~ Experimental and Applied Acarology 24: 191-211, 2000." (Ç)2000 Kluwer Academic Publishers. Printed in the Netherlands. 1 0 1 ~

Colonization of vineyards by phytoseiid mites: theirdispersai patterns in the plot and their fate

M.-S. TIXIER*, S. KREITER and P. AUGERENSA.M-INRA, Department of Plant Protection, Laboratory of Acarology, 2, place Viala,34 060 Montpellier cedex 1, France

(Received 14 April 1999; revised and accepted 4 January 2000)

Abstract. The effect of wind and woody margins on the dispersaI and population dynamics ofphytoseiid mites was studied in a vine plot for a period of two years. Mites were sampled in the plot andin the surrounding vegetation (crops and natural vegetation) in order to deterrnine phytoseiid miteabundance. The surrounding vegetation was considered to be a reservoir of phytoseiids from where thevine plot could be invaded. Directional and non-directional soi! and aerial traps were placed in the plotto deterrnine predatory mite exchange between the two areas. Colonization of the plot occurred in twostages: first, mite migration into the plot, followed by their establishment. The two-year study partiallyclarified the first of these two stages. Kampimodromus aberrans was the main species caught in the aerialtraps. Phytoseiid mite dispersaI within the vine plot seemed to be affected by both wind (direction,intensity and regularity) and phytoseiid mite density in the woody margin. However, the woody marginhad a large effect only over a short distance. Some observations pointed towards an effect of otherreservoir areas but it was not possible to characterize these. The population density of the phytoseiidmites in the plot increased from 1996 to 1998, but these increases are much smaller than one wouldexpect on the basis of the number of mites migrating by air in the plot. Moreover, blocks where mostmites were trapped were not the blocks where densities of phytoseiid mites on vine leaves were thelargest. It therefore seems likely that not all migrants were able to develop. Their settlement pattern wasnot deterrnined and this could constitute a potential research focus for the future.

Key words: Phytoseiidae, Kampimodromus aberrans, colonization, wind, dispersaI, field margins

Introduction

Predatory mites are effective in limiting phytophagous mite outbreaks that otherwisewould result in serious damage to vineyards (Sentenac et al., 1993). Typhlodromuspyri Scheuten and Kampimodromus aberrans (Oudemans) are the two main speciespresent in French vineyards (Kreiter et al., 1993). Phytoseiid mites, in particular thetwo species mentioned above, have been observed in wild vegetation surroundingcrop fields (Boller et al., 1988; Tuovinen and Rokx, 1991; Tuovinen, 1993a,b,c;Duso et al., 1993; Coli et al., 1994; Kreiter and Sentenac, 1995; Tixier et al., 1998).

* To whom correspondence should be addressed at: Tel: +4 99612374; fax: +4 67521554;e-mail: garcin@ensam.inra.fr

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192

Several authors showed that the presence of wild vegetation can even favour cropyield because of its richness and diversity in natural enemies (Dennis and Fry, 1992;Jones et al., 1995; Cooley and Autio, 1997; Kiss et al., 1997). Kreiter and Sentenac(1995) reported on the rapid restoration of predatory mite populations in vineyardsunder integrated pest management (phytoseiid mite densities increase with a con-comitant rise in the number of plots that they inhabit). This rapid restoration seemsto be due to natural colonization from surrounding areas. Tixier et al. (1998) showedthat this phenomenon is mainly infiuenced by the density of phytoseiids found inwild vegetation surrounding the fields and by wind. Colonization of crops by naturalenemies is of great significance as crop protection is an important factor in asustainable farming framework. However, very little is known about this process.The present study is a continuation of the work carried out in 1996 (Tixier et al.,1998). Its aims are the confirmation of earlier results, an in-depth analysis of factorssuch as wind on predatory mite dispersaI into vine plots and the description of areasthat may act as reservoirs of predatory mites. Moreover, this work focuses on thestudy of colonization over time (over two experimental years) and whether migrantscan develop within the plot.

Materials and methods

The experimental grape field, already studied in 1996, has been maintained under anintegrated pest management programme since 1995 (Tixier et al., 1998) (Figure 1).The selection of pesticides used is based on their side effects on predatory mites(Kreiter et al., 1998).

