A non-chemical method for monitoring adult olive weevil populations in Tadla-Azilal plain in Morocco.

EL JIATI, A.
Key words: olive- Otiorhynchus cribricollis – monitoring – non-chemical - weevil
Abstract 
The olive weevil becomes one of the major olive pests in Tadla-Azilal in Morocco. The chemical control is expensive and of low efficiency. Its application at night complicates the task for olive growers. An alternative non-chemical method has been developed using polyester fiber as a trap. It has a high efficiency, low cost and might be advised to organic olive farmers.
Introduction 
Adult black olive weevils (Otiorhynchus cribricollis Gyllenhal) are nocturnal feeding insects. They hide beneath bark, in debris, or litter around the base of the plant or some other protected places during the day and move up the tree to feed at night (Civantos, 1999; Alvarado et al. 2001). They primarily feed on leaves, and heavy infestations can cause considerable leaf loss. Adults feed along the leaf margin, causing a characteristic notched appearance ((Civantos, 1999; Alvarado et al. 2001; Daane, 2005). It is the most common damage observed, in both new and established olives. 
In the last years, this insect becomes one of the most principal olive pests in Tadla-Azilal. In 2005-2006, we led a survey to know the monitoring methods used by Tadla-Azilal olive farmers for this insect. 
Based on survey results, we decided on 2006-2007 to make an experiment using Polyester Fiber Trapping (PFT) commonly used in filling pillows in mass trapping as an alternative method to chemical monitoring. This is, in order to compare, in one hand its effectiveness in olive weevils management and in the other hand its cost compared to the former method. The idea comes from the fact that weevils have a roughened body surface. It is covered with dense, short, light-colored hairs with small patches of yellow hairs on the "elytra" or hard front wings (Daane et al., 2004). This characteristic might help in capturing weevils when climbing the trunks of the trees and crossing the polyester fiber.  Captured weevils remain there until their deaths.
Materials and methods
An orchard of 1 hectare containing 200 olive trees of the picholine de Languedoc variety has been chosen for the trial. Each trunk has been surrounded by a PFT band of 15 cm of width at about 60 cm from the sol surface. Some trees have more than one trunk. The PFT band has been attached to the trunk using rubber strapping. It is not recommended to use baling twine because it may cause damages to the tree as the trunk grows.
Two PFT bands, 15 cm away from one another, have been used to some trunks to show the capacity of weevils to overcome the first barrier. 
The cost of PFT per hectare has been calculated and the evolution of the weevil population trapped has been followed in 3 trees randomly chosen in the orchard.
Results and discussion
The survey led on 2005-2006 shows that only 10% of the olive growers (modern farmers) know something about olive weevils and use the nocturnal chemical monitoring. The remaining 90% (traditional farmers) do not use any monitoring method.
Tab. 1: Comparison between the annual cost per ha of two weevil monitoring methods.
	PFT	Chemical	
Cost 	500 dirhams ( about 45.5 €)	750 dirhams (about 68.2 €)	

It also shows that the majority of the 10% olive growers apply chemical treatment 3 times a year and that the average cost per hectare of the whole 3 treatments is about 750 dirhams ( equivalent to 68.2 €) (tab. 1). 
The curves of the evolution of the weevil population captured by PFT (Figure 1) all tend to a plateau (asymptote). This means that, with the time, the population surrounding each olive tree tends to extermination. In the case of chemical treatment, it is difficult to exterminate weevil population because, in one hand, of the nocturnal 

Figure 1: Evolution of the number of weevils trapped by 3 olive trees using PFT

habit of the insect: only a part of the population is touched by insecticides. In the other 
hand, black olive weevil is parthenogenetic (i.e., there are only females, no males) which makes the survival of the population easier than the case of gynogenetic species. That’s what explains the practice of chemical treatment 3 times a year by modern farmers surveyed.
Other trap designs have been developed on other fruit tree species: repellent polybutane sticky bands placed on the trunks of vines prevent weevils from entering the canopy (Fisher and Learmonth. 2006). The sticky band has the inconvenient of becoming a source of disturbance in the tree. The Circle trap and the pyramid or "Tedders trap" (Mulder et al. 2004, Neil Carter. 2004). Pyramid trap was developed from an intelligent idea: Weevils prefer to climb the dark plastic pyramid trap instead of the tree when light coloured trunks are used. But no one of these traps is used in mass trapping of weevils and all of them require a lot of labour. 
It is also interesting to mention that in all the trunks where we placed 2 PFT bands, we found weevils captured in these second bands. This means that some adult weevils were able to overcome the fiber barrier.
Recommendation 
At the light of this work we recommend the followings:
- To study the lifetime of PFT under field conditions. The longest the lifetime of PFT, the lowest the cost of this monitoring method will be.
- To study the eventual negative effect of PFT band on olive tree. The band may be used as a refuge of other harmful insects. It my also accumulate humidity after rain. The humidity may promote the development of fungi such as Armillaria for example.
Acknowledgments
I acknowledge especially Mr Mohamed Kabbaj for accepting his farm to be used as a field of this experiment. Without his cooperation this work would not have been possible. We also thank the farm manager Mr Mohamed Jalil and all the farm workers who contributed to this work. I appreciate their hard work and I’m extremely grateful to them. I would be failing in my duty if I do not thank my respondents who gave their valuable time and answered the survey questions with tremendous patience and understanding.
References 
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