HYMENOPTEROUS PARASITOIDS ASSOCIATED WITH CEREAL APHIDS IN WHEAT FIELDS AT ASSIUT الطفيليات من رتبة غشائية الأجنحة المصاحبة لحشرات من النجيليات فى حقول القمح فى أسيوط

HYMENOPTEROUS PARASITOIDS ASSOCIATED WITH CEREAL APHIDS IN WHEAT FIELDS AT ASSIUT

Alaa El-Deen A. A. Salem

Plant Protection Research Institute, Giza, Egypt

INTRODUCTION:

In Egypt, the major insect pests attacking wheat plants are the greenbug, Schizaphis graminum (Rondani), the bird cherry-oat aphid, Rhopalosiphum padi L. and to lesser extent the corn leaf aphid, R. maidis (Fitch.) and the English grain aphid, Sitobion avenae (Fab.) 
(El-Heneidy et al., 2001; Abdel-Samad and Gomaa, 2004; Mahmoud, 2005; Salem, 2007 and Salem and Mahmoud, 2012).

The damage caused by cereal aphids in both direct through feeding injury and indirect through the effects of honeydew in combination with fungi, which reduce rate photosynthesis and the available leaf area (Martin and Johnston, 1982; Ryan et al., 1987 and Royer et al., 1997).

The hymenopterous parasitoids attacking cereal aphids are considered to be of importance in natural control of their host populations. The reported parasitoid species attacking cereal aphids are Diaeretiella rapae (McIntosh), Praon spp., Aphidius spp. and Ephedrus spp. (Stary, 1976; Ibrahim, 1990; Elliot et al., 1995; Ali et al., 2001; Adly, 2002 and Salem, 2007).

The present study focused on survey and population densities of major cereal aphid species and their associated parasitoid species particularly D.rapae on wheat plants at Assiut Governorate.

MATERIALS AND METHODS:

The present study was carried out in Assiut province (Assiut district) located beside El-Azhar University at Assiut, during two successive seasons (2011 and 2012). An area of about 1/4 feddan was divided into 4 plots of equal size (approximately 250 m²) and cultivated with wheat seeds (Seds 1). Common cultural practices were applied and no chemical pesticides were used during the study and weeds were removed by hand.

1-Mummies collected from wheat plants:

Available mummified aphids were collected weekly from wheat plants (depending on aphid density). After collection, mummies transferred to the laboratory and put individually in small plastic vials (1.5 cm in diameter and 5.5 cm in high), then kept under the laboratory conditions. The mummies were inspected daily, and the emerging adult parasitoids were identified and counted. Aphid parasitoids emerged were identified according to Pike et al. (1997) and confirmed by aid Dr. Peter Stary, Academy of Science, Institute of Entomology, Republic of Czechoslovakia.

2-Sweep-net method:

                Sweep-net method was used to study the population of cercal aphids and the major parasitoid, D. rapae. Samples of cereal aphids and the associated parasitoid inhabiting wheat plants were taken regularly at weekly intervals from the beginning of January to the end of March. Twenty five net strokes were taken 4 times (100 net strokes) as the standard sample at weeks intervals. Each collected sample was empted carefully into labeled collected bag. Samples were transferred to the laboratory and the insects were killed by chloroform. The number of cereal aphids and parasitoid species were count and recorded.

                Dominance (D) and abundance (A) degrees of cereal aphids and parasitoid species were calculated according to Facylate, (1971).

D = t/T × 100

where:

 t =Total number of each species during the collecting period.

T = Total number of all species collected during the collecting period.

A = n/N × 100

where:

 n = Total number of samples in which each species appeared.

N = Total number of samples taken allover the season.