Phytoseiid mites population study

Surrounding uncultivated areas (woody areas and border hedges) were sampled (May27, 1997; July 08, 1997; August 11, 1997; May 26, 1998; July 28, 1998) in order todetermine the abundance of phytoseiid mites and diversity in species composition. Ateach sampling, a minimum of 30 leaves were collected randomly from at least oneplant of each species present. An index called WR (woody richness) proposed byTixier et al. (1998) was used to estimate phytoseiid mite abundance in uncultivatedareas. Samples (one leaf per 20 vine stocks) were also taken from grape fieldssurrounding the experimental plot (May 21, 1997; July 01, 1997; August 05, 1997;June 09, 1998; July 15, 1998). Considering the number of leaf samples to collect,phytoseiid mite species to identify and the number of mites to count, it wasimpossible to perform the various samplings in uncultivated areas and in vine plotsat exactly the same time. However, the sampling dates were as close as possible.

Pre-sampling was performed in April 1997 and 1998 in order to estimate the initialphytoseiid mite distribution inside the plot and their pattern over two years, as was

193

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done in 1996. The plot was divided into 56 blocks with 100 vine stocks in eachblock, of which 10 leaves were sampled. Phytoseiid mites were counted on each leafusing a binocular microscope. Between April and September 1997 and 1998, samples

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194

were taken every two weeks in 15 blocks (Figure 1) on the basis of data obtained in1996 in order to assess phytoseiid mite variation in time and place.

Phytoseiid mites were removed from leaf samples from uncultivated or cultivatedareas using the soak-wash extraction method of Boller (1984) or the Berlese-Tullgren method. Mites collected were counted and identified following the reportsby Moraes et al. (1986) and Chant and McMurtry (1994).

Phytoseiid mites dispersai study

Four kinds of traps, two for studying ambulatory dispersaI and two for assessingaerial dispersal, were used. Twenty-five soil traps similar to those used by Tixieret al. (1998) were placed in the grape field. Twenty direction al soi! traps, made oftwo parallel drugget strips (20 X 5 cm) in open-sided boxes (50 X 50 cm), were alsoused. As in the non-directional soil traps, drugget strips were covered with pinepollen. One of the two drugget strips was facing the plot and the other the surround-ing area. To study the direction of the ambulatory dispersal, glue was appliedbetween the two drugget strips and on the two sides perpendicular to the crop borderedges. The soil traps were only used in 1997 because very few phytoseiid mites werecaught and the species trapped were not of agricultural interest. Twenty-five aerialtraps made of funnels (31 cm diameter) similar to those used by Tixier et al. (1998)were placed approximately 1 m above the vines (Figure 1). ln order to studycolonization inside another vine plot (called V2) with high densities of phytoseiidmites and in the vicinity of the experimental vine field, nine other funnels were

placed inside this plot in 1998. Directional aerial traps were used in the experimentalgrape field to study the influence of wind direction on predatory mite dispersal. Thesewere made of funnels (31 cm diameter) topped with a lid glued to the external edges.The lid was opened when wind blew in one of the four cardinal directions (North-west = NW, Southeast= SE, Northeast = NE, Southwest= SW) using a windshield(50 cm X 20 cm polycarbonate shield) positioned perpendicularly to the wind. Thelid was attached to the funnels allowing a maximal opening of the trap of 40°. Agroup of four traps corresponding to the four cardinal points was placed at five sitesinside the vine plot (Figure 1). ln 1998, a non-directional funnel, hereafter called a'prime', was added to each site to measure the efficiency of the direction al traps.

The soil and aerial traps were inspected weekly. Predatory mites were extractedfrom the drugget strips by the Berlese- Tullgren method. The funnel contents werefiltered through a 100 /-lmsieve and the mites found were counted, mounted on slidesand identified.

Statistical analysis

Variance analysis, followed by a Duncan mean comparison test (SAS Institute,1985), was used to compare the number of phytoseiid mites trapped in the differentfunnels, and the phytoseiid mite density in the different blocks in the experimental

195

vine plot. A Kendall test (Statistica@, 1998) was used to test the correlation betweenvariation of the traps in the experimental plot and in V2.

