Evaluation of wheat ear insects in large scale field in central Germany

doi:10.4236/as.2010.12010

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Vol.1, No.2, 68-75 (2010)

doi:10.4236/as.2010.12010

Agricultural Sciences

Evaluation of wheat ear insects in large scale field in

central Germany

——Evaluation o f wh ea t ea r insects in winter wheat scale field

Nawal Gaafar*, Christa Volkmar

Institute of Agricultural & Nutritional Sciences, Martin-Luther-University Halle-Wittenberg, Halle, Wittenberg, Germany

*Corresponding au t h o r s: nawal_gaafar@yahoo.com

Received 16 June 2010; revised 3 July 2010; accepted 15 July 2010.

ABSTRACT

Wheat ear insects in large scale winter wheat

field in Salzmünde (Saxsony-Anhalt) central

Germany were evaluated. The present study

aimed at studying the abundance of wheat

blossom midges WBM, Sitodoplosis mosellana

(Géhin), Contarinia tritici (Kirby) and thrips, Li-

mothrips cerealium (Haliday) and Haplothrips

tritici (Kurdjumov). Infestation in winter wheat

during the growing seasons 2007, 2008 and

2009 was evaluated. Three methods were used

to determine population densities and damage

of wheat midges and thrips; pheromone traps,

inspection of ear insects and water traps. A

strong corre l atio n be tween midge’s catches and

weather conditions was obtained in field ob-

servations. A positive correlation between phe-

romone catches and ear infestation levels was

recorded; it was higher in 2008 than in 2009. On

the other hand, in 2007 there was no synchro-

nization; S. mosellana hibernated emerged too

late to coincide with the susceptible wheat

growth stages. The chemical treatment applied

at 2008 for highly infestation; there were sig-

nificant differences in thrips and midge num-

bers between treated and untreated. Thrips and

midge numbers were lo wer in the treated than in

control. The high midge populations in water

traps w ere recorded at growth stages 77-79 and

83 and the low populations were recorded at GS

75 and 75-77. This gives a reliable base for de-

cision making to midges control.

Keywords: Winter Wheat; Thrips; Wheat Midges;

Population Densities

1. INTRODUCTION

Wheat (Triticum aestivum L.) is one of the most impor-

tant cereal grain crops in the world and it is cultivated

over a wide range of climatic conditions [1]. Yields can

be improved if producers take ti me to inspect their fields

and control the insect pests during the growing season

[2]. Important pests that may reduce wheat yields are

wheat blossom midges and thrips. The orange wheat

blossom midge Sitodiplosis mosellana (Géhin) and the

yellow wheat blossom midge Contarinia tritici (Kirby)

(Diptera: Cecidomyiidae), have a very patchy spatial

distribution and infestations vary from year to year, be-

cause they have the capacity for extended diapauses and

only a portion of the larvae in the soil develop and pu-

pate each spring, depending on climatic conditions [3]. S.

mosellana and C. tritici cause direct damage by the lar-

vae feeding on developing grain, and secondary fungal

attack by Fusarium graminearium and Septoria nodorum

may occur [3]. During the past decade, infestations of

wheat midge seriously reduced the yield and quality of

wheat in the major wheat-producing provinces in Ger-

many [4,5], UK [6] Canada [7] and Finland [8]. The

highest wheat midge populations can be found in fields

where wheat was grown in previous years and in fields

that are next to them.

Pheromone traps gave a reliable indication of peak

midge emergence, onset of flight and abundance of

midges throughout the season. The wheat plants are

susceptible growth stages (GS) from the flag leaf sheath

opening up to the flowering half complete (GS 47-65),

[10]. Also weather conditions have to be favorable for

the insect to lay eggs within the florets [6,11,12]. The

critical risk factors are the proportion of diapausing

midge larvae that might develop in any given season, the

coincidence between emergence of adult midges and

susceptible stages [13] and the suitability of the weather

during adult midge activity coinciding with susceptible

N. Gaafar et al. / Agricultural Sciences 1 (2010) 68-75

growth stages for flight and oviposition [14,15]. A strong

correlation between maximum trap catches and crop

infestation levels has resulted in many studies [3,5].