RESULTS AND DISCUSSIONS:

1-Survey and relative abundance of cereal aphid parasitoid species by using mummies collection method:

Figure (1) show the numbers of mummies collected from wheat plants and the percentage of emerged parasitoids during 2011 and 2012 seasons. In 2011 season, from about 21200 mummified aphids collected from wheat fields there were 18760 adult parasitoid emerged from mummies represented 88.49% of the total collected mummies. On the other side, data also showed that of 26800 mummies collected from wheat fields in 2012 season, there were 24390 adult parasitoid emerged from mummified aphids represented 91.01% of the total collected mummies. Also, Figure (1) show that the percentage of emerged adult parasitoid in 2012 season was higher than that recorded in 2011 season with an overall average of 89.69%.

In this approach, Gilstrap et al., (1984) reported that from 7.700 greenbug and 2.200 from corn leaf aphid mummies which collected from sorghum, adult parasitoid emerged were 5.635 of the collected greenbug mummies and 1.468 of the collected corn leaf aphid mummies represented 73.18% and 66.72%, respectively.

A-Primary parasitoid recorded:

Family: Aphidiidae; Aphidius colemani Vereck, A. matricariae Haliday, Diaeretiella rapae (McIntosh), Ephedrus plagiator (Ness), Praon necans Mackauer and Trioxys sp.

Fig. 1: Total cereal aphid mummies parasitoids emerged and non parasitoid emerged collected
from wheat plants, Assiut, 2011 and 2012 seasons

B-Secondary parasitoid recorded:

Family: Cynipidae, Alloxysta australiae (Ashmead)

Family: Megaspilidae, Dendrocerus carpenter (Curtis)

Family: Chalcididae, Chalcids sp.

Family: Encyrtidae, Aphidencyrtus sp.

                The same results were obtained by Ali et al., 2001; El-Heneidy et al., 2001; El-Heneidy et al., 2002; El-Hussieni et al., 2003; Sobhy et al., 2003; Abdel-Rahman, 2005 and Salem and Mahmoud, 2012.

C-Relative abundance of parasitoid species:

Data in Table (1) show the percentage of each primary and secondary parasitoid species emerged from mummified aphids collected from wheat fields during 2011 and 2012 seasons. Among the recorded cereal aphid primary parasitoids, the highest percentage (87.92%) was recorded for D. rapae followed by P. necans (4.04%), A. colemani (3.01%), E. plagiator (2.45%), A. matricariae (1.85%) and Trioxys sp. (0.74) during 2011 season. Highest percentages during 2012 season reached 64.28, 15.50, 7.08, 6.08, 4.78 and 2.27% for D. raae, P. necans, A. matricariae, E. plagiator, Trioxys sp. and A. colemani, respectively. The highest percentage (94.01%) of total parasitoid species among the surveyed parasitoids was recorded in 2011 season and it was relatively higher than that of season 2012 (65.56%). This results agrees with the findings of Elliot et al., (1995) and pike et al., (1997) reported that the parasitoid D. rapae was recorded in high numbers on cereal aphids than other species. Salem and Mahmoud (2012) reported that D. rapae seem to be the most important primary parasitoid species as biological control agents due to their highest value of dominance and abundance degrees followed by A. colemani and P. necans.

Table 1: Relative abundance of the primary and secondary parasitoid species emerged from cereal aphid mummies collected from wheat plants, Assiut, 2011 and 2012 seasons

Concerning the secondary parasitoid species, the highest percentage of hyperparasi-tism (65.87%) was recorded for D. carpenteri followed by A. australiae (20.80%), Chalcids sp. (7.64%) and Aphidencertus sp. (5.69%) during 2011 season. Meanwhile, in 2012 season the highest percentages of hyperparasitism were 61.15, 29.50, 7.41 and 1.94% for A. australiae, D. carpenteri, Chalcids sp. and Aphidencyrtus sp. respectively. On the other hand, the highest percentage (34.44%) of the total parasitoids was observed in 2012 season and it was relatively higher than that of 2011 season (5.99%). Al-Dobi et al., (1999) stated that the secondary parasitoid, Dendrocerus sp. was the most abundant species on cereal aphid primary parasitoids in wheat fields.

