This question asks us to flex our CR muscles, but let's first take a moment to nail down the structure and purpose of this passage:

• In paragraph 1, the author explains that a small number of lepidoptera species exhibit population cycles, and states that the driving force behind these cycles is difficult to identify. The author concludes that this driving force may be intimately connected to the insect itself, rather than being connected to the insect's predators or parasites.
• In paragraph 2, the author presents recent work, which suggests that this driving force (a.k.a. the agent regulating population cycles) may be a virus.
• In paragraph 3, the author notes that the virus hypothesis is attractive because it seems broadly applicable across different lepidoptera species.

OK, now let's revisit the conclusion being highlighted:


The author concludes that whatever is regulating the population cycle is closer to the insect than it is to predatory birds or parasites. Here's how the author reaches this conclusion:

• Studying predatory birds and parasites, thought to be potential agents, has been unproductive in the case of lepidoptera.
• Recent study of viral disease, thanks to new techniques of molecular biology, has identified nuclear polyhedrosis viruses as a more likely agent.
• Polyhedrosis viruses follow an infectious cycle that is intimately linked to lepidoptera, in which lepidoptera ingest the virus in the form of crystals, incubate new virus particles within their bodies, and release these crystals back into the environment upon dying.

If we're looking to weaken this conclusion, we need an answer choice that either:

• contradicts what we've read about this link between polyhedrosis viruses and lepidoptera, or
• delivers evidence that some other regulating agent is even more likely to be the driving force of these population cycles.

Now let's use process of elimination to go through all five choices.


This choice actually strengthens the conclusion, because it adds evidence that other potential agents (predatory birds and parasites) have significantly decreased in number in recent years. This reinforces what the author stated plainly in paragraph 1. Eliminate (A).


This choice introduces new evidence that the regulating agent we're all looking for might be intimately linked to the habitats of lepidoptera, after all. Most importantly, this choice tells us that whatever these experiments did to habitats, that action definitely resulted in the shortening of lepidoptera population cycles. We'll keep choice (B) because it provides proof that another agent--habitat--can regulate population cycles.



Knowing that the nuclear polyhedrosis virus is present in a number of predators and parasites doesn't change anything we've read about the nuclear polyhedrosis virus and its link to lepidoptera. The author's conclusion does not depend on this virus existing only in lepidoptera. And the language of this answer choice doesn't include any dismissal of the intimate link between polyhedrosis viruses and lepidoptera. It simply points out that the virus is also present in predators and parasites of lepidoptera.

(C) doesn't contradict the impact of nuclear polyhedrosis viruses or provide an alternative agent, so we'll eliminate (C).



Choice (D) does identify a potential alternative agent: weather. However, the logical statement made here is quite weak. (D) tells us that it's difficult to assess the effects of weather on lepidoptera population cycles. So even if it's possible for weather to affect these population cycles, we have no way of confirming whether that effect is actually taking place.

Likewise, I could say, "It's difficult to assess the effects of the lunar cycle on lepidoptera," but this wouldn't make me jump up and say that lunar cycles are likely to be more of a driving force of lepidoptera population cycles than nuclear polyhedrosis viruses. We're not interested in hypothetical effects that are difficult to assess. We're interested in proof that an alternative agent is truly at work. That's why we eliminate (D).


This choice doesn't affect the conclusion. Much like Choice (C), it tells us a little more about the presence of viral disease in lepidoptera populations, but it doesn't mention polyhedrosis viruses and doesn't provide evidence of any alternative agent regulating population cycles. Eliminate (E).

Whew! This may have taken me an entire population cycle to write. (Yes, more bad Dad jokes. I'm practicing them. There's no "negative kudos" button, so you're stuck with them.) After all of that, (B) is the only choice that presents evidence of a regulating agent that is NOT intimately linked to the insect (instead, the agent is linked to the insect's habitat). So we keep (B) and move on.

I hope this helps!