A grower noticed that some Colorado potato beetles remained alive after a potato crop had been sprayed. Fig. 1 shows an adult beetle feeding on a potato lea...
Question 1 Report
A grower noticed that some Colorado potato beetles remained alive after a potato crop had been sprayed. Fig. 1 shows an adult beetle feeding on a potato leaf. The grower had used the same insecticide for six years. Beetles were collected from three fields and kept in separate boxes with fresh potato leaves. Each box received a measured spray of insecticide. After 24 hours, the technician counted living beetles.
Table 1 shows the results. Field C had not received this insecticide in the previous six years because the farmer had used a different method to control pests. The three fields had similar temperature, water supply and potato plants. The insects were not tested on the same day.
| Field | Number of beetles at start | Number alive after 24 hours |
|---|---|---|
| A, sprayed for 6 years | 80 | 49 |
| B, sprayed for 6 years | 80 | 44 |
| C, not sprayed for 6 years | 80 | 7 |
The resistance allele can be passed from adult beetles to their eggs. A resistant beetle may have cells that make an enzyme which breaks down the insecticide. The grower is considering alternating insecticides and leaving some unsprayed strips of plants.
(a) Use Table 1 to calculate the percentage of beetles from field B that were killed by the insecticide. Show your working. [2]
(b) Describe the difference between the results from fields A and C. [2]
(c) Suggest why the result from field C provides evidence that some beetles from fields A and B are resistant. [2]
(d) Describe how repeated spraying changes a beetle population so that resistance becomes more common. [5]
(e) Suggest two reasons why alternating insecticides could slow the increase of resistant beetles. [3]
Answer Details
(a) First find the number killed:
\[80-44=36\text{ beetles}\]
Then calculate the percentage killed:
\[\frac{36}{80}\times100=45\%\]
Therefore, 45% of beetles from field B were killed. [2]
(b) More beetles from field A survived than from field C: 49 compared with 7. Therefore, fewer were killed in field A, 31 compared with 73 in field C. [2]
(c) The insecticide killed most beetles from field C, but many beetles from fields A and B survived exposure. This is evidence consistent with resistance in some A and B beetles, rather than all beetles being equally susceptible. [2]
(d) The beetle population has inherited variation, so some beetles carry a resistance allele. Spraying kills susceptible beetles, while resistant beetles survive. The survivors reproduce and lay eggs. Resistant parents pass the resistance allele to offspring, so the frequency of the resistance allele increases in later generations. [5]
(e) Alternating insecticides can slow resistance because a beetle resistant to one insecticide may be killed by a different insecticide. Different insecticides therefore impose different selection pressures, reducing the number of resistant survivors that reproduce after every treatment. Leaving unsprayed strips can also maintain susceptible beetles, which can breed with resistant beetles. [3]
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