2-Population of cereal aphids and the associated parasitoids by using sweep-net method:

Wheat plants in Upper Egypt were susceptible to damaging infestations mainly by three cereal aphid species, S. graminum, R. padi and R. maids. Seasonal occurrence periods of different species were recorded as follows; from the beginning of January to the end of March for R. padi and S. graminum and from the 1^st wk of January to the 1^st wk of March for R. maids. The fourth and fifth species, S. avenae and, Metopolophium dirhodum (Walker) which occurs late in the season were found in very low numbers during course of this study. Available literatures in Egypt confirmed that most of the surveyed cereal aphid species were, R. maidis, R. padi, S. graminum and S. avenae as recorded by El-Hariry, 1979; Abou-Elhagag and Abdel-Hafez, 1998; El-Serafy, 1999; Ali and Abdel-Rahman, 2000; Ali et al., 2001; Abdel-Rahman, 2005; Mahmoud, 2005 and Salem, 2007. In addition, El-Fatih, 2000; Salem, 2007 and Salem and Mahmoud, 2012, they recorded M. dirhodum on wheat plants at Giza, Beni-Suef and Assiut regions.

A-Dominance and abundance degrees of cereal aphids and associated primary and secondary parasitoid species:

* Cereal aphid species:

                Data in Table (2) show that, R. padi and S.gramimim seems to be the most important economic cereal aphids as indicated by the highest value of dominance (88.91 and 90.73%) as well as abundance degrees (100%) in 2011 and 2012, respectively, followed by R.maidis which had low dominance (3.77 & 2.48%) and abundance (69.23 & 76.92%) during 2011 and 2012 season respectively. Similar results were reported by Ali and Abdel-Rahman (2000) reported that R. padi and S. graminum were the most important economic pests by the highest value of dominance and abundance degrees followed by R. maidis. Salem (2007) reported that S. graminum was the most abundant cereal aphids species followed by R. padi .The corn leaf aphid was less abundant.

*Primary parasitoid species:

Data in Table (2) show that D. rapae seems to be the most important primary parasitoid species as indicated by the highest values of dominance (48.43 & 47.15%) and abundance (100.00%) during 2011 and 2012 seasons, respectively. However, the high abundance degrees (100.00%) of the parasitoid A. colemani and P. necans indicate that these species could be of economic importance if the environmental conditions charged in their favor. Meanwhile, the primary parasitoid species which had low values of dominance and abundance were
A. matricaiae, E. plagiator and Trioxys sp. represented (12.12 & 19.99%), (11.90 & 9.23%) and (3.42 & 3.53%) for the dominance degrees and (61.54 & 69.23%), (46.15 & 69.23%) and (30.77 & 53.85%) for the abundance degrees during 2011 and 2012 seasons, respectively. Salem and Mohmoud (2012) reported that D. rapae was considered the most dominate and abundance primary parasitoid species and may play an important role in controlling the cereal aphids in wheat fields.

*Secondary parasitoid species:

Data in the same Table (2) show that highest percentage of the secondary parasitoid species (34.00%) was recorded to D. carpenteri in the first season, while it was (47.43%) to A. australiae during the second season. Lowest dominance degrees were recoded to Chalcids sp. (19.95 & 16.42%) and Aphidencyrtus sp. (13.24 & 1.99%) during 2011 and 2012 seasons, respectively. Salem (2007) reported that Alloxysta sp. was the most common and abundant species among the secondary parasitoid species collected.

Table 2: Dominance and abundance degrees of cereal aphids and their parasitoid species collected by sweep-net technique inhabiting wheat plants, Assiut, 2011 and 2012 seasons

B-Population of cereal aphids and the associated primary parasitoid, D. rapae:

Data in Table (3) show that, in 2011 season, S. graminum was appeared on the plants during the 1^st wk of January (6.8 aphids/25 net strokes). The greenbug showed its peak of activity during the 4^th wk of January (230.2 aphids/25 net strokes). In spit of the bird cherry-oat aphid was first observed in the field in the 1^st wk of January (1.2 aphids/25 net strokes) and reached its peak (368.0 aphids/25 net strokes) during the 1^st wk of March. As regards, R. maidis was recovered in wheat plant during the 1^st wk of January (3.2 aphids/25 net strokes) and continued on wheat plant until the 1^st wk of March given the population peak during the 4^th wk of January (25.4 aphids/25 net strokes). On the other side, in 2012 season, the greenbug population followed nearly the same trend of the first season. Its population increased from 3.4 aphids/25 net strokes in the 1^st wk of January to reach a peak of 318.6 aphids/25 net strokes during the 1^st wk of March. Concerning the aphid, R. padi was recorded two weeks later than S. graminum (1.2 aphid/25 net strokes) and reached its peak during the 1^st wk of March (410.2 aphids/25 net strokes). On the other hand, R. maidis was noticed during the 1^st wk of January (5.2 aphids/25 net strokes) and reached a peak (16.2 aphids/25 net strokes) during the 4^th wk of January and decline from the wheat fields during the 3^rd wk of March. On the other side, the main peak of total aphid species was recorded during the 1^st wk of March (529.0 and 729.0/25 net strokes) during 2011 and 2012 seasons, respectively. Generally, cereal aphids were encountered on wheat plants during the 1^st wk of January and left wheat plants on the end of March throughout the study period. El-Heneidy et al., (2003) reported that the active period of cereal aphids in wheat fields in Egypt usually starts from the late tillering growth stage (during January) and continued through the stem elongation, booting, heading and ends during the ripening stage (during April).

Table 3: Population densities of cereal aphid species and associated parasitoid species collected by sweep-net inhabiting wheat plants, Assiut, 2011 and 2012 seasons

*S. graminum                                      ** R. padi                             ***R. maidis

Concerning the primary parasitoid, D. rapae (Table 3) was more common throughout the season. In 2011 season, two peaks of D. rapae were observed, the first peak recorded at the 3^rd wk of January (10.8 parasitoids/25 net strokes) and the second which was higher than the former, was observed in the 2^nd wk of March (408.0 parasitoids/25 net strokes) coincided with the peak decline of the aphid populations. Similar results was recorded in 2012 season except for the first peak was recorded during the 2^nd wk of February (102.0 parasitoids/25 net strokes). Generally, D. rapae was appeared on wheat plants during the 1^st wk of January and increased gradually until reached the maximum peak during the 2^nd wk of March throughout the two seasons coincided with aphid populations decline. Alichi et al. (2008) found that peak populations of the aphid parasitoids occurred after the peak of aphid populations by 1-3 weeks. Cai et al. (2009) stated that the density peak of parasitoid populations was 9-12 days behind the density peak of the aphid populations. On the other side, El-Heneidy (1994) found that highest population of the primary parasitoids was counted during February and March synchronizes the peak population of cereal aphids on wheat plants

Finally, the present work and the aforementioned literatures indicated that wheat fields are rich with aphid parasitoid species which of course play a significant role in suppressing aphid infestation. Thus, spraying insecticides particularly early in wheat growing season should be avoided to conserve parasitoids that are more efficient when aphid populations are low.

REFERENCES:

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El-Heneidy, A. H.; Gonzalez, D.; Stary, P.
and Adly, D. (2002): Significance of hyperparasitization of primary cereal aphid parasitoids in “Egypt” Hymenoptera, parasitical. Egypt. J. Biol. Pest Control, 12 (2): 109-114.

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HYMENOPTEROUS PARASITOIDS ASSOCIATED WITH CEREAL APHIDS IN WHEAT FIELDS AT ASSIUT

Alaa El-Deen A. A. Salem

Plant Protection Research Institute, Giza, Egypt

INTRODUCTION:

In Egypt, the major insect pests attacking wheat plants are the greenbug, Schizaphis graminum (Rondani), the bird cherry-oat aphid, Rhopalosiphum padi L. and to lesser extent the corn leaf aphid, R. maidis (Fitch.) and the English grain aphid, Sitobion avenae (Fab.) 
(El-Heneidy et al., 2001; Abdel-Samad and Gomaa, 2004; Mahmoud, 2005; Salem, 2007 and Salem and Mahmoud, 2012).