- To study the initial predatory mite spatial patterns inside the vine plot, tests wereconducted to fit two statistical distributions: the Poisson distribution, corresponding

to a random spatial pattern, and the negative binomial distribution, corresponding toan aggregative spatial pattern (Sherrer, 1984; Statistica@, 1998). Two aggregationindexes were studied: the common K of the negative binomial distribution (South-wood, 1991) and the slope of Taylor's power law (Taylor, 1984). As the distributionof K. aberrans in the -plot follows a negative binomial distribution, data wereconverted using the following equation: y* = y(1-b/2). A Kruskall and Wallis non-

parametric analysis of variance (NPStat@,1995) was then used to compare phytoseiidmite density of the 56 pre-sampling blocks.

ln order to compare phytoseiid mites diversity in the different traps, a specificdiversity index (ls: Simpson index) and an equability index (Es) were used. When Isis near 1, only one species is dominant. When high densities are occurring for onlyone species, Es = O. Es is near 1 when all the species have the same abundance(Barbault, 1992).

The influence of wind on the number of phytoseiid mites trapped was studiedusing the dispersaI index (DI) proposed by Tixier et al. (1998). The Frenchmeteorological office (Météo France, Pézenas) provided the wind data. This indexaccounted for wind direction, velocity and frequency parameters and the richness ofthe woody margin (WR) (Tixier et al., 1998). Correlation tests were performedbetween the DI for the NW and SE directions (the two directions of the strongestdominant winds in Languedoc region) and the number of phytoseiid mites trapped inaerial traps (directional and non-directional). The number of phytoseiid mites trappedwas converted to the number of phytoseiid mites per leaf in order to estimate thepotential of migrant mites that could reach a vine leaf in the plot. This conversionwas based on two factors: the estimate of the vegetation area that could actually bereached by phytoseiid mites, and the transformation of this surface into a leaf numberwhere the migrants (once present in the vegetation) are able to develop. The estimateof the actual landing surface of phytoseiids was based on the work of Carbonneau(1995). This author defined the foliar surface potentially exposed to solar radiationfor modelling the photosynthetic yields of vine plants. ln our study, this surface wasassumed to be similar to that exposed to the wind. The height and width of thevegetation was 1.5 m and 1 m, respectively. The estimate of the actual landingsurface that could be reached by phytoseiid mites for a vine stock was 3.55 m2.Based on the large number of samples, the mean number of leaves per rootstock wasestimated at 200 (M. Weber, pers. comm.) and the funnel area was 0.0707 m2. Thenumber of phytoseiid mites caught per leaf and per week was ca1culated as follows:

Mean number of phytoseiid mites reaching a vine leaf per week = (mean number of

phytoseiid mites trapped in a funnel X 3.55) / (0.0707 X 200).

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196

Results

!he surrounding vegetation

Results based on the occurrence of predatory mites in the surrounding wildvegetation near the plot are presented in another report (Tixier et al., manuscript).Data of this study of interest for an understanding of the colonization process are thefollowing: (i) K. aberrans was the main species collected in these areas during thetwo-year period. Species density was especially high on Celtis australis (0.58phytoseiid per leaf: Ph lear 1; 99% of K. aberrans), on Quercuspubescens (0.51 PhleaCI; 95% of K. aberrans), on Euphorbia characias (0.24 Ph learl; 97% of K.aberrans) and on Ulmus sp. (0.18 Ph leaf-1; 95% of K. aberrans). (ii) The woodymargin and hedge no. 1 showed the highest abundance of phytoseiid mites over the2 years. (iii) The WR index usually decreased between May and July, except forhedge no. 3 in 1997 (Figure 6).

Among the four grape fields surrounding the experimental plot, two (VI and V2)had high densities of phytoseiid mites per leaf (1.43 Ph 1eaCI and 3.15 Ph leaCIrespectively). K. aberrans accounted for 99% of the phytoseiid mites found in thesevine fields except for V4 where Kampimodromus aberrans was not observed.

Phytoseiid mite dispersai

Numbers of phytoseiid mites trappedDespite the fact that there were 40 additional soil traps, the number of phytoseiidmites collected in 1997 was similar to the numbers found in soil traps in 1996 andlower than the number trapped in aerial traps (Table 1). The number of phytoseiidmites collected in directional soil traps was identical to that trapped in non-directional soil traps although their trapping surface was twice as large (P = 0.65).