White water traps are often used to sample migrating

and flying insects. Larvae are caught in their migrating

way from wheat ears to soil at the end of the season.

Insects are attracted visually by colour of the traps and

are then captured in the water. Studies have demon-

strated the preferences of a certain cultivar of insect to a

particular coloured trap, as well as weather condition,

especially rainfall [16].

Thrips infesting cereals are usually found behind the

sheath of the flag leaf, feeding on the stem; however,

leaves, and heads also were attacked [17]. Adults and

nymphs can cause damage and, if present in large num-

bers, may cause the tissue on which they are feeding, to

turn into a silver coloration. The stage of growth at the

infestation time seems to determine the extent of yield

loss [18]. The most important thrips species in the world,

damaging wheat and barley heads are Limothrips cere-

alium (Haliday) and Haplothrips tritici (Kurdjumov).

They are species of wide ecological plasticity, and able

to build up populations with notable individual numbers

in cooler zones of Europe [19-21].

The objective was to determine the abundance of

WBM and thrips infestation in large scale wheat fields

through three monitoring method s to establish econo mic

thresholds. To address the growers need for monitoring

systems against wheat ear insects to prepare an expert

system should help wheat farmers in dry region in cen-

tral Germany.

2. MATERIAL AND METHODS

2.1. Winter Wheat Fields

The winter wheat varieties Tommi, Manager and Im-

pression were chosen to cultivate in 2007, 2008 and

2009, respectively. These varieties are commonly culti-

vated and with high quality properties [22], they were

sown in sandy loam soil in the previous October every

year in Salzmünde (Latitude 51°4’N, Longitude 11°55’E)

central Germany. The crop rotation in the experiment

sites was winter wheat after winter wheat and the plots

size was 7.5 hect ar es .

2.1.1. Monitoring WBM Adults Using Pheromone

Traps

Pheromone monitoring kits were obtained from AgriSen-

seTM (UK). Each trap consisted of a pheromone lure;

Dispenser: Septa; Material: Natural rubber; Packaging:

Individually Sachet Packed; Sachet Material: Foil Lined

Laminate [23]. Two traps were set up when winter wheat

was at growth stages 45 (flag leaf sheath swollen) and

were taken off at GS 77 (late milky) in the stud ied years.

The traps were placed at the same height as the wheat

ears at a distance of 20 m from field borders and sepa-

rated by 10 m [24,25]. Trap catches were recorded twice

a week. Trapped WBM adults and debris were removed

from the traps; and depending on the density of the

caught insects the cards were changed.

2.1.2. Inspection of Thrips and Midges in Wheat

Ears

Ten ears were collected in method of liner observation

[26] at flowering stag e (GS 65) and milky stage (GS 73)

[27] when the most larvae are already practically grown

up, but still not left the spikes, they transpo rted in sealed

bags and stored at –20℃. By mean of a binocular the

numbers of larvae per ear was counted and classified as

S. mosellana or C. tritici and thrips Limothrips cereal-

ium (Haliday) and Haplothrips tritici (Kurdjumov). In

addition, kernel damage was registered as reformatted,

cherviled or cracked.

2.1.3. Surveying WBM Larv ae Using Water Traps

The migrated midge’s larvae from wheat ear were

monitored using white water traps as expectation factor

for the following years. The traps consisted of white

plastic dishes; 12.5 cm diameter and 6.5 cm deep. Two

traps were placed on the ground among wheat plants at

milky stage (GS 73) and were taken off at gold dough

(GS 89), and were partly filled with water (200 ml) plus

1ml of detergent (Fit). Traps were examined twice a

week and larvae were counted using a magnifying glass.

