Pests of Acorns

---

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 diseaseor insect damage in the few acorns they could find. The two most commoninsects 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 insectspecies. Insect populations and corresponding damage levels can vary widelyfrom year to year due to differences in weather, the previous year's foodsupply, and predator and parasite populations. It is likely that the 1996bumper 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 typicallyis more common in acorns that have been held for storage than in recentlycollected acorns. The humid conditions necessary to maintain acorn viabilityalso provide optimal conditions for fungal growth. Because these fungi typicallydecay acorns in the cool, wet winter months, they are well-adapted to causedecay in acorns held in cold storage.

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

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


prepared and edited by Richard B. Standiford and Pamela Tinnin