The number of phytoseiid mites caught in the aerial traps was higher in 1998 thanin 1997 (Table 1) (P < 0.000), especially in the aerial traps for NW and SW direc-tions and for sites 2 and 5 (Figure 2). On the other hand, the number of phytoseiidmites trapped at site 3 in 1997 did not differ from those trapped at this same site in1998 (Figure 2). The number of phytoseiid mites trapped in the neighbouring grapefield (V2) was lower than those trapped in the test plot (P = 0.40) (Table 1).

Using the conversion to ca1culate the number of migrants reaching a vine leaf, thenumbers of phytoseiid mites trapped in this study would present an increase of 0.05and 0.17 phytoseiid mite per leaf and per week for 1997 and 1998 respectively, basedon the assumption that settlement was successful. ln 1998, the number of phytoseiidmites trapped in aerial funnels located near the woody margin between May andSeptember corresponded to a total 'landing' of 14 phytoseiid mites per leaf.

The total number of phytoseiid mites trapped in the four direction al funnels wasalways larger than the number of phytoseiid mites trapped for every site in the prime

funnel (Figure 2). At sites 1, 2, 3 and 4, the number of phytoseiid mites in the fourdirectional funnels was, respectively, 8, 12.5, 13 and 22-times larger than the numberof mites trapped in the prime funnels. However, this ratio was only 2 at site 5 locatednear the woody margin.

Species of phytoseiid mites trappedK. aberrans was the main species caught in aerial funnels (directional and non-directional) (Figure 3, Table 2). However, this dominance is less prominent fordirection al traps and funnels located in V2 (Table 2). K. aberrans was the mainspecies captured in only four traps of nine present in V2. Is is also larger and Es lessimportant in directional traps and in funnels located in V2 (Table 2). Very fewindividuals of this species were collected in soil traps. Typhlodromus phialatusAthias-Henriot was the main species trapped in non-directional and directional soiltraps in 1997.

Sex-ratio and age structure of phytoseiid mite populations trappedA large number of females was trapped in aerial funnels during the two-year studyperiod. Although some variation was observed in 1997, the sex ratio was quiteconstant (Table 1). The percentage of immature mites captured in the direction al andnon-direction al funnels was approximately 30% over the two years. This percentagewas always lower in soil traps.

197

Table 1. Numbers, sex-ratio and immature percentage of phytoseiid mites captured in the different kindsof traps in 1997 and 1998

Undirectional Directional Prime Aerial traps Undirectional Directional

aerial traps aerial traps funnels in V2 soil traps soil traps

1997 Number of 3.36 2.75 0.88 1.35 (0.67phytoseiid mites per strip)captured per trap

Number of traps 25 20 20 40Standard error 2.9 1.62 3.3 1.81

% Immatures 37 33 10 15Sex-ratio 82:18 58:42 88:12 92:8

(female:male)

1998 Number of 13.04 7.35 8.2 5.9

phytoseiid mitescaptured per trap

Number of traps 25 20 5 9Standard error 24.9 6.9 12.2 3.7

% Immatures 31 27 26 38Sex-ratio 74:26 74:26 73:27 71:29(female:male)

1998

2 3 4 5

IIIDSE INW

Figure 2. Number of phytoseiid mites trapped in the four different directions and in the prime funnel atthe five sites of directional traps in 1997 and 1998.

Spatial pattern of phytoseiid mite capturesNo difference was observed between the two strips in the directional soil traps (onestrip facing the plot and one strip facing the surroundings) (P = 0.72) and between

the different soil traps with respect to their location (P = 0.35). However, the numberof phytoseiid mites caught in directional soil traps was relatively high near thewoody margin and the neighbouring grape field (VI). K. aberrans was the onlyspecies captured in soil traps located inside the plot.

The largest number of captured phytoseiid mites was observed in aerial trapslocated near the woody margin (Figure 3). Unlike 1998 (P -<0.000), the number ofphytoseiid mites trapped in 1997 was so low that no significant difference (P = 0.07)between the different funnels was observed. The number of phytoseiid mite specieswas also larger in funnels placed near the woody margin (Figure 3).

