Efficacy of different Insecticides in Controlling Pollen Beetle (Meligetes aeneus F.) in Rapeseed Crop

Predrag Milovanović1, Petar Kljajić2, Goran Andrić2, Marijana Pražić-Golić2 and Tatjana Popović3 1Galenika-Fitofarmacija a.d., Batajnički drum bb, 11080 Belgrade, Serbia 2Institute of Pesticides and Environmental Protection, Banatska 31b, 11000 Belgrade, Serbia 3Institute for Plant Protection and Environment, Teodora Drajzera 9, 11000 Belgrade, Serbia (milovanovic@fitofarmacija.rs) Received: September 3, 2013 Accepted: December 9, 2013


INTRoduCTIoN
The pollen beetle Meligethes aeneus (F.) is a major pest of rapeseed crops all over the world (Hansen, 1996;Alford et al., 2003) and also in Serbia (Sekulić and Kereši, 2007;Milovanović, 2006Milovanović, , 2007;;Milovanović et al., 2008Milovanović et al., , 2011)).M. aeneus adults injure rapeseed inflorescences by feeding, reducing considerably the crop's seed yield.Rapeseed is most sensitive at the stage of green and yellow buds, when M. aeneus causes the most serious damage.The flowering period is significantly longer in seasons followed by cold springs when damage may go up to 70% unless control of M. aeneus is undertaken.Even though M. aeneus populations are active every year, damage caused by this species is sometimes below economic threshold and insecticides are not applied at all or the number of treatments is lower.This is especially important as rapeseed is a major pasture for honeybees, and most insecticides used to control M aeneus are highly toxic to bees (Frearson et al., 2005;Williams et al., 2007).
Pyrethroid insecticides are the most frequent choice for M. aeneus control (Hokkanen et al., 1998) because their activity is better in cold weather (negative temperature coefficient) when M. aeneus is normally controlled.Besides pyrethroids, organophosphate insecticides are also used (Węgorek et al., 2009), as well as neonicotinoids (Wegorek, 2005).Years of repetitive application of insecticides, often not fully justified, have lead to reduced susceptibility or resistance of various populations of M. аeneus (Hansen, 2003), such as reported from different parts of Europe, e.g.France (Delorme et al., 2002), Germany (Heimbach et al., 2006;Glattkowski et al., 2008;Müller et al., 2008), Denmark (Hansen, 2003(Hansen, , 2008)), Switzerland (Derron et al., 2004;Philippou et al., 2010), Austria, Sweden (Kazachkova, 2007) and Poland (Węgorek, 2005;Węgorek et al., 2006Węgorek et al., , 2007Węgorek et al., , 2009;;Wegorek andZamojska, 2006, 2008;Philippou et al., 2010).As pyrethroid insecticides are primarily used against this pest species in our country and globally, resistance to that group of insecticides has become most evident (Derron et al., 2004;Heimbach et al., 2006Heimbach et al., , 2007;;Hansen, 2008).On the other hand, susceptibility of M. aeneus populations to insecticides in Serbia has not been studed and there is little data on the efficacy of insecticides that are being used against this pest species (Vuković et al., 2007;Milovanović et al., 2008) The present three-year (2008)(2009)(2010) study focused on testing the efficacy of several insecticides with different modes of action against M. aeneus in winter rapeseed crops at three locations (Kovin, Smederevo and Požarevac) as a basis for thorough examination of the susceptibility of different populations of M aeneus to insecticides.

MATERIAL ANd METHodS
Efficacy testing was conducted using the label rates of several insecticide products registered in Serbia for control of M. aeneus (Janjić and Elezović, 2010) and a thiacloprid-based product registered for M. aeneus control in other countries but not yet registered for that purpose in Serbia (Table 1).The efficacy of insecticides was asessed in fields trials at locations listed in Table 2 during three successive vegetation seasons (2008, 2009 and 2010).Trials were set up according to the methods PP 1/178(3) and PP 1/152(3) (OEPP/EPPO, 1999; OEPP/EPPO, 2004) and a random block design was used with four replicates and experimental plot size of 25 m 2 (Figure 6).The insecticides were applied during the winter rapeseed development stage of visible flower buds but still closed (BBCH 55-59) (Lancashire et al., 1991).A Solo backpack sprayer and 300 l/ha of water were used for treatments (Figure 6).No insecticide was applied on control plots.The trials were set up on April 1 st , 2008, April 7 th 2009 and April 9 th , 2010, after counting the present of M. aeneus imagoes.
The efficacy of insecticides was evaluated according to the method PP 1/178(3) (OEPP/ EPPO, 2004).Ten top inflorescences of rapeseed were evaluated in each plot representing an insecticide treatment and each replicate.The surviving imagoes of M. aeneus were counted three and seven days after treatment on the following dates: April 4 th and 8 th , 2008, April 10 th and 14 th , 2009, and April 12 th and 16 th , 2010, respectively.The data were statistically analyzed by One-way ANOVA and the significance of mean differences was determined by Fisher's LSD test at P < 0.05 (Sokal and Rohlf, 1995).

