CETIOM (CEntre Technique Interprofesionnel des Oléagineux
Métropilitains), BP n°4, F-78850 Thiverval-Grignon.
In France and for a long period, difficulties related to aphids in winter oilseed rape crops have been restricted to occasional pullulations of the Mealy Cabbage Aphid (MCA), Brevicoryne brassicae L., in borders of post-flowering fields. Before 1990, more steady autumnal presences of the Green Peach Aphid (GPA), Myzus persicae SULTZER, were remarked. Then, the subsequents related virus diseases have been examined. After a few years with weather conditions unfavorable to aphids the present background is highly complex.
(1) Unexpected attacks of MCA occurred in spring 1995. In most of the cultivating areas, aphids populations first developed during flowering and then pullulated till harvest. Fields were globally infested, plants fully covered by insects. A such phenomenon, perhaps not so intense, occurred again in 1998. (2) Autumn 1997 was completely atypical. Aphids pullulated in numerous regions. For the first time in France, farmers were confronted to obvious direct nuisibility of the GPA concretized by killed plants and crop destructions. (3) As, in some places, GPA pullulated in previously insecticide treated fields, the question of insecticide resistances was asked. Laboratory tests conducted in 1997 and 1998 confirmed resistance phenomenons to pyrethroids. (4) Hereafter, even with slight aphid populations, viruses can be detected in most of the fields. (5) In this late period, field experiment programs for better control of aphids and/or virus diseases have been performed. In the context, others aphids species have been collected, i. e., Rhopalosiphum padi L and Lipaphis erysimi KALTENBACH.
KEYWORD : mealy cabbage aphid, green peach aphid, virus, insecticide resistance, aphid pest.
As an old crop, rape is well known in France as a crop especially confronted to coleopteran pest species. From the first improvements of rape cultivation (1945-1948 : 32.000-36.000 ha) to the recent period (1990 : 689.000 ha), aphids have not been considered as major pests of rape. Last decade was characterized by a widening of the pest register and the development of new difficulties and aphid-virus problems.
For a long time, MCA had been considered as the only aphid species susceptible to damage crops in France but without standing as a major pest of rape. In the past, especially in the southern areas of production, it was possible to be confronted to late spring attacks and limited effects on yield in the borders of the fields.
In spring 1995, a new scheme occurred based upon four unusual characteristics : (1), the problem was enlarged to the whole french area of production ; (2), instead of declining as plants matured, aphid populations pursued on their development until harvest ; (3), in many cases, attacks were not restricted to borders, fields were globally infested ; (4), it was possible to find plants fully covered by insects, from basal collars to distal extends of stems and pods.
The situation was also critical in 1998. But, as aphids began to pullulate earlier farmers sprayed flowering crops without hesitation.
For a long time, never more than slightly present in the fields, GPA had not been considered like a potential pest of rape in France. Its present status is linked to two new sources of difficulties : virus transmissions to rape (see § 5) and, recently, direct nuisibility.
Usually, on rape, GPA populations increase in October - possibly pullulate - before becoming scarce at the beginning of November (rains, first colds). In autumn 1997, GPA pullulated in many crops in most of areas of rape cultivation as if the agricultural contexts were very different (large crops, grasses, orchards, ...). From place to place, large damage was observed with killed plants and destroyed crops. For the first time in France a direct obvious nuisibility of GPA appeared.
In autumn 1997, in Lorraine, as they were confronted to unusual attacks of diamondback moths (Plutella), farmers reacted by cypermethrin sprays. After that, as GPA pullulations occurred in previously treated crops, the question of resistance to insecticides was asked.
As GPA is well known as a species having developed resistances to several chemical families of insecticides, as if such topic is not of present interest with the classical pests of rape, this hypothesis have been considered.
In autumn 1997, first laboratory tests were performed with cypermethrin (Ballanger and Delorme, 1998). In autumn 1998, confirmations came from new series of tests managed with deltamethrin and lambda-cyhalothrin (Ayala et al, 1999).
The level of resistance observed is slightly less than the ?``R2''? type and can be considered to correspond to the ?``French-R''? type (Devonshire, 1977, Devonshire and Moores, 1982). Thus this level of resistance in not so high, experts considered it to be sufficient to explain lakes of efficacy in the field as : CE/CL50 < 20 [CE : registered concentration / letal concentration 50] (Ayala et al., 1999).
It was also confirmed that GPA has not developed resistance to pyrimicarb and that the phenomenon does not concern MCA (Ayala et al., 1999).
Studies take in account three viruses : (1) TuYV, Turnip yellow Virus (Graichen and Schliephake, 1996), or BWYV, Beet Western Yellow Virus ; (2) CaMV, Cauliflower Mosaïc Virus ; (3) TuMV, Turnip Mosaïc Virus (Kerlan, 1991).
After an unexpected record of virus in rape plant coming from the field - in England - (Gilligan et al, 1980) and as GPA seems to be more present in the crops, special care was put on this new difficulty. At first, results were indicating BWYV was not so damaging to crops. In spring 1989, severe symptoms attributed to CaMV and TuMV were observed in the fields. A year later, new results coming from 2 field trials estimated the damage to crops at 0.5 or 0.7 t/ha (Regnault and Maisonneuve, 1994).
Lots of field tests have been performed from 1990-91 to 1993-94. It was concluded that BWYV is widely overspread and easily detected in most of the crops and that the two other viruses are not so frequent. It was also concluded effects on yield are not so negative with BWYV as with CaMV or TuMV and yield losses due to aphid-virus complex can be measured in insecticide field trials at almost 0.9 t/ha (Maisonneuve et al., 1995).
After some years without significant infestations (1993-1997), the question was asked again in 1997-98 (see § 4) and suggested new investigations. Field tests revealed again general attacks of TuYV and larger TuMV-CaMV occurrences than expected (Ballanger and Hosotte, 1998). In 1998-99, as if aphids infestations stood at very low levels, viruses were easily detected in the fields.
In France, beside MCA and GPA, crucifers are susceptible to be colonized by other aphids like Aulacorthum solani KLTB, Lipaphis erysimi KLTB, Macrosiphum euphorbiae THOS, Myzus ascalonicus DONC, Myzus ornatus LAING or Sminthurus betae WESTW (Leclant). Till the recent past, these species were not considered as pests in France.
As field experiment programs have recently been developed for better control of aphids and virus diseases and as more attention was focused on aphids, in some occasions, insects collected in infested winter oilseed rape fields have been labelled as : Lipaphis erysimi KALTENBACH and Rhopalosiphum padi L.
In France, aphids and/or viruses are new difficulties farmers have to deal with. This new evolving situation comes to disturb long well established strategies of crop protection against pests efficaciously based on pyrethroid insecticides.
Without good expressions of symptoms in infested fields, even if real nuisibility of the less damaging virus (BWYV) may be expected higher than supposed at first (Jay and Smith, 1995), it is still difficult to mobilize farmers against virus diseases. Actual preconisation is : spray if 20 % of the plants are infested by aphids during the first 6 weeks period of the crop (Anonyme, 1998). This preconisation is not so conforted by our last investigations and evidence of high level virus infestations in fields quite without aphids.
In autumn, GPA may be damaging by itself. Integrated management can come from field observations but needs to take in account resistance to pyrethroïds and managements of coleopterous pests control.
In spring, MCA also demands new investigations, concerning dynamics of populations, and new recommendations, concerning aphid control strategy. In this last case, as if MCA is not resistant to pyrethroïds these contact products are certainly not so well adapted to the actual cover of modern rape varieties.
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