No statistically significant differences were found in the catches of mites trappedin the funnels located in V2 (P = 0.11). The numbers of phytoseiid mites trappedin 1997 in directional aerial traps were not significantly different between any siteand direction studied (Psite= 0.23; Pdirection= 0.88). ln 1998, the same results were

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Figure 3. Number and species of phytoseiid mites trapped in undirectional aerial traps during twoyears.

obtained for the analysis of the effect of the direction (P = 0.42). However,phytoseiid mites moving from the SW were mainly trapped at site 2, those comingfrom the SE at site 4 and those coming from the NW at site 5. The numbers ofphytoseiid mites trapped at the five sites were significantly different in 1998(P = 0.034). Captures were significantly higher in site 5, located near the woody

margin. Moreover, the numbers of phytoseiid mites trapped in prime funnels weresignificantlyhigher at site 5 than at the four other sites (P = 0.006).

Temporal patterns of phytoseiid mite captures

There were very irregular temporal variations in soil trapping (Figure 4). For aerialnon-direction al and directional traps, the numbers of phytoseiid mites trapped in1997 were low throughout the year, whereas in 1998 the number of predatory mitescaptured over time increased reaching a maximum number in early July (Figure 4).Temporal variations in trapping were very important, especially during 1997 (co-efficients of variation for 1997: 218% and for 1998: 151%). A non-parametriccorrelation (Kendall-test) shows that temporal variations of aerial trapping in V2 andin the experimental plot were not significantly different (P = 0.04) (Figure 4).

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Wind and woody margin influences

The DI of the prevailing wind (NW) was not correlated with the numbers ofphytoseiidmites trapped each week in 1997 and 1998 (r1997= 0.04, P = 0.53; rZ1998 = 0.49,P = 0.08). No correlation was observed with the SE DI either (rZ1997= 0.02,P = 0.59; rZ1998 = 0.19, P = 0.89). There was no correlation between DI of NW andSE winds and the numbers of phytoseiid mites trapped in V2 (rZNW= 0.057,P = 0.50; rZSE = 0.16; P = 0.24). However, in 1998, the DI of NW wind was

significantly correlated with the numbers of predatory mites trapped in NW-facingdirectional funnels and at site 5 (site 5: rNW= 0.62; P = 0.04; NW direction:rNW = 0.70, P = 0.017). Moreover, a statistically significant correlation (r2= 0.88,P = 0.000) was observed between the numbers of phytoseiid mites trapped in non-directional traps and those trapped in the directional traps of NW direction in1998.

Phytoseiid mite populations within the grape fieldK. aberrans was always the main predatory mite species present in the plot duringthe three years. The numbers of phytoseiid mites observed at the first samplings

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(May) were similar for the two years (Figure 6), but the succeeding variations did notshow any consistent pattern between years. Mean densities per leaf increased from1997 to 1998 in the whole plot (Figure 7), but was especially large in the centralnorthern zone (zone 2).

The predatory mites densities observed in the 15 blocks were significantly

different (P97= 0.0001, P98= 0.0001, Figure 5). During the two years, the highestpredatory mite densities were observed in the blocks located near V2 (Pl, P2)(0.85 Ph lear1) and VI (P7) (0.8 Ph leaC1). Block P13 located near the woodymargin showed medium densities, while those located in the middle of the plotshowed low densities (0.2 Ph leaf-1). The coefficients of variation in 1997 and 1998

were 77% and 75%, respectively. Temporal variations in the numbers of phytoseiidmites were the lowest in blocks located close to the VI and V2 neighbouring grapefields, and near the woody margin and the highest in blocks located close to the

hedges.There was no correlation between the number of phytoseiid mites trapped and

collected inside the plot for the two years (r = 0.011,P = 0.70) (Figure7). Duringboth years, blocks where the number of phytoseiid mites trapped was the highestwere not the ones where the phytoseiid mite populations and their increase inside the

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plot were the largest. Similarly, the blocks where the greatest population increaseoccurred were not the ones where the number of predatory mites was the highestinitially in 1996 (zone 1) (Figure 7).

Kampimodromus aberrans distribution in AprilThe distribution of K. aberrans on leaves and in the different blocks (for 1996, 1997

and 1998) did not follow a random spatial pattern (Poisson statistical distribution)(P < 0.005) but a negative binomial statistical distribution (P > 0.995) (Table 3). ln1996 and 1998, pre-sampling was undertaken when the vines were of the same

phenological stage. ln 1997, the vegetation was further developed. Therefore, itwould only be correct to compare the results for 1996 and 1998. Variation in these

aggregation parameters indicated that the spatial pattern was less aggregated in 1998than in 1996. For each year, the number of phytoseüd mites present in the 56 blockswas different (P = 0.000). Blocks located near the V2 neighbouring grape field hadthe largest density of phytoseiid mites per leaf.