2.2. Chemical Control

The wheat midge’s management was conducted by using

Karate (Lambda cyhalothrin), a pyrethroid insecticide, at

a rate of 0.75l/ ha [28]; insecticide application was sp-

rayed on 3rd June 2008 (GS 59), and only a 4/5 of the

wheat field was sprayed. Insect populations were sam-

pled before the insecticide application, thereafter, 3, 7,

10, 15 and 20 days afte r treat ment.

2.3. Statistical Analysis

Numbers of captured insects and ear insect’s evaluation

were analyzed by linear model (a repeated measures

analysis of variance (Statistix 9) [29]. Tukey test was

used to compare means of varieties. Significances were

noted at P < 0.05 for all trials. Thrips and midge num-

bers per ear were correlated with infested kernels by

using the Pearson’s correlation coefficient.

3. RESULTS

3.1. Monitoring S. Mosellana Adult s Using

Pheromone Traps

Populations of S. mosellana adults started slowly till

N. Gaafar et al. / Agricultural Sciences 1 (2010) 68-75

milky stage and the first peak was recorded at GS 73

(1496 midges/trap) in 2007 (Figure 1). In 2008 large

variations in numbers of midges in the pheromone traps

and in time of peak catches were found; the highest

number of males was 173 midges/trap recorded in GS

(55-59) (Figure 2). There was one peak in 2009 (32

midges/trap) at GS 59-61 (Figure 3).

The lowest number of midges were1, 13 and 2.5

midges/trap in 2007, 2008, and 2009, respectively (Fig-

ures 1-3). Coincidence of adult activity and susceptible

growth stages was more obvious in 2008 than in 2009 as

shown in o val shape in Figures 2 and 3. The susceptible

stages of wheat coincided with suitability for flight and

oviposition. There was also a strong correlation between

peak pheromone trap catches and weather conditions,

rainfall and temperature (r = +0.892 and r = +0.742) in

2008 & 2009, respectively. On the other hand, there was

no correlation (r = +0.38) in 2007, possibly because the

midge activity started later than the susceptible stage.

Figure 1. Mean Sitodiplosis mosellana adults caught in phero-

mone one trapsamd their realtion with temperature and rainfall

in Salzmünde 2007.

Figure 2. Mean ± SE of Sitodiplosis mosellana adults catches

in pheromone traps and their relation with temperature and

rainfall in 2008. Oval refers to coincidence of adult activity

and susceptible growth stages. Different letters indicate sig-

nificant differences.

Figure 3. Mean ± SE of Sitodiplosis mosellana adults catches

in pheromone traps and their relation with temperature and

rainfall in 2009. Oval refere to coincidence of adult activity

and susceptible growth stages. Different letters indicate sig-

nificant differences.

3.2. Inspection of Thrips and Midges in

Wheat Ears

3.2.1. 2007

3.2.1.1. To t al Thrip s

In the most impor tant growth stage GS 65&73 ther e was

significant difference in thrips populations (P = 0.0047)

(P = 0.0484) and (P = 0.0451) in thrips adults, larvae

and total thrips, respectively. The thrips adults were 0.7

and 1.5/ear in the same way. The corresponding records

in thrips larvae were 1.3 and 2.0/ear. The total thrips/ear

were 2.1 and 3.5 , respectively (Figure 4).

3.2.1.2. Wheat Midges

There was a significant difference (P = 0.0357) in total

midges between both growth stages (flowering and

milky). Total midges (S. mosellana & C. tritici) were 0.2

and 1.8 larvae/ ear, respectively (Figure 4).

3.2.1.3. Infested Kernels by Thrips and Midges

There was significant difference (P= 0.0 391) in infested

kernels (deformated, cherviled or cracked kernels) be-

tween the growth stages 65 and 73, these values were 0.2

and 1.8 infested kernels/ear, respectively (Figure 4).

