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Fruit Notes |
Eyes On Plum Curculios: Watching Them
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Fruit Notes |
Ronald Prokopy and Catherine Wirth |
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Fruit Notes |
Shortcomings of black pyramid traps for monitoring plum curculios described in the preceding article have stimulated us to take a closer look at the behavior of individual curculios in hopes of discovering why black pyramid traps perform less and less satisfactorily as the curculio season progresses. In the 1997 Winter issue of Fruit Notes, we described some preliminary studies that led us to postulate that perhaps curculios bypass pyramid traps under moderate and high temperature conditions, flying directly into tree canopies rather than crawling or flying onto tree trunks or trunk-mimicking pyramid traps. Here, we describe two studies conducted in 1997 in which we made extensive direct observation of movements of plum curculios toward host trees and pyramid traps under conditions as natural as possible for typical curculio behavior, while still permitting effective observation. Materials and Methods Curculios to be observed were tapped from branches of unmanaged apple trees onto a 40x40-inch white bedsheet (first study) or a 16x16-inch white bedsheet (second study) held taut by staples driven into a wooden frame beneath. Apple foliage was scattered across about 15% of the surface area of each framed cloth to provide hiding places for fallen curculios. Plum curculios fall frequently from limbs of host trees (sometimes more than once per day) onto the ground beneath in response to perceived danger or adverse weather. We reasoned that tapping curculios from branches and allowing them to fall on cloth provided with shelter effectively mimicked natural behavior and conditions. As soon as four or five curculios (first study) or eight curculios (second study) accumulated on the cloth, we quickly but gently carried the frame and cloth to a position half-way between the trunk and canopy edge of a nearby semi-dwarf unmanaged apple tree (first study) or plum tree (second study). There were no curculio traps of any sort in the vicinity of the apple tree. One unbaited black pyramid trap was placed next to the trunk and another at the edge of the canopy of the plum tree, with the framed cloth centered between and equidistant (4 feet away) from either trap. Dressed in white cap, shirt, and shorts that hopefully were invisible to curculios, one of us knelt down nearby the cloth and quietly observed the proportion of curculios that departed the cloth by flight or by crawling and the proportion that moved to hide beneath foliage or rested on the cloth. Observation periods lasted 1 hour. They were evenly spaced among 1-hour intervals beginning at 8 AM and ending at 8 PM in the first study, or among 1-hour intervals beginning at 2 PM and ending at 6 PM in the second study. In both studies, observations commenced a day or two after petal fall and extended over a 3-week period thereafter. We recorded the direction taken by each adult upon departure from the cloth and continued to track adult destination until it was lost from sight. Results Of the 166 plum curculios observed beneath the apple tree, 52 (31%) left the framed cloth by flight and 27 (16%) by crawling. The remainder moved to hide beneath foliage on the cloth (18%) or rested in place (35%). Among curculios that flew, significantly more (54%) flew toward the tree canopy above than flew toward inter-tree space (25%), the tree trunk (17%), or grass beneath the canopy (4%) (Table 1). Among curculios that crawled, significantly more (88%) crawled toward the tree trunk than toward all other directions combined (12%) (Table 1). Of the 104 plum curculios observed beneath the plum tree, 39 (38%) left the framed cloth by flight and 19 (18%) by crawling. The remainder moved to hide beneath foliage on the cloth (13%) or rested in place (31%). Among those that flew, significantly more flew toward the tree canopy above (36%) or toward inter-tree space (38%) than toward the pyramid trap at the tree trunk (15%), the pyramid trap at the canopy edge (3%) or grass beneath the canopy (8%) (Table 2). Among curculios that crawled, significantly more (74%) crawled toward the pyramid trap at the tree trunk than toward the pyramid trap at the canopy edge (10%) or toward other directions (16%) (Table 2). Temperatures taken beneath each tree at the time of observed curculio movement indicated that no flight occurred at temperatures of 67°F or less. Also, there was a significant positive correlation between temperature and proportion of total observed curculios that flew. On the other hand, a substantial proportion of those curculios observed to crawl off the cloth (about 20%) did so at temperatures of 67°F or less, and there was no correlation between temperature and proportion of total observed curculios that crawled off. Conclusions The more robust data reported here confirm and extend the preliminary data reported in the 1997 Winter issue of Fruit Notes. Combined data indicate that when the air temperature is 67°F or less beneath the canopy of a host tree, the plum curculio adults that have dropped from the tree canopy (as they normally do on a frequent, even daily, basis) are reluctant or unable to fly but are able to reenter the tree canopy by crawling. Crawling is almost exclusively toward the tree trunk, or if a black pyramid trap is adjacent to the tree trunk, then toward such a trap, which is thought to be a visual mimic of a tree trunk. At temperatures of 68°F or greater, curculios exhibit an increasing propensity to reenter the tree canopy by flight. Most flights are into the tree canopy. Only a small proportion (15-17% according to our findings here) is toward the tree trunk or a black pyramid trap next to the tree trunk. Hence, at temperatures of 68°F or greater, there is only a small chance of capturing a tree-reentering curculio using a black pyramid trap. Other data that we collected in 1997 show that curculio damage to tree fruit increases with increasing temperature. It is therefore doubtful that any prospective unbaited curculio traps placed in association with the tree trunk will be able to monitor curculio entry into or abundance in the tree canopy in a way that reflects accurately the probability of curculio damage to fruit. Acknowledgments This work was supported by a grant from the USDA Northeast Regional IPM Competitive Grants Program and the New England Tree Fruit Research Committee.
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