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204

V2

*

0.5

.~

*

0.44'

*

96 97 98 96 97 98 96 97 98 96 97 98 96 97 980

zone 1 zone 2 zone 3 zone 4 zone 5c::=:::::J Samplings

"* - L::..Trappings*: standard error of sampJings

Figure 7. Average number of phytoseiid mites sampled per leaf and per week in five zones in the plotand number of phytoseiid mites captured per trap and per week in these five areas during three years.

Table 3. Common K and parameters of aggregation of Taylor's power law

Discussion

Dispersal of phytoseiid mites in the experimental plot

This two-year study confinns the most important results obtained in 1996 (Tixieret al., 1998) on aerial and soil dispersaI of phytoseiid mites, in particular of K.aberrans:

zone5 zone4

zone1

zone2 zone3

b.

woodymargin

3

"C

2.5 :J-a.m2

01/) a.Q)-'Ë -...

1.5 :g Q)

'(6a.

1/)0-

1 >-.!::a.-0...Q).cE:JZ

Standard DevelopementTaylor' s power law deviation stages of vine plants

Common K equation r2 of the slope (Eichhorn and Lorenz)

1996 1.55 Y= 1,36x + 0,35 0.94 0.0017 9-121997 0.25 Y= 1,79x + 0,60 0.95 0.0013 16-17-181998 90 y = 1,13x + 0,08 0.82 0.0053 9-12

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205

1. Aerial dispersaI was found to be the most important way of dispersaI into the plotfor phytoseiid mites trapped in this study.

2. K. aberrans was the main phytoseiid mite species captured, both in aeriaI trapsand observed in the plots studied (uncultivated area and grape fields). However,during the two-year study, K. aberrans was hardly collected in soil traps. As K.aberrans mainly inhabits trees (Moraes et al., 1986; Tixier et al., manuscript),aerial dispersaI could be easier via air currents. lndeed, the species trapped in thedrugget strips are soil or herbaceous stratum species (Moraes et al., 1986).Moreover, ambulatory dispersaI requires more energy than this species couldprobably supply to allow movement between the field margins and the experi-mental plot. Larger and/or more mobile species would be able to adopt ambulatorydispersaI (Coop and Croft, 1995). It is assumed that K. aberrans could not surviveor disperse by ambulatory means when it reaches the soil after aerial dispersai.

3. Aerial and ambulatory dispersai did not seem to be sex and development-stagedependent. The very low number of phytoseiid mites trapped during that yearcould explain the minor variations observed in 1997. However, this result,observed over three years, seemed to contrast with most other results to date,which indicate that gravid females disperse the most (Sabelis and Dicke, 1985).These results point at passive air dispersaI of K. aberrans, the trembling of leavesdue to wind making it airbome, independent of age or sex.

4. Woody margins affect the number and the diversity of phytoseiid mites capturedboth in aeriaI andsoil traps, as the largest number of phytoseiid mites was trappedclose to this area. As K. aberrans is the main species present in the environment,it is also the species mainly trapped in funnels. As T.phialatus was more abundantin the surrounding vegetation and in the experimental vine plot in 1997 than in1996, it was also more captured in the soil traps in 1997 than in 1996.

Some results however differ from those obtained in 1996 and the new experimentaltools, used during 1997 and 1998, bring new and complementary elements:

1. The directional and non-directionaI soil traps show the same efficiency. lncontrast, a larger number of phytoseiid mites were trapped in direction al aerialtraps than in non-directional ones. These traps seem to be more successful forintercepting phytoseiid mites that disperse by air.

2. Soil directional traps did not show an oriented ambulatory dispersai (into or awayfrom the experimental vine plot). For aeriaI dispersaI, phytoseiid mite numberstrapped in the four directions are not significantly different. However, thenumbers of phytoseiid mites captured in the NW and NE funnels are twice as highas those captured in SE and SW traps. Furthermore, a higWy significant correla-tion exists in 1998 between the numbers of phytoseiid mites trapped in the NWfunnels and those trapped in the non-directional ones. Therefore, phytoseiid mitestrapped in funnels during the experimentation seem mainly to originate from theNW direction.

