Synthetic pyrethroids are pesticides recommended for use in apple orchards to control leaf miners, insects whose larvae feed on the leaves of apple trees. Synthetic pyrethroids also kill predators of spider mites, however. Thus these pesticides do more overall harm than good in an apple orchard.

Which of the following, if true, most strengthens the argument above?

Premise: Larvae of LM insects feed on leaves of apple trees and pesticide SP helps control the the LM. SP also kill predators of spider mites.
Conclusion: SP does more harm than good to apple orchard.

So, there's a contrasting finding that one SP does one good thing by controlling/killing LM insects. But when it helps kill predators of spider mites all good done turns into harm. Clearly, predators of SM must be doing something good for the apple orchard. If that's the case, then SM might be doing harm to Apple orchard.

A. Spider mites are beneficial to apple trees. - WRONG. Goes in opposite direction. Weakens the argument.
B. Predators in apple orchards do more harm than good. - WRONG. Again its in the opposite direction.
C. Many consumers avoid buying apples that have been treated with pesticides. - WRONG. Looks promising so we can keep it, initially, till we find a better one.
D. Spider mites are potentially more serious pests in apple orchards than are leaf miners. - CORRECT. If SM are more dangerous than LM then predators of SM helped control the SM. Killing the predators would be least which someone would demand. So, killing predators did more harm than good. Note the comparison of 'more seriously', although 'harm' done by SM is obvious as it is compared with LM which harm apple orchard.
E. Leaf miners are sometimes parasitized by other insects. - WRONG. 'Sometimes' is problematic here.

Note: This problem has two steps that must be noted.
1. SP does one good thing by killing LM.
2. Other one of SP is bad for apple orchard when it kills SM's predators. Here again two things are layered in the passage. We don't know, initially, whether SM or its predators do good or bad(although we know it would most likely be SM) for Apple orchard which is what we need to find along with the relationship of SM and LM.

IMO. Answer D.