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The curculio is capable of wiping out nearly an entire crop when fruitlets are pea-sized and up.
The curculio is capable of wiping out
nearly an entire crop when fruitlets
are pea-sized and up.

Curculio Riffs

by Michael Phillips

This will interest growers throughout the eastern half of the North American continent who ponder options for the infamous plum curculio. Read between the lines if you live elsewhere and you will be able to pick up pointers for similar hard-backed pests of the beetle persuasion.

This listing of Plum Curculio Fact Sheets by State is a good place to start if you're not sure of the life cycle quirks of this orchard pest. Recognizing points of vulnerability is always the key for nudging pest dynamics favorably.

Trap Tree Shenanigans

Plum curculio has definite varietal preferences based principally on regional reproductive success. An intelligent grower can skip a few such trees when applying orchard-wide repellents. This "pushes and pulls" the pest to these untreated locations where ground level strategies can be employed to knock back next year's population. The most formidable barrier protection to accomplish this is refined kaolin clay sold under the brand name Surround. The micronized clay particles irritate curculio to distraction-thereby causing these weevil-like bugs to stop feeding and ovipositing-and instead seek out a more reasonable place to be.

The regional "curculio preference map" found in Apple Grower shows where different fruit types merge in this respect. Asian hybrid plums are a given throughout. Cherries are especially relevant towards the Great Lakes just as peaches take on curculio aura as one heads south. Lush-leafed apple varieties like Liberty find favor in the Northeast whereas everything seems to merge throughout the Mid-Atlantic region. Research gives a particular edge to Formosa and Santa Rosa plums as being eleven times more attractive than apple regardless.

Mean No. PCs Detected Per Tree over Immigration Period

The real story here concerns spring emergence and which fruits are of proper size for unfettered joviality on the part of the curculio hoard. That bar graph reflects West Virginia experience. An early start to the season will favor activity on stone fruits whereas a cool start to spring results in a stronger apple focus. Typically, it's wise to provide clay coverage on peaches, apricots, cherries, and plums sooner than is necessary for apple. Places where apple turns out to be equally favored will require full white coverage be in place come petal fall week.

Grow your orchard library at our Bookshelf
Grow your orchard library at our Bookshelf.

Ground Level Strategies

Curculio pupates in the soil. Blocking egress can take the form of a physical barrier. Old carpets are quite effective at snagging fruitlets to shrivel in the hot sun (just be sure to take up the carpet sometime in July to prevent vole build-up). Running piglets ala Jim Koan in Michigan to devour June drop has its own intensity. Chickens do it for me, brought in for trap tree duty, kept in place with portable electric fencing. All such plans depend upon effective push and pull. Some growers make similar headway shaking curcs out at dawn onto gathering tarps below.

Allies Down Below

The biopesticde strategy being used by the Mark Whalon team in Michigan involves infecting grains of rice with a parasitic fungi called Beauvaria bassiana. These are seeded into the soil beneath varietal vectors (trap trees!) prior to larval drop from fruit. The fungus then goes on to kill the larvae-becoming-pupae. . . the stage that appears the most vulnerable of all.

At ARS's Southeastern Fruit and Tree Nut Research Station in Byron, Georgia, entomologists found that soil applications of the nematode Steinernema riobrave can suppress plum curculio larvae by 78 to 100 percent. "Nonfeeding infective juvenile nematodes seek out larval hosts," says Dr. Shapiro-Ilan. "When one finds a larva, it penetrates its body. Once inside, it releases a bacterium that multiplies rapidly and kills the host. The nematode then reproduces while feeding on the bacteria and insect tissues." Again, note the larvae-becoming-pupae timing.

The Black Towers of Mordor

Position tower traps parallel to the silhouette of an actual tree trunk to prod curcs up in search of fruit.
Position tower traps parallel to the silhouette of an
actual tree trunk to prod curcs up in search of fruit.

