Watching for Weevils: Trapping Plum Curculio

Most monitoring for plum curculio (PC) involves looking for damage on developing fruit, but traps can be used to monitor for this pest as well. Beating trays are often recommended for finding the adult weevils on trees but I have had little success finding PC with beating trays even in problem areas for the weevil. Traps can be most useful when there are known problem areas where PC cause damage every year and especially where PC populations are high. Traps are good tools to indicate when the first PC start moving into an orchard; knowing when these weevils are active helps in accurately timing the first insecticide application for PC, rather than waiting until damage is found. Although trapping PC indicates when PC are active, the number of PC caught in traps is not a good predictor of the amount of eventual fruit damage. Any type of PC trap should be used in conjunction with fruit damage monitoring, not as a replacement for monitoring.

A couple of years ago, I wrote about "Circle traps" for PC designed by Kansas pecan grower, Edmund Circle. (See Feb 2003 Hort Matters). Another type of PC trap is available and has been used for several years in the U.S. Their PC problems are often more consistent and severe than ours, and they use various PC traps in different crops. The trap pictured here is sometimes called a "Tedders trap" (in 1994, W.L. Tedders and B.W. Wood published their results of using these traps to monitor pecan weevil). However, these traps are more commonly referred to as "pyramid traps" or "pyramid trunk traps". Work in 2002 by Andrea Coombs, Mark Whalon, and Larry Gut showed that pyramid traps generally caught more PC than other styles of traps in Michigan (although the differences were not always statistically significant).

In Niagara orchards, PC is a localized, sporadic pest, so monitoring for it in stone and pome fruit orchards each year is a good strategy. When PC is present in an orchard, it is often a problem only on one or more borders so a border spray may be sufficient for control. Plum curculio is a small (4-6 mm), warty, grey - brown weevil (a type of beetle) with a long snout. Female weevils lay their eggs under the skin of fruit beside a characteristic crescent-shaped scar - that crescent-shaped scar is what you look for on fruit when you are monitoring for PC. Plum curculio overwinter in brush or wooded areas near to orchards and become active after a period of warmth (3 or 4 days over 15°C) and rain in the spring.
The pyramid traps look like tree trunks to the weevils and the weevils climb up them into the trap at the top. This is true for many weevils, so you will likely have other similar insects in the trap (along with spiders, wasps, click beetles, etc.). Plum curculio will walk into orchards when the temperature is mild but some will fly when temperatures reach 20°C and higher. Hence, any traps such as this are less effective in warmer weather because some weevils may fly over them and into the trees.

The best placement for pyramid traps is adjacent to tree trunks at the border of the orchard near to likely overwintering sites for PC. If the traps are being used only for early season monitoring, the pyramid traps can be removed before mowing. Pyramid traps and Circle traps can be used unbaited or baited with "plum essence" which is usually available from wherever you buy your traps. Some lures may include components of the aggregation pheromone for PC (such as "grandisoic acid" and "benzaldehyde"). A list of pest monitoring equipment suppliers can be found in OMAFRA Publication 360 - Fruit Production Recommendations. There is very little difference in the price of the two styles of trap (Circle versus pyramid) in the Great Lakes IPM catalogue, so groundcover management might be your only concern with the pyramid traps.

Figure 1. Weevils prefer to climb the dark plastic pyramid trap
instead of the tree when light coloured trunk guards are used.

Learn more:

• Trapping Plum Curculio- Feb 2003 Hort Matters
• OMAFRA Publication 360 - Fruit Production Recommendations

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