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Stuart B. Hill, Ph.D.
Québec apple growers are all familiar with. the plus curculio (PC) and with the moon-shaped egg-laying scars it makes on their apples in spring and early summer, but few of them know what to do about it other than keep spraying pesticides. In fact, most pest control experts are in the same position. The reason is that this weevil is insects to study, particularly because no technique has yet been discovered to trap and 'monitor it. My own association with it began in 1978 when Dr. Rudolph Paradis, the leading orchard pest researcher in Québec at that time, suggested an investigation into methods of trapping and monitoring PC would be suitable suitable line of research for my new graduate students, Jean-Pierre LeBlanc..
After investigating over a dozen techniques over the next four years we had to admit defeat. Actually, some techniques did work, like attaching Granny Smith' apples to the forks of major branches on trees adjacent to woodlots in spring, before apples appeared on the trees, and counting the number of egg-laying scars, but few growers would be likely to adopt such a technique.
- We also found that funnel traps hung under the trees effectively collect PC as they fell in response to Bled or changes in temperature or light, but again this is hardly a convenient way to detect this insect's presence. However, in the process of studying this insect, we found out enough about its biology and ecology to suggest some more profitable lines of study. These centered around describing in detail the seasonal movements of PC, particularly its dispersal to overwintering quarters and return to the orchard. For this we had to label it with a radioactive isotope, release it into the environment and track its movement with a scintillation counter (like a sophisticated Geigèr counter). This work was done at the Frelighaburg experimental farm of the' St-Jean-sur-Richelieu research station of Agriculture Canada 'in conjunction with Or Charles Vincent, and with Or Nyana Barthakur at Macdonald College. Gerald Lafleur was the first student involved and his studies started in 1981. As a result of a series of very successful experiments we came to the following conclusion. regarding the behaviour of PC in the Frelighaburg orchard.
1. In fall PC were found to be most active at night.
2. At this time PC tend to disperse In a~southwesterly direction toward high tree silhouettes in the adjacent woodlot.
3. Where the vegetation ' and litter are unsuitable (e.g., coniferous), PC return to the orchard where 85` of more of them may die during the winter; If the litter is thin they either remain at the edge of the woodlot or return to the orchard. However, if the litter is suitable (e.g., a thick layer of maple leaves) they disperse into the woodlot and hide among the leaves. Here as many as three quarters of them may survive to return to the orchard the next spring.
4. In Spring some of them return to the orchard, most stopping on the ground under the perimeter row of trees adjacent to the woodlot for up to three weeks before venturing into the trees. In this location they sometimes collect into small groups, probably for mating, and can be heard to make a faint chirping noise.
5. They tend to disperse first to early apple varieties that have the thickest foliage (e.g., Melba); also to plums and other stone and pome fruits that flower before apples.
Along the way other students joined the team; Laurie Shell did an undergraduate project on litter preference for overwintering PC, and Richard Blanchet did an undergraduate project on the method and timing of the dispersal in spring from the forest to the orchard. Gaetan Racette continued our studies on daily activity and Gerald Choulnard the studies on dispersal behaviour. Jean-Plerre Brossard initiated some new studies testing nematodes as biological control agents. Again, all of these studies produced interesting findings that suggest approaches than can-be taken to control PC. Based on our discoveries and those of other researchers, I would make the following suggestions with respect to the control of PC.
1. Because of PC's preference for maple woodlots an overwintering sites and its low winter survival In other locations, it seems advisable to establish orchards as far removed as possible from maple woodlots. Furthermore, because PC also attacks crabapple, wild apples, pears, plums, cherries, peach, apricot, quince and hawthorn, apple orchards should be kept away from these trees, and wild host trees should be removed from the surrounding area.
2. Early- apple varieties that have a lush, leafy foliage, such as Melba, are more attractive to PC and should be avoided or used as trap trees.
3. Trap trees, such as plums, can be used to give early warning of PC's presence as thy characteristic egg-laying scars may be seen earlier on the small plums than on apples. In the absence of plums, inspecting small apples for egg-laying scars is presently the most reliable technique to detect presence of PC.
4. A stone mulch about 1 ft radius around the tree trunk with a thick spoiled hay mulch out to the dripline is likely to encourage many predators of PC. Efficient mice guards must be used in conjunction with mulches.
5. A diverse array of ground vegetation with small flowers is likely to attract parasitic wasps which may parasitize not only PC: but also many other pests. This would need to be mown during apple blossom to avoid attracting the bees away from the apple pollination. Bare soil in the orchard is probably also unattractive to PC, but Its dependence on heavy herbicide use and associated soil erosion makes it an unsuitable solution.
6. Sites adjacent to maple woodlots could be made less attractive by planting one or two~rows of conifers along the edge to dlscourage PC from entering the woodlot in the fall. Coniferous leaf litter scattered along the woodlot edge might also repel the PC.
7. Alternatively, isolated groups of suitable trees could be located along the edge of the woodlot-and the PC that congregate under them be controlled by physical, chemical or biological means.
8. Ground-feeding birds (chickens, quail, peacocks or wild birds) could be caged or attracted to the area under the perimeter rows of trees adjacent to a woodlot in the spring (throughout Hay in S.W. Québec). An easy way to do this is to have several mobile chicken houses that can be moved along the edge of the orchard. Incidentally, all successful organic orchards that I have ever visited had several 100 chickens in thee as pest control agents.
9. Removal of June drop apples can considerably reduce PC damage, particularly if it can be done soon after the apples fall, before the grubs have a chance to move into the soil to pupate. This can be done with a machine (similar to ones some golf clubs use to vacuum up golf balls), or with animals (possibly geese, pigs or sheep). Ducks in an orchard will also eat some pests as will certain insectivorous birds, which' can be Encouraged by placing appropriate size bird boxes on stakes or dead trees. Flickers, for example, may pick PC out of the bark of trees..
10. Biological controls are-not yet commercially available, although our research indicates that several species of nematodes may be useful. These should-be sprayed on the ground under the perimeter rows of trees in Spring, under the trees at June drop and under the tall -trees or all the trees at the edge of any adjacent woodlots in the fall.' It may be possible to apply suitable bacteria, fungi or viruses in the same way as these become available. In the absence 'of these materials insecticides (imidan, zolone or guthion) may be used in the same way, or applied to the perimeter rows of trees at the usual time of spraying for PC (at 90 petal' fall and if needed 8 to 10 days , later). As PC become active at dusk at this time of year, spraying | should be caroled out at this time because susceptibility to pesticide increases with activity.
Although I would like to tell you that this work is progressing and that reliable, safe' and economic controls are around the corner, I have to report' that for the past two years I have received no research funding to continue this work. By letting those who give out research money know what you would like to have investigated, you the grower can play an important role in ensuring that you have answers to your problems.
S.B. Hill Dept. Entomology Macdonald College, Ste-Anne de Bellevue, QC September 1989 '
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