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Pests of Acorns

Oaks ‘n’ Folks – Volume 13, Issue 1 – February, 1998

The fall of 1997 was a lean season for acorns in many parts of California.To make matters worse, some oak planters have noticed high levels of disease or insect damage in the few acorns they could find. The two most common insects that attack acorns of California oaks are the filbertworm (Cydialatiferreana) and several species of filbert weevils (Curculiospp.). Virtually all native California oaks are attacked by these insect species. Insect populations and corresponding damage levels can vary widely from year to year due to differences in weather, the previous year’s food supply, and predator and parasite populations. It is likely that the 1996 bumper acorn crop helped produce a bumper crop of acorn-eating insects,and these hungry little critters are eating up the 1997 meager acorn crop,at least in some locations.

Various fungi and certain bacteria also can colonize and decay acorns,although these pathogens have not been studied in much detail. Decay typically is more common in acorns that have been held for storage than in recently collected acorns. The humid conditions necessary to maintain acorn viability also provide optimal conditions for fungal growth. Because these fungi typically decay acorns in the cool, wet winter months, they are well-adapted to cause decay in acorns held in cold storage.

Insect and disease damage to the acorn crop can significantly reduce the reproductive potential of most oak species in a given year. However,because damage levels vary from year to year, such damage is not likely to completely inhibit regeneration. Oaks compensate for fluctuations in the acorn crop by producing persistent seedlings. Small oak seedlings in the understory can be many years old, and these constitute the potential or “advance” regeneration that replaces trees as they die or are cut. Unfortunately, these small seedlings often are prematurely killed by livestock, rodents, and drought induced by weed competition. Over the long term, these factors generally are much more likely to limit oak regeneration than pest damage to the seed crop.

Jays and acorn woodpeckers are major acorn consumers, and they generally seem to know which ones are good. If you set out a tray containing sound acorns and acorns with substantial internal insect damage, scrub jays generally will get the good ones and leave most of the bad ones behind. Taking a cue from the birds, human acorn planters also should sort their acorns and plant only the good ones. Acorns that have insect exit holes (open holes about the diameter of a pencil lead) typically are quite chewed up and should be tossed. Smaller, closed blemishes typically are oviposition (egg-laying)wounds but can be found on sound acorns. Acorns that are very lightweight or deform easily when squeezed are usually abortive or decayed. However,even fairly solid acorns sometimes have high levels of internal decay. If you have doubts about acorn quality, slice a few acorns open to check for damage or discoloration inside. The famous “float test” (toss’em in water and keep the sinkers) is especially useful when screening large numbers of bad acorns for the few good ones. Hand sorting at planting is usually easier when the opposite situation exists.


prepared and edited by Richard B. Standiford and Pamela Tinni

Ted Swiecki, Phytosphere Research, Vacaville