Year Dose to kill 50% of aphids / mg 1 5 3 12 5 28 7 60 A farmer sprayed a field of cabbages with the same insecticide every year to control aphids, which a...
Question 1 Report
| Year | Dose to kill 50% of aphids / mg |
|---|---|
| 1 | 5 |
| 3 | 12 |
| 5 | 28 |
| 7 | 60 |
A farmer sprayed a field of cabbages with the same insecticide every year to control aphids, which are small insects that feed on the crop. The same field was treated in the same way each year, and over seven years the insecticide gradually became less effective at controlling the pest. Fig. 4.1 shows the dose of insecticide needed to kill half of the aphids in each year that was tested.
(a) Describe the trend shown in Fig. 4.1. [2]
(b) Explain, using ideas about natural selection, why the aphids became resistant to the insecticide. [5]
(c) State where the allele for resistance originally came from. [1]
(d) Calculate how many times greater the dose needed in year 7 is than the dose needed in year 1. [1]
(e) Suggest two methods, other than using more insecticide, that the farmer could use to control the aphids. [2]
(f) Explain one way in which the insecticide could harm other organisms living in the field. [2]
Answer Details
(a) The trend in the data is that the dose needed to kill half the aphids increases each year [1], and the increase becomes larger in later years, so the line rises more and more steeply [1]. Plotting the values makes this clear:
(b) Natural selection explains resistance as follows: the aphids show variation [1]; a (random) mutation gave some aphids an allele for resistance to the insecticide [1]; when the field was sprayed, the non-resistant aphids were killed but the resistant ones survived [1]; the surviving resistant aphids reproduced and passed the resistance allele to their offspring [1]; over the years the proportion of resistant aphids in the population increased, so more insecticide was needed [1].
(c) The resistance allele originally arose from a (random) mutation [1].
(d) The dose in year 7 compared with year 1 is: \[\frac{60}{5} = 12\] so it is 12 times greater [1].
(e) Any two other control methods (1 each): introduce a natural predator (biological control); use crop rotation; grow resistant/pest-resistant crop varieties; remove weeds that harbour aphids; use physical barriers or netting [2].
(f) One way the insecticide can harm other organisms: it may kill useful insects such as pollinators or the aphids' natural predators [1]; and it can be passed along food chains / accumulate (bioaccumulate) and harm animals such as birds that eat the poisoned insects [1].
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