Resisting the "Biotic Potential" of Grasshoppers

Brenda Frick, Ph.D.

We are depressingly aware of the damage that grasshoppers cause in field crops. We are much less aware of the damage that some field crops can 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, grasshopper populations 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 diet is a piece of the puzzle that we can influence.

Grasshoppers grow more quickly, are more likely to survive, and lay more eggs when they eat some crops than when they eat others. This is measured as "biotic potential", the average number of offspring per grasshopper. In simple terms, if 100 grasshoppers eating wheat grow and develop, and produce 100 offspring, their biotic potential = 1 on that food source. If they produce 300 offspring, the biotic potential = 3. Biotic potentials of less than one mean that the grasshopper populations will decline over time.

According to Owen Olfert, of the Saskatoon Research Centre of Agriculture and Agri-Food Canada, the biotic potentials of grasshoppers that are fed the leaves of bread wheat are in the range of 1.1 to 3.0. Eating wheat generally allows grasshopper populations to grow. The hopeful news is that grasshoppers that eat some types of wheat, such as Sinton or HY320, have a biotic potential less than one. As a result, these cultivars are considered to have a significant level of biotic resistance to grasshoppers.
In comparisons of a number of historic wheat cultivars, Hinks and Olfert 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 the cost to conventional producers of grasshopper control products, renewed interest in grasshopper resistance in cereals such as wheat is warranted.

Grasshopper resistance can also be employed 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 oat or pea. Grasshoppers prefer not to feed on these crops, but this is only part of their advantage. According to Olfert and colleagues, the biotic potential of Harmon oats, for example, is only 0.5, and that of Sirius pea is a low 0.08. Grasshoppers will feed on these crops if nothing else is available, but the level of damage will be relatively lower than on preferred crops. In addition, grasshopper populations in the following year will be greatly reduced because of the low biotic potential. Thus, these crops give some advantage in the current year, and greater advantage in coming years.

Grasshoppers most often lay their eggs in roadside ditches, and field margins. The biotic potential of the food plants 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 oat. Grasshoppers prefer not to feed on these plants, and will move from them to neighbouring crops once they are large enough to travel the distance. 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 this year, and fewer grasshoppers will be present next year.

The use of grasshopper resistant plants has great potential for long-term grasshopper management. Grasshopper outbreaks are, in part, a response to our plant selections. Unfortunately, many plants that are yummy and healthy to us are yummy 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.

Brenda Frick, Ph.D., P.Ag., is the Prairie Coordinator for the Organic Agriculture Centre of Canada at the College of Agriculture, University of Saskatchewan. She welcomes your comments at 306-966-4975 or via email brenda.frick@usask.ca .

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Additional organic information can be found at http://organic.usask.ca/

Much of the scientific evidence is this article comes from the writings of Owen Olfert. I thank him for his review of this article.

References:

C.F. Hinks and O.Olfert, 1992. Cultivar resistance to grasshoppers in temperate cereal crops and grasses: A review. Journal of Orthoptera Research 1: 1-9

D.F. Hinks and O. Olfert, 1993. Growth and Survival to the second instar of neonate grasshopper nymphs, Melanoplus sanguinipes (F.) fed cultivars ancestral to hard red spring wheat. Journal of Agronomic Entomology 10: 171-180

C.F. Hinks and O. Olfert. 1999. Growth and survival of early-ionstar grasshoppers on selected perennial grasses, with observations on plant constituents which may influence performance. Journal of Orthoptera Research 8: 237-242.

O. Olfert, 2000. 3. What is the role of grassland vegetation in grasshopper population dynamics? J.A. Lockwood et al (eds.) Grasshoppers and Grassland Health, 61-70. Kluwer Academic Publishers. Netherlands

O. Olfert, C.F. Hinks, V.O. Biederbeck, A.E. Slinkard and R. M. Weiss. 1995. Annual legume green manures and their acceptability to grasshoppers (Orthoptera: Acrididae). Crop Protection 14: 349-353.

This article first appeared in The Western Producer, and is published here on the OACC website with permission.