Acorn Weevils Inquiry

Acorn Weevils Inquiry

Designed By: Lisa Stiver, Lakota School District

Age Level: 5-8

Materials Needed

  • acorns
  • magnifying glasses
  • pencil
  • penknife or other sharp object
  • tray

Background Information

The colorful days of autumn are a bustling time for woodland animals as many prepare their winter homes. Squirrels, blue jays, and woodpeckers collect and store acorns, an important winter food. But behind the scenes, there is another inconspicuous group that vastly outnumbers the energetic squirrels and jays and also feeds on acorns - insects.

Over 100 species of insects feed on acorns, which are produce by oaks, and other nuts of North American trees. Many play important ecological roles as consumers of acorns, as food for insect-eating animals, or as pioneers that hollow out acorns that other insects and small animals can use as homes. While many of these nut-eating insects don’t cause problems for humans, some, like the acorn weevil, acorn moth, filbertworm and gall wasp, destroy so many nuts that it can be difficult for trees to reproduce.

The most common acorn insects are the half-an-inch long acorn weevils. There are two types, or genera: long-snouted short-snouted. The long-snouted acorn weevil’s snout may be equal to or greater than the length of its body. The short-snouted weevil’s snout is one-half or less than its body length.

Adults of both genera use their small, saw-like teeth to feed on acorns, but only the long-snouted weevils can chew or drill into the shells to feed on the meat inside. They also generally feed on acorns still attached to oak trees. The short-snouted weevil feeds on the bits of meat they can get from cracked acorns lying on the forest floor.

After eating their fill, females often lay eggs in the acorns on which they’ve fed. Long-snouted females lay their eggs in the holes drilled into acorns during feeding, while short-snouted females lay eggs between cracks in the acorn shells. Legless, grub-like acorn weevil larvae hatch from eggs a few days after they are laid. There may be one to several larvae in each acorn. Larvae typically go through five growth stages, or instars. Each instar ends with the molting, or shedding, of the old skin, providing larvae with room to grow and develop. After a few weeks, larvae chew their way out of the acorn, burrow into the soil to pupate, and eventually emerge as adults in one or more years. Short-snouted weevils usually exit from a single hole that already exists in the acorn, but long-snouted larvae may chew their exit from a single acorn shell.

Acorn Inquiry

Like larvae of the short-snouted weevil, acorn moths lay eggs in damaged acorns, sometimes in the emergence holes of acorn weevil larvae. However, distinguishing the acorn moth larva, which is a caterpillar, from acorn weevil larvae is easy: The acorn moth larva has three pairs of legs near the head and is generally longer than the legless, fat larvae of acorn weevils. Larvae of the acorn moth feed on acorns (and probably the fungi that often grow in damaged acorns) and usually pupate inside them.

The filbertworm, a moth whose larva closely resembles the acorn moth, is also often found in acorns. Unlike the acorn moth, which usually attacks fallen acorns, the filbertworm attacks acorns in trees.
Gall wasps are some of the oldest acorn eaters. Fossilized gall wasps, thousands of years old, have been found in acorns from the famous La Brea Tar Pits of California. Adult female gall wasps lay their eggs in female flowers of oaks in the spring. These eggs cause galls, abnormal growths in plant tissue, to develop in or on the acorn. The gall wasp larva then lives in and feeds on the plant tissue inside the galls.
The acorn plum gall is quite conspicuous. It is a blood-red globe about an inch in diameter that is attached to the acorn cup. Many of these external galls drop off the acorns in the autumn. Other galls are inconspicuous and can be found only by opening acorns and examining the contents. Internal galls usually consist of small, stony cells embedded in the acorn meat. While gall wasps usually do not cause extensive damage to acorns, in 1989 the pip gull wasp destroyed millions of acorns in the Midwest.

Activity Description

  1. Provide each child with an acorn, a magnifying glass, and a data sheet.
  2. Encourage discussion without giving too much information.
  3. Have students record their observations, questions, and hypotheses.

 

Wrap Up

  1. Discuss students’ observations, questions, and hypotheses.
  2. Give students some time to see if they can discover what made the hole inside the acorn.
  3. Pull students back together read and discuss related articles (see articles section).
  4. Try opening acorns to see if any of the larvae can be found. There are two methods to test for larvae; the bounce method and the water method.

Teacher Comments

My class was very curious about the holes that were in some of the acorns. They found it fascinating to learn about the acorn weevil and the other animals that rely on the acorn as a food source. I used this as a way to introduce food chains and food webs. My kids really liked testing acorns to see whether or not any of the acorns would bounce. We were hoping to see weevil larvae when we opened some of the acorns, but were not successful. It would have been nice to try the float or sink method to see if we would have had success finding some larva that way.