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| Temperature effects on insect populations | |
| The speed of development of insects is largely dependent on temperature. This fact is important in integrated pest managament and affects both insect pests and the predatory insects used to control them. Growers should pay more attention to these temperature effects in deciding their overall energy saving production strategies. At 16°C the predator Encarsia develops more quickly than the greenhouse whitefly it is there to control while at 20°C the opposite is true. If the night temperature is below 16°C, populations of the gall midge Aphidoletes aphidimyza will not predominate while at higher temperatures they will rapidly increase. Leafhopper populations increase much faster in the summer at a temperature of 20°C and above while at 16°C they will multiply much more slowly. Research has shown that each insect or mite has its own specific minimum threshold temperature at which development starts. The difference between this value and the actual temperature determines the speed of development. Many insects also have a maximum threshold temperature above which development is suspended. The temperature will be that of the leaf or of the soil in the case of pupae of thrips, leaf minors, moth larvae, nematode worms etc. On the basis of the average temperature over a 24hr period it is possible to roughly calculate the speed of development of a given insect or mite. Each species needs a fixed number of degree days to complete one life-cycle. This has been determined for many insects and mites in laboratry trials. For example, if the leaf temperature over a 24 hour period of the host plants is 18°C and the minimum threshold temperature of Bemisia tabaci is 10°C, this translates to 8 degree days. It is known that this insect needs 316 degree days to complete its life cycle. Divide 316 by 8 and we obtain 39.5 days as the period in which one life-cycle can take place (at a constant 18°C). In crops where the temperature is lower that the threshold value, degree days are calculated by counting only the period at which the temperature exceeds this threshold value. Temperature also affects the flying behaviour of insects. In Encarsia for example, flying starts at around 15°C and by 30°C decreases strongly declines. It increases at 15-20°C and remains optimal and constant 20-30°C. This sort of data is very important to the introduction and spread of beneficial insect populations. Ed Moerman, “Fruit & Veg Tec” 2006 www.HortiWorld.nl |
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