..U. -- .

206

Unlike data for 1996, no correlation was found between the DI of the NW wind

and the number of phytoseiid mites trapped each week in non-directional funnelsin the entire plot during 1997 and 1998. This could be due to the lower NW windintensities in 1997 and 1998 compared with those for 1996. Furthermore, themean richness of the woody margin was low in 1997.

3. A significant correlation between phytoseiid mites trapped and NW DI wasobserved in 1998 when only phytoseiid mites trapped near the woody marginwere considered. Furthermore, captures were significantly higher in directional,non-directional and prime funnels located nearby the woody margin than in thoselocated in other parts of the plot. The influence of the woody margin seemed to bewind dependent and only important on the edge of the experimental plot locatednear this area. This short distance effect could be due to trembling of leavesinduced by air currents, causing phytoseiid mites to faH near the woody margin,and to the orientation of the woody margin (not directly crossed by the prevailingwinds). Fauvel and Cotton (1981) also observed this short distance effect on aerial

trappings of phytoseiid mites in an apple orchard. This short distance effect couldaIso be linked to the low dispersaI ability of K. aberrans, the main speciestrapped.

Do phytoseiid mites trapped elsewhere in the plot also corne from the woodymargin? The fact that variation in phytoseiid mites trapped in the experimentaIplot and V2 is not significantly different might show that the phytoseiid mitestrapped in these two plots could originate from the same areas. However, thecatches of K. aberrans are lower in V2 than in the experimental plot. Theremoteness of the woody margin and the low dispersaI ability of K. aberranscould be involved.

4. Some results seem to show the effect of other areas, but it is impossible tocharacterize these:

(i) Typhlodromus pyri was essentially captured in the SE direction al trap locatedat site 4. This seemed to be associated with the large number of this speciesfound on Ulmus sp. in hedge no. 2 at 5-10 m from the trap. This result isconsistent with previous data on low dispersaI capacity of T. pyri (Dunleyand Croft, 1990, 1992).

(ii) Some species of phytoseiid mites trapped in funnels (Euseius finlandicus(Oudemans), Neoseiulus reductus (Wainstein), Neoseiulus barkeri Hughes,Typhlosciulus simplex (Berlese), Galendromus longipilus (Nesbitt), Typhlo-dromus (Anthoseius) rhenanoides (Athias-Henriot), Neoseiulus bicaudus(Wainstein) and Proprioseiopsis messor (Wainstein» were not found in thesurrounding vegetation. These species could have moved in from otherreservoir areas. Some phytoseiid mite species are able to disperse over longdistances (Hoy et al., 1984; 1985).

(iii) Although the number of phytoseiid mites trapped in 1997 was lower than in1998, numbers captured in some traps were higher in 1997 than in 1998. ln

- - H- H

207

the same way, the number of phytoseiid mites trapped in funnels located nearV2 were high in 1996, despite the larger richness of the woody margin in1998. These observations could be due to the involvement of other reservoir

areas but there is no proof for this assumption. Other factors, such astopography or/and wind speed could also be involved.

Phytoseiid mite populations inside the experimental plot

Kampimodromus aberrans was the main species collected in the experimental vineplot over the two years. The diversity of phytoseiid mites sampled in the vines didnot change while that of phytoseiid mites in the traps increased. This trend contrastswith other results that show the influence of field margins on the diversity of predatorspecies found along the edges of the crop fields near these areas with wild vegetation(Dennis and Fry, 1992; Kiss et al., 1997). Species other than K. aberrans did notseem to be able to develop into the vine plot. Several factors linked to the bettercompetition properties of this species could be considered. The species is one of thesmallest among the Phytoseiidae and can probably develop well on vine cultivarswith very hairy leaves (Tixier et al., 1999; Duso and Pasqualetto, 1993). Kampimo-dromus aberrans is a generalist predator, and can survive better in the absence ofprey than other species (McMurtry and Croft, 1997). This species probably developsbetter on food and especially on pollen on vine 1eaves or on the prey Eotetranychuscarpini (Oudemans) (Duso et al., 1991), the most important phytophagous mite onvine in South of France. However, the occurrence of K. aberrans cannot only beexplained by the presence of E. carpini, as the density of the latter was very low in1997 and 1998. It could also be assumed that K. aberrans is less susceptible to cropmanagement (in particular to chemical treatments), than the other phytoseiid mitespecies trapped.

