From the ArcaMax Publishing, Gardening Newsletter:
http://www.arcamax.com/news/gardening/s-156832-316216
Question: Last year my asparagus plants were stunted and the foliage
formed tight balls instead of the regular ferns. What was the problem,
and how can I prevent it from happening again?
Answer: Balled asparagus foliage is a sign of the asparagus aphid. The
pest overwinters as small, black eggs on dead asparagus stalks and
foliage. In spring, when new stalks begin to emerge from the ground,
the eggs hatch and the young begin feeding exclusively on the stalks
and young volunteer asparagus seedings. During feeding the aphid
injects a substance that causes the asparagus foliage to grow into
balls, and heavy infestations can reduce production. The aphids feed
until fall, when they mate and lay eggs. To control the aphids, remove
asparagus foliage after it dies in the fall. This reduces the number
of overwintering eggs. Weeding your patch of the small volunteers,
which young aphids feed on first in spring, will also reduce the
damage. For heavy infestations, spray insecticidal soap when the
spears begin to send out branches and the fernlike leaves. Repeat the
spraying a few times during the growing season, whenever you notice
damage to the leaves.