Conclusion: Painted spiders are better predators than other spiders. We have to weaken this.
(A) Not all of the species of insects living in the painted spider’s habitat are flying insects. - As long as some species of insects are flying insects and painted spider's trapping record is good, it won't weaken the conclusion
(B) Butterflies and moths which can shed scales are especially unlikely to be trapped by spider webs that are not very sticky. -- We are not told whether Butterflies and moths are preys of spider. Out-of-scope/irrelevant info
(C) Although the painted spider’s venom does not kill insects quickly, it paralyzes them almost instantaneously. -- extraneous info. Has no bearing with conclusion
(D) Stickier webs reflect more light and so are more visible to insects than are less-sticky webs. -- This weakens the conclusion by saying that even though the webs are stickier, they reflect more hence negating the efficiency of stickier webs. Keep it
(E) The webs spun by the painted spider are no larger than the webs spun by the other species of spiders in the same habitat. -- This is not a web area measuring competition

. Strengthener