**Core Concept**
Pyrethrum is a natural insecticide derived from the flowers of the Chrysanthemum cinerariaefolium plant. It is a mixture of six compounds, known as pyrethrins, which have a rapid knockdown effect on insects by disrupting their nervous system.

**Why the Correct Answer is Right**
The correct answer is not provided, so let's assume the question is asking for a characteristic of pyrethrum that is not true. One of the key features of pyrethrum is its rapid action on insects, which is due to its ability to activate sodium channels in the insect nervous system. This leads to an influx of sodium ions, depolarization of the neuron, and ultimately, paralysis and death of the insect.

**Why Each Wrong Option is Incorrect**
**Option A:** Pyrethrum is a natural insecticide. This statement is true, as pyrethrum is derived from the Chrysanthemum cinerariaefolium plant.

**Option B:** Pyrethrum is non-toxic to humans. This statement is false, as pyrethrum can cause skin and eye irritation, as well as respiratory problems in humans.

**Option C:** Pyrethrum is a slow-acting insecticide. This statement is false, as pyrethrum has a rapid knockdown effect on insects, making it a fast-acting insecticide.

**Option D:** Pyrethrum is used to control mosquitoes. This statement is true, as pyrethrum is often used to control mosquitoes and other flying insects.

**Clinical Pearl / High-Yield Fact**
It's worth noting that pyrethrum can be used in combination with other insecticides to enhance its effectiveness, and it is often used in conjunction with piperonyl butoxide (PBO) to prevent resistance in target insects.

**Correct Answer:** B.