3.2.2. 2008

3.2.2.1. To t al Thrip s

Thrips population was 10.0 thrips/ear before the insecti-

cide application, while after 3 days post treatment; they

were 8.8 and 26.0 thrips/ear in the treated and control,

respectively.

In flowering stage (GS 65): Significant differences

were found (P = 0.0083) in the number of total thrips

between treated and control. On the 7th day, thrips num-

ber in control plants were higher than in treated 26.4 and

6.8/ear, respectively; the corresponding numbers on the

N. Gaafar et al. / Agricultural Sciences 1 (2010) 68-75

Thrips adults

Thrips larvae

Total thrips

Total midges

Infested kernels

Thripse & midge larave/ ear

Flowering stage

Milky stage

Figure 4. Mean ± SE of thrips (adults, larvae & total) and total

midges in two growth stages in 2007. Different letters indicate

significant differences.

10th day were 27.6 and 6. 8 t h ri p s/ ear (Figure 5(a)).

In milky stage (GS 73): There was significantly dif-

ferent (P = 0.0041) in thrips number between treated and

untreated. Thrips numbers were lower in the treated than

control. They were 6.8 and 31.6 thrips/ear, respectively

after 15 days post treatment; the corresponding records

on 20th day were 18.4 and 34.4 thrips/ear (Figure 5(a)).

3.2.2.2. Wheat Midges

There was no wheat midge larvae recorded before treat-

ment (Figure 5(b)), while 3 days after treatment; they

were 0.0 and 4.4 midge larvae/ear in treated and control

plots, respectively.

In flowering stage (GS 65): There was no significant

difference (P = 0.0672) in the number of midge larvae

(S. mosellana & C. tritici) between treated and untreated

plots. On the 7th day, midge larvae numbers in treated

were lower than in control 0.8 and 2.0/ ear, correspond-

ingly; the equivalent records on the 10th day were 2.4

and 3.6 thrips/ear (Figure 5(b)).

In milky stage (GS 73): There was significantly dif-

ferent (P = 0.0245) in wheat midge larvae between

treated and untreated. Midge larvae n u mbers were highe r

in control than in treated plants. They were 4.0 and 1.2

larvae/ear, respectively after 15 days post treatment; the

corresponding numbers on 20th day were 4.8 and 2.4

larvae/ear; this mean that treated had an half population

which recorded in control (Figure 5(b)).

3.2.2.3. Correlation between Thrips, Midges and

Infested Kernels

There was significant difference (P = 0.0485) in infested

kernels by thrips and wheat midge. Treated wheat had

lower infested kernels than control plants. There was a

positive correlation coefficient between wheat midge

larvae and infested kernels (r = +0.56 and +0.76) in GS

65 and 73 stages, respectively; while there was no

Figure 5. Mean ± SE of total thrips (a) andl midge larvae (b) in

treated and untreated winter wheat during season 2008. Dif-

ferent letters indicate significant differences.

significantly correlation between total thrips and infested

kernels (r = +0.121 and +0.175) in both stages (Figures

5 (a) and (b)).

3.2.3. 2009

3.2.3.1. To t al Thrip s

There was a significant difference in thrips populations

in GS 65 and 73; the significant value was (P = 0.0030)

in thrips adults and (P = 0.0484) in total thrips. While

there was no significant difference (P = 0.891) in thrips

larvae between both stages. The thrips adult were 0.3 in

GS 65 and 1.1/ear in 73, respectively. The corresponding

records in thrips larvae were 3.2 and 3.4/ear. The total

thrips were 3.5 and 4.5/ear in GS 65 and 73, respectively

(Figure 6).

3.2.3.2. Wheat Midges

There was a significant different (P = 0.0263) in total

midges populations (S. mosellana & C. tritici) between

GS 65 and 73. The total midges were 0.3 and 0.8 larvae/

ear in GS 65 and 73, respectively (Figure 6).