The damage caused by cereal aphids in both direct through feeding injury and indirect through the effects of honeydew in combination with fungi, which reduce rate photosynthesis and the available leaf area (Martin and Johnston, 1982; Ryan et al., 1987 and Royer et al., 1997).

The hymenopterous parasitoids attacking cereal aphids are considered to be of importance in natural control of their host populations. The reported parasitoid species attacking cereal aphids are Diaeretiella rapae (McIntosh), Praon spp., Aphidius spp. and Ephedrus spp. (Stary, 1976; Ibrahim, 1990; Elliot et al., 1995; Ali et al., 2001; Adly, 2002 and Salem, 2007).

The present study focused on survey and population densities of major cereal aphid species and their associated parasitoid species particularly D.rapae on wheat plants at Assiut Governorate.

MATERIALS AND METHODS:

The present study was carried out in Assiut province (Assiut district) located beside El-Azhar University at Assiut, during two successive seasons (2011 and 2012). An area of about 1/4 feddan was divided into 4 plots of equal size (approximately 250 m²) and cultivated with wheat seeds (Seds 1). Common cultural practices were applied and no chemical pesticides were used during the study and weeds were removed by hand.

1-Mummies collected from wheat plants:

Available mummified aphids were collected weekly from wheat plants (depending on aphid density). After collection, mummies transferred to the laboratory and put individually in small plastic vials (1.5 cm in diameter and 5.5 cm in high), then kept under the laboratory conditions. The mummies were inspected daily, and the emerging adult parasitoids were identified and counted. Aphid parasitoids emerged were identified according to Pike et al. (1997) and confirmed by aid Dr. Peter Stary, Academy of Science, Institute of Entomology, Republic of Czechoslovakia.

2-Sweep-net method:

                Sweep-net method was used to study the population of cercal aphids and the major parasitoid, D. rapae. Samples of cereal aphids and the associated parasitoid inhabiting wheat plants were taken regularly at weekly intervals from the beginning of January to the end of March. Twenty five net strokes were taken 4 times (100 net strokes) as the standard sample at weeks intervals. Each collected sample was empted carefully into labeled collected bag. Samples were transferred to the laboratory and the insects were killed by chloroform. The number of cereal aphids and parasitoid species were count and recorded.

                Dominance (D) and abundance (A) degrees of cereal aphids and parasitoid species were calculated according to Facylate, (1971).

D = t/T × 100

where:

 t =Total number of each species during the collecting period.

T = Total number of all species collected during the collecting period.

A = n/N × 100

where:

 n = Total number of samples in which each species appeared.

N = Total number of samples taken allover the season.

RESULTS AND DISCUSSIONS:

1-Survey and relative abundance of cereal aphid parasitoid species by using mummies collection method:

Figure (1) show the numbers of mummies collected from wheat plants and the percentage of emerged parasitoids during 2011 and 2012 seasons. In 2011 season, from about 21200 mummified aphids collected from wheat fields there were 18760 adult parasitoid emerged from mummies represented 88.49% of the total collected mummies. On the other side, data also showed that of 26800 mummies collected from wheat fields in 2012 season, there were 24390 adult parasitoid emerged from mummified aphids represented 91.01% of the total collected mummies. Also, Figure (1) show that the percentage of emerged adult parasitoid in 2012 season was higher than that recorded in 2011 season with an overall average of 89.69%.

In this approach, Gilstrap et al., (1984) reported that from 7.700 greenbug and 2.200 from corn leaf aphid mummies which collected from sorghum, adult parasitoid emerged were 5.635 of the collected greenbug mummies and 1.468 of the collected corn leaf aphid mummies represented 73.18% and 66.72%, respectively.

