A mast year brings a bountiful harvest to acorn weevils, Curculio glandium
The acorns weevil’s proboscis is remarkable by any standard.
In forests and landscapes here in the DMV, exceptionally humongous crops of acorns were produced by several species of red and white oaks this autumn. These large acorn crops occur periodically and in many types of plants such uber-productive seasons are called mast years. The exact climatic conditions underlying a mast year are not yet fully understood, but what is well-known is the fact that eaters of acorns including small and large mammals such as chipmunks, squirrels, deer, and bears, and many species of birds such as turkeys, quail, blue jays, and woodpeckers enjoy an autumnal food bounty when oaks go into acorn overdrive. Do any insects take advantage of this bounty? You bet! Enter the amazing acorn weevil.
Legless weevil larvae emerge from circular holes in acorns and enter the soil in advance of winter.
The backstory to this episode begins with Stephanie, collector of native plant seeds, who was on a mission to gather seeds of our native pin oak tree. Soon after the acorns were collected, dozens of creamy, white, legless grubs appeared in the bottom of the collecting bag. At first glance this seemed like some incarnation of spontaneous generation – the appearance of life from non-life. However, on closer inspection, tiny, perfectly circular holes were visible on the hulls of many of the acorns. This was the work of the acorn weevil. Acorn weevils are remarkable creatures noted for their exceptionally long snouts. At the tip of the snout, or proboscis, are jaws that a female weevil uses to cut a hole in the husk of the developing acorn while it is still attached to a branch in the treetop. Into this hole, she deposits eggs that hatch and release tiny grubs which eat the nutritious meat of the acorn. In autumn, when the acorns drop to the ground, fully grown larvae chew small holes in the husk to escape their oaken nursery. They enter the soil and burrow several inches underground to pupate. They may remain in the soil for several years before emerging as adults in spring, with the return of leaves to the oak and the production of a new crop of acorns. Weevils emerge from the soil and climb to the canopy of the tree to feed and deposit a new batch of eggs.
Periodically oak trees produce amazing crops of acorns in seasons called mast years. In addition to many birds and mammals, beetles called acorn weevils take advantage of the bounty. Female weevils deposit eggs inside acorns which hatch into larvae called grubs. Grubs develop during summer and escape in autumn through exit holes and enter the soil to pupate. A year or more later they emerge as adults and using jaws at the tips of their ridiculously long snouts cut holes in acorns, where the next batch of eggs are laid.
To observe these remarkable insects first hand, simply go outdoors to your favorite oak and collect a batch of acorns. According to one account, if you place these acorns in a container such as a pitcher or cooking pot filled with several inches of water and the acorns sink, they are usually intact and unlikely to be infested. Acorns that float have airspace within and are likely to contain weevil grubs or other interesting bugs. Crack a few of these open and observe the grubs inside. If you are really ambitious and desire a glimpse of the bodacious adults, you might try placing infested acorns in a pot with several inches of soil. This will provide grubs with the necessary habitat to complete their circle of life. The pots can be placed outdoors to expose the grubs to natural conditions of temperature and rainfall. Cover the pot with a bit of netting and, who knows, in a few years you may actually capture some fantastic beetles with schnozzolas so enormous, they would make Jimmy Durante jealous, ha cha cha cha.
Acknowledgements
Bug of the Week gives special thanks to Stephanie for providing the specimens and inspiration for this week’s story. The interesting article “The overwintering biology of the acorn weevil, Curculio glandium, in southwestern Ontario” by Hiroko Udaka and Brent J Sinclair was consulted for this episode.
To learn more about acorn weevils, please visit the following websites:
https://www.uky.edu/Ag/Entomology/ythfacts/allyr/yf804.htm
https://content.ces.ncsu.edu/acorn-weevil
This post appeared first on Bug of the Week
