Ulrich Neumann
Landwirtschaftliche Versuchsstation der BASF, 67114 Limburgerhof, Germany
Key words - sex pheromone, mating disruption, controlled release dispensers, technology transfer, apple, pome fruit orchard, Cydia pomonella, Adoxophyes orana, Tortricidae, Lepidoptera
Introduction
[top]
Meanwhile it has been recognised, that mating disruption by pheromones - besides for pests of grapes and peaches - can also be used successfully to control the major pests of apple. The number of communications on this subject presented at the Montpellier meeting is proof enough. Using exemplary results, I am particularly going to discuss the complex and difficult-to-define factor "population density", which is most important for successful applications of the mating disruption technique.
Codling moth (South Tyrol)
[top]
First of all I would like to mention the work carried out by the South Tyrolean Advisory Service for Fruit- and Winegrowing (W. Waldner) with BASF dispensers (RAK 3+4) in 1991. The RAK 3+4 dispenser contains 420 mg E8,E10-12OH and 420 mg Z11-14Ac/dispenser and is being applied at 500 dispenser/ha. It was on the basis of the results obtained with codling moth, Cydia pomonella (Figure 1; Neumann et al. 1992), that the Advisory Service recommended the mating disruption technique for commercial use.
Figure 1 Control of codling moth, C. pomonella, by mating disruption in South Tyrol. Top: Average infestation of apples on a total area of 94.6 ha (1991). Bottom: Fruit attack in relation to infestation levels during the previous season (1990/91)
Quite obviously, the success of the mating disruption technique depends to a large extent on the population density. A good indication for the population density of adult moths is the degree of larval attack during the previous generation. It is therefore not surprising that mating disruption was not successful in an unsprayed plot with a previous attack of 25% (Figure 1). Even with a previous attack rate of 5% the results were poor, despite low levels of infestation according to first larval counts. As a general rule, the threshold infestation level for successful control of codling moth by mating disruption is 1%. The reason for this could be a continuous emergence of moths over a longer period of time, which is often observed with codling moth, and which would lead to a critical level of the population density.
Summerfruit tortrix (South Tyrol)
[top]
Results from South Tyrol on mating disruption of the summerfruit tortrix, Adoxophyes orana, with RAK 3+4 may be of general interest, since there is some doubt whether Z11-14Ac is efficacious for control of summerfruit tortrix (Waldner unpubl. res.; Figure 2). The shoot infestation in 1991 reaches 10% in some cases, indicating a rather high population density. Threshold levels for shoot infestation should probably not exceed 1%, according to our experience with Oriental fruit moth, Grapholita molesta, where only one larva/100 shoots is allowed (Audemard et al. 1989). Nevertheless, in none of the plots the fruit infestation exceeded 1%.
Figure 2 Control of summerfruit tortrix, A. orana, by mating disruption (South Tyrol, 1991)
The trial in South Tyrol demonstrates that a level of fruit attack of 2% or less (1990; Figure 2) leads to the erroneous impression of an extremely low population density.
Summerfruit tortrix (Germany)
[top]
Results from a trial in Jork, Germany, show a similar picture (Figure 3). A shoot infestation of up to 10% resulted in a fruit infestation below the economic damage threshold - but probably at the same time in an overall population density which would have been too high to be controlled by mating disruption in the following season. This means that mating disruption would have no longer decreased shoot infestation to a level of below 10%, resulting subsequently in insufficient protection against fruit infestation - especially if at the same time there is a regional increase in population density, as this was in my opinion the case in the South Tyrol.
This year the mating disruption technique has been employed for the first time on large areas in Germany. Before starting, the farmers were asked about the level of attack in 1995. No one was able to answer this question! Naturally none of them knew about the levels of shoot tip infestation or leaf attack either.
Pheromone traps in the mating disruption plots registered several times high captures of codling moth, but generally a corresponding degree of damage was not observed. It seems reasonable to assume that there had been a pronounced migration of male moths.
Figure 3 Control of summerfruit tortrix, A. orana, by mating disruption (Jork, Germany, 1990)
Another example of mating disruption against summerfruit tortrix is shown in Figure 4. An initially good control of codling moth and summerfruit tortrix turned into a rapid increase in infestation in September, once the experimental formulation (120 g/ha E8,E10-12OH, 130 g/ha Z11-14Ac) with too high release rates was exhausted. Fruit infestation increased to 2.9% in the case of summerfruit tortrix; the corresponding leaf infestation was probably much higher.
Figure 4 Control of codling moth, C. pomonella, and summerfruit tortrix, A. orana, by mating disruption. Fruit infestation (%; bars); amount of codlemone remaining in dispenser (%; line)
Figure 5 shows a typical example. With summerfruit tortrix, a better result was obtained with pheromone than with insecticides, although the differences are not very important. The threshold of damage was exceeded in all cases. The damage to the apples in the mating disruption plots would certainly still be acceptable for the fruit-grower, but with respect to successful mating disruption employment in the following season the prognosis is negative, since it must be assumed that there was an high degree of leaf infestation. If the mating disruption trials were to be continued in the corresponding plots, additional measures would have to be taken to lower the population density.
Figure 5 Control of codling moth, C. pomonella, and summerfruit tortrix, A. orana, by mating disruption (Kriftel, 1988)
Figure 6 The influence of high trees on mating disruption of codling moth, C. pomonella (Öhningen, 1989)
Last not least, the presence of large apple trees in the neighbourhood of pheromone-treated orchards should be taken into account. Codling moths tend to fly out of such orchards towards tall trees (Zelger, pers. comm.); the results shown in Figure 6 suggest that even dispersal of mated females from these trees into the orchard can take place.
Conclusion
[top]
I hope that the points I have made will help to shed some light on the problems which have been observed, particularly in the case of summerfruit tortrix. In my opinion it is, also in the case of summerfruit tortrix, the population density, as evidenced by infestation levels of both fruits and foliage, which remains the key factor, but is often underestimated.
References
[top]
Audemard H, Leblon C, Neumann U, Marboutie G (1989) Bilan de sept années d'essais de lutte contre la Tordeuse orientale du pêcher Cydia molesta Busck. (Lep., Tortricidae) par confusion sexuelle des mâles. J appl Entomol 108, 191-207
Neumann U, Harries V, Gasser A, Waldner W, Kast WK (1992) Recent advances with the mating disruption technique in apples and grapes - Factors influencing the success of pheromones. Brighton Crop Protection Conference - Pests and Diseases 1992, 3, 1045-1050
[top]