Efficacy testing of insecticides in 2008
Three days after treatments, the efficacy of the tested insecticides ranged from 90-95%, 90-100% and 93-99% at the locations Kovin, Smederevo and Požarevac, respectively (Tables 3-5).In the group of pyrethroids, lambdacyhalothrin showed the highest efficacy of 99% and 100% at the locations Smederevo and Požarevac, respectively, and bifenthrin had the lowest statistically significant efficacy, while no statistically significant differences were recorded between the pyrethroids at Kovin.Thiacloprid achieved the lowest efficacy at all locations three days after treatment.After seven days, the efficacy of the tested insecticides was significantly lower at all three locations, and the number of M. aeneus adults significantly increased, compared to the data after three days.In that interval, the highest efficacy was achieved by pirimiphos-methyl, 87%, 91% and 92% at Kovin, Smederevo and Požarevac, respectively, while the combination of chlorpyrifos and cypermethrin achieved 88%, 91% and 92% efficacy.

Efficacy testing of insecticides in 2009
In 2009, all tested insecticides achieved efficacy that exceeded 90% at Kovin and no statistical differences were detected among them, thiacloprid being the only examption (Tables 6-8).At the location Smederevo, the efficacy of all insecticides except thiacloprid exceeded 90% three days after treatment, and the highest was found in rapeseed plots treated with lambdacyhalothrin (98%), pirimiphos-methyl (98%) and the combination chlorpyrifos+cypermethrin (96%).At Požarevac, all insecticides reached high efficacy of 95-99%.Seven days after treatment, the insecticides showed a significantly lower efficacy at all three locations, compared to the evaluation after three days, i.e. an increased number of M. aeneus adults.In that interval, the highest efficacy was demonstrated by pirimiphos-methyl and chlorpyrifos+cypermethrin, 89-92% and 90-93%, respectively, at all three locations.The efficacies of pyrethroids and the neonicotinoid thiacloprid showed no statistically significant difference, ranging from 79-82% at Kovin, 81-85% at Smederevo and 87-91% at Požarevac.

Efficacy testing of insecticides in 2010
No statistically significant differences were detected between the tested insecticides at the locations Smederevo and Požarevac three days after threatment, while only thiacloprid achieved significantly lower efficacy at Kovin than the other insecticides (Tables 9-11).At all locations, the highest efficacy of 93-95% was achieved three days after rapeseed treatment with the combination of chlorpyrifos and cypermethrin.As in 2008 and 2009, the efficacy of the tested insecticides seven days after treatment was again lower than it was after three days.However, no statistically significant differences were detected between the insecticides at different locations in that interval.