3.2.3.3. Infested Kernels by Thrips and Midges

There was significant difference (P = 0.0169) in infested

kernels between growth stages 65 and 73, these values

N. Gaafar et al. / Agricultural Sciences 1 (2010) 68-75

were 0.3 and 0.8 infested kernels/ear, respectively (Fig-

ure 6).

3.3. Monitoring WBM Larvae Using Water

Traps

3.3.1. 2007

Yellow wheat midge larvae were only recorded on GS

75 (1 larva/trap). S. mosellana larvae were significantly

higher (P = 0.039) on growth stage 85 than other growth

stages. The population densities of S. mosellana were 6,

4 and 13 midge larvae/trap at growth stages 75, 83 and

85, respectively. The last WBM larvae were caught on

growth stage 87-89 (1 larva/trap) (Figure 7).

3.3.2. 2008

Populations of wheat midge larvae (S. mosellana & C.

tritici) were significantly higher (P = 0.00 23) on contro l

than treated. Population density was significantly lower

(P = 0.0353) on the first two stages (75 and 75-77 (18th

& 22nd June)) than other two growth stages (77-79 and

83 (26th & 30th June)). The results indicated that S. mose-

Thrips adults

Thrips larvae

Total thrips

Total midges

Infested kernels

Thripse & midge larave/ ear

Flowering stage

Milky stage

Figure 6. Mean ± SE of thrips (adults, larvae & total) and total

midges in two growth stages in 2009. Different letters indicate

significant differences.

Figure 7. Mean ± SE of red-orange and yellow midge larvae

by white water traps and their relation to temperature and rain-

fall during winter wheat season 2007. Different letters indicate

significant differences.

llana & C. tritici populations could be divided into two

groups; the high populations were recorded on 26th &

30th June and the low populations were recorded on 18th

& 22nd June. Mean of low populations of S. mo sellan a &

C. tritici 1 and 2 midge larvae/trap in treated and un-

treated plants. Mean of high populations of both wheat

midge larvae were 4 and 12 larvae/trap in treated and

control plants, respectively (Figure 8).

3.3.3. 2009

Yellow wheat midge was only recorded on GS 77 & 87

(1 & 2 larvae/trap, respectively). Population density of

orange wheat midge was significantly higher (P = 0.028)

on growth stages 83 and 89 than the others. S. mosellana

numbers were 4 larvae/trap in both stages. The last

WBM larvae were caught on growth stage 89 (Figure 9).

4. DISCUSSION

Large variations in adult midge’s numbers caught in the

pheromone traps and in timing of peak catches were found

between years (ca. fivefold) in farm scale studies under-

Figure 8. Mean ± SE of red-orange and yellow midge larvae

catches in treated and untreated plots by white water traps and

their relation to temperature and rainfall during winter wheat

season 2008. Different letters indicate significant differences.

Figure 9. Mean ± SE of red-orange and yellow midge larvae

by white water traps and their relation to temperature and rain-

fall during winter wheat season 2009. Different letters indicate

significant differences.

N. Gaafar et al. / Agricultural Sciences 1 (2010) 68-75

taken in Salzmünde. This suggests that it is more useful

for farmers to put traps in neighboring fields which were

cultivated wheat in the year. There was no coincidence

in 2007 between wheat midge activity and susceptible

stages of wheat. Therefore wheat plants had escaped

from midge’s infestation, because the hibernated midges

emerged later due to the warm weather in spring; it was

ca. > 10℃ as stated by Oakley et al. [12]. In general, in

2008 and 2009 the peak of midge flight synchronized

with the susceptible stage of the crop, it was more ade-

quately in 2008 than in 2009, da mage leve ls tended to b e

higher in 2008 than in 2009, because there was correla-

tion between total numbers of males caught during the

susceptible period and infestation as confirmed by Ellis

et al. [30]. Pheromone traps were very valuable in indi-

cating midge’s emergence and for decision making. This

is a significant benefit with other systems for monitor-

ing wheat midges as mentioned by Gaafar and Volkmar

[5,32]

