There was an MCQ question about a farmer who had discovered caterpillars on his plants, and he thus sprayed “a lot” of pesticides on the plants. The question then asked whether the number of plants had increased/decreased, and whether the fruit yields had increased/decreased.
This topic can generate a few hours of discussion and cannot be answered so simplistically. The answer would depend greatly on the context of the situation that the examiner has in mind. A Primary 6 child answering this question would not only need to understand the concepts but also has to read the mind of the examiner as to what is the context of the question. Please let me explain:
SIZE OF THE LAND: Historically, a farmer that uses DDT on his crops and fruit trees would ensure that his plants are not eaten by insects such as caterpillar (and hence his number of plants would increase), and he would also have an increase in the yield of fruits. In our Asian context, the farm would generally be a small holding of not more than one kilometre square. Thus, even if all the insects were killed, including those required for pollination, insects from the surrounding areas would still provide sufficient coverage for pollination to occur. (Note: Bees are known to fly as far as 5km for food.)
However if the farm is very large, then the pesticide could reduce pollination to such an extent that the yield of fruits would decrease. This would be the case in America where large farms are the norm and which may be tens to hundreds of kilometres in size.
TYPE OF INSECTICIDE: Most insecticides that are used in our modern context are those that target a specific species or only the caterpillar stage of the butterfly. These are called “narrow-spectrum” pesticides. More importantly, they are developed so as not to harm bees as the bees are the most important form of pollinators for farmers. So even if the farmer uses a lot of insecticide, he would not harm the bees. Neither would narrow-spectrum pesticides kill the plants/crops or affect the yield adversely since this is, after all, why farmers use pesticides in the first place – to have healthy plants and boost yield.
TIMING OF THE INSECTICIDE SPRAYING: If the insecticide was sprayed after pollination, then the yield of fruit would increase. If the insecticide was applied just before pollination began and the farm was very large (and assuming that broad-spectrum insecticide was used and all bees were killed), then the yield of fruit would decrease. If the fruits were ripening on the tree, then the yield of fruit would increase. If the tree was not flowering yet or had just produced a batch of fruits, then killing all the bugs and caterpillars would not make much of a difference as they would just return the next season. Then the answer would be plants increased and fruits, no difference. There are so many permutations because there are just so many parameters that are not given in the question. How would the student know which assumption to make?
If the answer is expected to be plants decreased, fruit decreased, then we are assuming that the farmer is so stupid or ignorant of farming technology that he sprays so much broad-spectrum insecticide until he poisons the plants leading to a decrease in the number of plants and hence fruit yield.
So what exactly are the parameters and context of the question? How big was the farm – small Asian farms or large American farms? What kind of insecticides were used – narrow-spectrum or broad-spectrum type that killed all insects? When was the insecticide applied? How would the children know if all these parameters are not clearly stated in the question?
The assumptions underlying this question and debate thereof could be the subject of a long JC-level General Paper discussion. It certainly cannot be a multiple-choice question for P6 students at PSLE level.
This question should not have been set in the first place without clear information on the context and parameters provided.
It is therefore little wonder that there is so much dissent and debate on this Question in this forum.
Regards.
Father of PlasmaStorm 
(Formerly known as kohjl Jr )