K. aberrans populations showed a patchy distribution at springtime as was alsonoted in previous studies (Malison et al., 1995). Aggregation dec1ined between 1996and 1998. The influence of the colonization phenomenon on this decrease could onceagain be questioned. Slone and Croft (1998) also suggested that the dispersaI ofAmblyseius andersoni Chant from an uncultivated surrounding area into appleorchards could explain a decline in the aggregated distribution of this species.However, no correlation between the number of phytoseiid mites in 1996 and theirincrease was observed. Migration could explain why phytoseiid mites' densitiesincrease in blocks where densities were low in 1996. Thus, other factors could be

involved, such as climatic condition and the use of pesticides harmless for predatorymites (Kreiter et al., 1998). Although the numbers of phytoseiid mites sampled in theexperimental vine crop increased between 1996 and 1997, and between 1997 and1998, this augmentation is lower than the putative augmentation due to migrants.Moreover, in blocks where the number of phytoseiid mites trapped was high, bothphytoseiid mites' numbers on vine leaf and their increase were low. This could,

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208

indeed, highlight the low effectiveness of the colonization phenomenon as wasobserved for inoculative introductions of K. aberrans. Several hypotheses could beformulated to explain a bad settlement:

1. It is possible that not all dispersing mites settled in the plot. Nymphs, males andnon-gravid female mites cannot constitute a colony alone, i.e. they will disappearwithout increasing the settled population if they do not encounter other individ-

uals. Only 49% of the migrant mites were females, and able (if gravid) to develop.This could limit population increases that could occur via colonization by two-fold. Moreover, the ca1culation adopted in this study to assess the number ofmigrant phytoseiid mites that could settle on a vine leaf did not take into accountthe mortality rates of migrant mites during dispersaI.

2. Dispersing mites did not land in an uncolonized environment, as K. aberranspopulations were already present. There could thus be competition betweendispersing and already settled mites (Roderick, 1992; Wade and McCauley, 1988;Hopper et al., 1993).

3. Finally, dispersing mites change environments and have to adapt to new livingconditions to be able to settle successfully: new host plant, new microc1imateconditions, different or new crop-related constraints, e.g. pest control treatments(Wade and McCauley, 1988; Roderick, 1992, 1996; Maurer and Holt, 1996).Some authors emphasize that populations introduced into an inundative biologicalcontrol framework often remain extremely rare for long periods (Force, 1967;Hopper et al., 1993). Furthermore, the reproductive success rates of migrant mitesas compared to those of mites already settled in the environment were notdetermined.

Conclusion

The present results should help to c1arify the dispersaI of these mites from the areasurrounding the plot (aerial dispersal of K. aberrans, which is unlikely to bedependent on sex, development stage and the influence of the woody margin, interms of numbers and phytoseiid species present) and to determine other parameterssuch as wind influence and the sphere of influence of the woody margin. However,some questions concerning the possible other origins of migrants and particularlyof K. aberrans still remain unanswered. Our study also highlighted the aggregateddistribution patterns of this species in the vine plot.

It seems difficult for migrant mites to settle once they have reached the plot.Indeed, putative increase due to immigration is more important than the effectiveaugmentation of phytoseiid mites observed into the plot. Furthermore, areas wherehigh numbers of phytoseiid mites are trapped do not have great densities ofphytoseiid mites per leaf. Sampling tools used do not permit us to differentiateK. aberrans individuals relative to their origins or to determine the percentage of

209

dispersing mites that could actually settle in the plot. It might be possible to obtainanswers to these questions through genetic labelling of mite populations in the plot,surroundings and traps.

Aknowledgments

We are very grateful to C. Rouquette, the farmer involved in this study, to MarcWeber (Service de la Protection des Végétaux du Languedoc Roussillon) for activehelp; and to Marc Baillod (SFRAC Changins, Suisse) and Gilles Caraux (ENSAM,Chaire de statistiques) for statistical advice. We also thank Sylvie Agret, FabienneCanaud, Filomena Camboa and Latifa Allam (ENSAM, Chaire de Zoologie) fortechnical assistance, and Ai-Linh Tran for translation help. This study was financedin part by the French Research Ministry (grant number 95 G 0129).

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