The peaks of pheromone trap catch for the whole sea-

son occurred when the wheat was past the susceptible

growth stage ex. 2007, but for setting the economic

threshold the peak catch during the susceptible period is

more relevant. There was also a strong correlation be-

tween peak pheromone trap catches and temperature and

high rainfall. These results agree with those obtained by

Oakley et al. [6], Bruce et al. [3], Volkmar et al. [31],

Gaafar and Volkmar [32], who studied pheromone traps

in different sites. Routine use of this monitoring method

should eliminate most unnecessary applications of

insecticides, and help assure that the benefits of insecti-

cide applications exceed the cost.

Levels of midge infestation were higher in 2008 than

in 2009 in both methods; evaluation ear insects and

white water traps. This meant that there was a good rela-

tion between pheromone trap catches and midge’s infes-

tation. Therefore, chemical control was applied in 2008

and did not apply in 2007 or 2009 in case of low levels

of midge infestation in 2007 and 2009. In terms of the

impact of meteorological conditions, although soil mois-

ture levels were favourable, weather conditions (tem-

perature > 10℃ and 1mm rainfall) in the 2007 season

were warmer than usual, which delayed the emergence

of wheat midges in wheat ears as well as in white water

traps. As a consequence, although the pheromone traps

indicated that WBM had emerged, the time of arrival of

many of the egg-laying females in th e crop was not syn-

chronized with the susceptible growth stage. This ex-

plains why there was a poorer correlation between

pheromone trap catches and subsequent infestation in

2007 or 2009 than in 2008 . Similar resu lts were r eco rded

by Ellis et al. [30], who reported that difference in wea-

ther condition between 2004 and 2005 had direct affect ed

on wheat midge’s populations.

Although levels of midge infestation were generally

higher in 2008 than in 2007 and 2009, there is evidence

to suggest that the proposed thresholds to the control

decision are a good basis with which to predict the risk

of midge attack. If cumulative trap catches exceed 30

midges/trap/day after heading (GS 59-65), then this in-

dicates an economic risk to the wheat crop and an in-

secticide application may be necessary. The correspond-

ing record in water trap was more than 10 midge larvae/

trap (at late milky stage). As well as for ear evaluation,

three or four maggots per kernel will destroy the kernels

in that ear. Similar results were found by Olfert et al.

[33,10], Oakley et al. [6] and Ellis et al. [30] in Canada

and UK, they confirmed th at if one or more adult midges

are observed for every 4-5 heads or 3-4 midge larvae/ear;

insecticide treatment is recommended. An economic

thresho ld for L. cerealium was 25 thrips/ear, while Lars-

son [18] reported that this value was 35 thrips/tiller in

his studies on winter barley. Pheromone traps, ear

inspection and midges captured indicate the class of risk,

while the threshold indicates the need for control.

5. CONCLUSIONS

The sequential sampling plans (pheromone traps, ear

insect’s evaluation and water traps) described in this

paper should provide a method for more efficient midges

monitoring. If pheromone trap catches indicate that a

significant number of adults and suitable weather (tem-

perature is > 16℃ and heavy rain is ca. 8 mm) during

the susceptible stage of the wheat crop, need to be

closely monitored at growth stages 47-65 [34]. Ear in-

sects evaluation should be conducted in the milky stage

(GS 73-75, when most larvae are already practically

grown up, but have still not left the spike), while water

traps should be also monitored carefully after the heavy

rain, especially at late milky stage. A strong correlation

between midge’s catches and weather conditions was

obtained in field observations; this gives a reliable base

for decision making to midges control.

6. ACKNOWLEDGEMENTS

The authors want to thank to Dr. H. Hussein (Academy of Science,

Czech Republic) for his beneficial notes on this manuscript, and Dr. N .

El-Wakeil for his help in statistical analysis.

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