those that eat other crops.

This is measured as “biotic potential,” the average number of offspring per grasshopper. In simple terms, if 100 grasshoppers eating wheat produce 100 offspring, that’s a biotic potential of one for that food source.

Three hundred offspring results in a biotic potential of three. Biotic potentials of less than one mean that grasshopper populations will decline.

According to Owen Olfert of the Agriculture Canada Saskatoon Research Centre, the biotic potentials of grasshoppers that are fed the leaves of bread wheat are in the range of 1.1 to three. Eating wheat generally allows grasshopper populations to grow. The hopeful news is that grasshoppers that eat Sinton or HY320 wheat have a biotic potential of less than one. These cultivars are considered to have significant biotic resistance to grasshoppers.

Research comparing several historic cultivars done by Olfert and Chris Hinks, a retired Agriculture Canada scientist, found that, as a group, cultivars released between 1883 and 1928 were more resistant to grasshoppers than the group released between 1935 and 1980.

This suggests that important sources of resistance may be found in the wheat germplasm, but that breeding for grasshopper resistance has not been a major focus.

In light of the cost of grasshopper damage to organic crops and pesticides to conventional producers, renewed interest in grasshopper resistance in cereals such as wheat is warranted.

Grasshopper resistance can be used in rotation. In insect management, as in so many areas of organic farm management, the value of a good rotation cannot be overemphasized. When forecasts indicate that grasshoppers are likely to be severe, producers are advised to plant crops such as oats or peas because grasshoppers show less preference for them.

But this is only part of their advantage. According to Olfert and his colleagues, the biotic potential of Harmon oats, for example, is only 0.5, and that of Sirius peas is 0.08.

Grasshoppers may feed on these crops, but the number of hoppers in the following year will be greatly reduced. Thus, these crops give some advantage in the current year, and greater advantage in following years.

Grasshoppers often lay their eggs in roadside ditches and field margins. The biotic potential of the feed in these areas can also be important. Several perennial grasses, including big bluestem, little bluestem, smooth brome, sheep’s fescue and especially blue grama grass have even lower biotic potentials than oats. Grasshoppers do not prefer these plants, and will move to neighbouring crops once they are large enough to travel. However, the time they spend in these areas slows their development, and reduces their overall biotic potential. Grasshoppers will move more slowly into adjacent fields and fewer grasshoppers will be present the next year.

The use of grasshopper resistant plants has great potential for long-term grasshopper management. Grasshopper outbreaks are a response to our plant selections.

Unfortunately, many plants that are attractive and healthy to us are also attractive and healthy to grasshoppers.

By paying careful attention to our plant selections in grasshopper-prone areas, we can reduce grasshopper populations without relying on insecticides.

Frick is the prairie co-ordinator for the Organic Agriculture Centre of Canada located at the University of Saskatchewan. Frick can be reached

at 306-966-4975, at brenda.frick@usask ca, or opinions expressed in this column are not necessarily those of The Western Producer.



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