Tracey Leskey at the Appalachia Fruit Research Station in Kearneysville, West Virginia, is working to identify compounds that can be used to improve the attractiveness of lures for plum curculio. Volatiles like benzaldehyde and grandisoic acid are high on the list. Pure plum essence that's naturally-derived is the surest draw yet but harder to stabilize. Tower rigs for the funnel trap can be purchased from Great Lakes IPM or cut from plywood sheets and painted black. Should all this get perfected, the majority of curculios may well wind up trapped in funnels of ill repute.

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Weakening the Beast

Harry Hoch in Minnesota uses Surround in a slightly different manner. He has specific varietal blocks favored by curculio and watches those for first signs of activity. Now the clay serves to dissipate curculio stamina. . . there's no safe haven provided in the form of trap trees. . . leaving the pest more worn out and thus subject to neem applied with the clay. Harry uses Azadirect, a patented formulation of the azadirachtins found in pure neem oil, which would be "too fatty" otherwise to mix with clay directly.
Both egg  insertions and feeding stings lead to a gumming response by the baby peach.
Both egg insertions and feeding stings lead
to a gumming response by the baby peach.
Previous trials with patented neem by itself have indicated little effectiveness with curculio. And yet, in this orchard-with woods nearby filled with wild apple and wild plum-it's enough to drive back the hoard. "The more PC moves around, cleaning themselves, looking for a place to oviposition, the more of their fat reserves get burned up," reports Harry, "means far fewer apples get hit overall." This insight ties directly to moderate curculio pressure in the Upper Midwest and a degree of grower tolerance to hand thin out damaged fruit.

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Nail in the Coffin

Growers in other places have definite need to end the game. Using PyGanic orchard-wide is not ideal, however. This natural pyrethrum works as a lethal contact poison on quite a few species, including beneficials. Typically, it can take several twilight-applied doses to make significant impact on curculio. Here's where trap trees or untreated border rows can be used to congregate PC to spare the ecosystem as a whole. Come those first "curculio nights" when temps hover near 70°F as the sun sets -- once fruitset scarring is evident -- PyGanic proves a singly effective whump for bugs already drained by the repellant plan on other trees.

Coverage Overlap

The fast-acting compounds in Entrust (produced by aerobic fermentation of the actinomycete, Saccharopolysora spinosad) acts through ingestion or by direct contact to stimulate the nervous system of the insect, causing loss of muscle control. Entrust is known to be very effective on leafroller caterpillars, fly larvae, leafminers, and thrips. Those pests are highlighted on the label. This organically-approved product has less impact on internal -- feeding caterpillars (like codling moth) and apple maggot -- direct contact being less likely -- yet enough that those pests are on the label as well. Entrust provides poor control of sucking leafhoppers and true bugs. Curculio would seem to be included in this last grouping, as pest numero uno is not on the label.

But hold those horses. A fruit advisor friend was told by a product rep that "even though spinosad lacks efficacy enough to put curculio on the label, it does indeed have substantial activity against PC." The rep then want on to say spinosad gave about 50% control. That said, Cornell rates Entrust a big fat zero for PC. Penn State gives it a "poor" rating meaning it does have a tad of recognizable impact by itself. A dose applied with the refined kaolin clay is what makes the difference.

Stone fruit growers can abate curc interest early on with this synergistic mix. The best timing is just prior to split shuck, thereby avoiding residual clay on soft fruit harvested in midsummer. Apple growers who might already be using Entrust every other year (to knock back European apple sawfly) may find a discernible PC boost if it's plenty warm during petal fall week. Curculio needs to be active in the trees when this singular strategy gets employed.

Entrust comes with resistance issues. Use it too often and you favor super-pests with inbred resistance. This product must be rotated so as not to treat back-to-back generations of the same pest with the same mode of action.

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Curculio Grange

Feels like we just had a grange meeting like was done 150 years ago to discuss the state of the art around a major orchard pest. Take what's useful here in the context of your orchard layout and observed pressure patterns. Nor should we neglect the intercepting contributions of ground beetles and wolf spiders. Every orchardist weaves a web made from threads like these to get a handle on this rather confounding pest. This is the scope of what we know. . . so let's just hope there are no curculios listening in!

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