41.   The creature is the Asian long-horned beetle, which kills trees by eating them from the inside and which has no natural enemies in the United States.

42.   The Fed battles inflation, the natural enemy of money, by making money more expensive to borrow or to lend, thus cutting off its growth and circulation.

43.   The dog he names Scraps is his alter ego, hanging in there, not to be deterred by nightstick-wielding cops or other natural enemies.

44.   The grasses brought to America from Eurasia more than a century ago have no natural enemies and little can stop their spread across empty desert pavement.

45.   The natural enemies of beach dudes.

46.   The most hopeful strategy is to import some natural enemies of invasives.

47.   The wandering albatross has no significant natural enemies, and this has enabled it to survive with such a low reproduction rate.

48.   There is another, more natural enemy of coral, one that is particularly destructive to reefs in parts of the Pacific and Indian Oceans.

49.   They have different jobs, and in many ways they are natural enemies, like cats and dogs.

50.   They have few natural enemies and multiply at a rate that would make rabbits seem chaste.