dISCuSSIoN
Analyzing the results of our field testing of the effectiveness of different insecticides, we found their efficacies to range: from 90-100% in evaluation three days after treatment and 80-92% seven days after treatment in 2008; 89-99% in evaluation three days after treatment and 79-93% seven days after treatment in 2009; 85-95% three days after treatment and 76-87% seven days after treatment in 2010.The combination of chlorpyrifos and cypermethrin (94-99%) and pirimiphosmethyl (93-99%) showed the highest efficacy during the three years of testing at all locations, while the pyrethroids achieved lower efficacy (87-100%), and thiacloprid the lowest (85-95%).
As the weather was favourable during trials, i.e. warm and dry, M. aeneus populations recovered relatively fast in winter rapeseed crops, and the efficacy of the tested insecticides consequently declined.Data from other similar studies in our country (Vuković et al., 2007) had also indicated a high level of efficacy (90-100%) of the pyrethroid insecticides alfa-cypermethrin, gammacyhalothrin and tau-fluvalinate three days after treatment, while their efficacies were significantly lower 7-8 days after treatment, i.e. 14-61%.Petraitiene et al. (2008) reported that pyrethroids (zeta-cypermethrin 100 g/l, deltamethrin 25, 50 and 100 g/l, alfa-cypermethrin 50 g/l, beta-cyfluthrin 25 g/l, lambda-cyhalothrin 5 g/l) and a neonicotinoid-pyrethroid combination (thiacloprid + deltamethrin 100 + 10 g/l) had an efficacy range of 86-100% one day after application, but it decreased significantly after four and seven days, and the abundance of M. aeneus beetles increased.Węgorek andZamoyska (2006, 2008) reported a drop  Vaitelyte et al. (2011) showed in a study that several pyrethroids (beta-cyfluthrin, lambda-cyhalothrin, tau-fluvalinate), a combination of a neonicotionoid and a pyrethroid (thiacloprid +deltamethrin), a combination of a pyretroid and an organophosphate (chlo-rpyrifos+ beta-cyfluthrin), an organophosphate (chlorpyrifos) and an oxadiazine (indoxacarb) were able to significantly reduce the number of M. aeneus imagoes in winter rapeseed crops and that tau-fluvalinate was the most effective pyrethoid.In our evaluation three and seven days after treatment, the numbers of M. aeneus adults were not significantly different between the plots treated with lambda-cyhlothrin, alfa-cypermethrinom and bifenthrin, except at the Smederevo location in 2008 and 2009, and Požarevac in 2008 when rapeseed treated with lambda-cyhalothrin had a significantly lowest number of beetles three days after treatment, while the number was highest in rapeseed treated with bifenthrin.
Pyrethroid and organophosphate insecticides have been used in Serbia (Janjić and Elezović, 2010) for many years to control pests in rapeseed crops, while neonicotinoids have not yet been introduced but their efficacy is the same in some other European countries.In our study, the efficacy of thiacloprid three days after treatment was significantly lower than the efficacy of all other tested insecticides, and the same was observed after seven days, except in 2010 when thiacloprid showed similar efficacy as the other insecticides seven days after treatment.
Research based on the optimization of chemical protection of rapeseed crops against M. aeneus has been conducted in recent years in most European countries, such as France, Great Britain, Sweden, Denmark, Germany, Poland, the Check Republic, Austria and Switzerland (Alford et al., 2003;Hansen, 2004).Węgorek et al. (2009) recommended at least 3-5 chemical treatments to control M. aeneus in Poland, the first one at the plant development stage BBCH 51-54 using insecticides with chlorpyrifos as an active ingredient; the next one at the stage BBCH 55-59 using pyrethoid or neonicotinoid insecticides (e.g.acetamiprid) or, if two treatments are needed during that period, a compound not belonging to the groups used before.They inferred that chlorpyrifos application at that stage was justified only when infestation was massive or resistance detected to pyrethroids or neonicotinoids.At the beginning of rapeseed flowering (beginning with stage BBCH 60), they recommended insecticides that include tau-fluvalinate or acetamiprid in order to rotate the insecticide chemical groups (Węgorek et al., 2009).The combination of chlorpyrifos and cypermethrin was found in that study to be the most effective at all locations during three years of experiment.Also, the efficacy data indicated that another insecticide treatment was needed after seven days.Petraitiene et al. (2008) measured rapeseed yields for eight years and found that seed yield in untreated plots was 102-447 kg/ha lower than in plots in which insecticides had been applied.Rapeseed growing has been intensified in Serbia in recent years, and increased usage of insecticides is expected in order to ensure high and stable yields.However, years of insecticide treatments may cause a change in susceptibility/resistance to insecticides in M. aeneus populations, especially in regions where rapeseed is grown intensively.In our three-year trials, the efficacy of insecticides was found to range from 85-100% three days after treatment, but it decreased significantly to 75-92% seven days after treatment, which indicates a need to organize monitoring in Serbia.

Table 2 .
Locations of efficacy evaluation of insecticides

Table 3 .
Efficacy of insecticides atKovin site, 2008 * Means within columns followed by the same letter are not significantly different (p>0.05)Legend: BT -before treatment; 3DAT -three days after treatment; 7DAT -seven days after treatment; Ms -mean; Sd -standard deviation; E -efficacy

Table 6 .
Efficacy of insecticides at Kovin site, 2009 * Means within columns followed by the same letter are not significantly different (p>0.05)Legend: BT -before treatment; 3DAT -three days after treatment; 7DAT -seven days after treatment; Ms -mean; Sd -standard deviation; E -efficacy

Table 7 .
Efficacy of insecticides at Smederevo site, 2009 * Means within columns followed by the same letter are not significantly different (p>0.05)Legend: BT -before treatment; 3DAT -three days after treatment; 7DAT -seven days after treatment; Ms -mean; Sd -standard deviation; E -efficacy

Table 9 .
Efficacy of insecticides at Kovin site, 2010 * Means within columns followed by the same letter are not significantly different (p>0.05)Legend: BT -before treatment; 3DAT -three days after treatment; 7DAT -seven days after treatment; Ms -mean; Sd -standard deviation; E -efficacy