A-Primary parasitoid recorded:

Family: Aphidiidae; Aphidius colemani Vereck, A. matricariae Haliday, Diaeretiella rapae (McIntosh), Ephedrus plagiator (Ness), Praon necans Mackauer and Trioxys sp.

Fig. 1: Total cereal aphid mummies parasitoids emerged and non parasitoid emerged collected
from wheat plants, Assiut, 2011 and 2012 seasons

B-Secondary parasitoid recorded:

Family: Cynipidae, Alloxysta australiae (Ashmead)

Family: Megaspilidae, Dendrocerus carpenter (Curtis)

Family: Chalcididae, Chalcids sp.

Family: Encyrtidae, Aphidencyrtus sp.

                The same results were obtained by Ali et al., 2001; El-Heneidy et al., 2001; El-Heneidy et al., 2002; El-Hussieni et al., 2003; Sobhy et al., 2003; Abdel-Rahman, 2005 and Salem and Mahmoud, 2012.

C-Relative abundance of parasitoid species:

Data in Table (1) show the percentage of each primary and secondary parasitoid species emerged from mummified aphids collected from wheat fields during 2011 and 2012 seasons. Among the recorded cereal aphid primary parasitoids, the highest percentage (87.92%) was recorded for D. rapae followed by P. necans (4.04%), A. colemani (3.01%), E. plagiator (2.45%), A. matricariae (1.85%) and Trioxys sp. (0.74) during 2011 season. Highest percentages during 2012 season reached 64.28, 15.50, 7.08, 6.08, 4.78 and 2.27% for D. raae, P. necans, A. matricariae, E. plagiator, Trioxys sp. and A. colemani, respectively. The highest percentage (94.01%) of total parasitoid species among the surveyed parasitoids was recorded in 2011 season and it was relatively higher than that of season 2012 (65.56%). This results agrees with the findings of Elliot et al., (1995) and pike et al., (1997) reported that the parasitoid D. rapae was recorded in high numbers on cereal aphids than other species. Salem and Mahmoud (2012) reported that D. rapae seem to be the most important primary parasitoid species as biological control agents due to their highest value of dominance and abundance degrees followed by A. colemani and P. necans.

Table 1: Relative abundance of the primary and secondary parasitoid species emerged from cereal aphid mummies collected from wheat plants, Assiut, 2011 and 2012 seasons

Concerning the secondary parasitoid species, the highest percentage of hyperparasi-tism (65.87%) was recorded for D. carpenteri followed by A. australiae (20.80%), Chalcids sp. (7.64%) and Aphidencertus sp. (5.69%) during 2011 season. Meanwhile, in 2012 season the highest percentages of hyperparasitism were 61.15, 29.50, 7.41 and 1.94% for A. australiae, D. carpenteri, Chalcids sp. and Aphidencyrtus sp. respectively. On the other hand, the highest percentage (34.44%) of the total parasitoids was observed in 2012 season and it was relatively higher than that of 2011 season (5.99%). Al-Dobi et al., (1999) stated that the secondary parasitoid, Dendrocerus sp. was the most abundant species on cereal aphid primary parasitoids in wheat fields.

2-Population of cereal aphids and the associated parasitoids by using sweep-net method:

Wheat plants in Upper Egypt were susceptible to damaging infestations mainly by three cereal aphid species, S. graminum, R. padi and R. maids. Seasonal occurrence periods of different species were recorded as follows; from the beginning of January to the end of March for R. padi and S. graminum and from the 1^st wk of January to the 1^st wk of March for R. maids. The fourth and fifth species, S. avenae and, Metopolophium dirhodum (Walker) which occurs late in the season were found in very low numbers during course of this study. Available literatures in Egypt confirmed that most of the surveyed cereal aphid species were, R. maidis, R. padi, S. graminum and S. avenae as recorded by El-Hariry, 1979; Abou-Elhagag and Abdel-Hafez, 1998; El-Serafy, 1999; Ali and Abdel-Rahman, 2000; Ali et al., 2001; Abdel-Rahman, 2005; Mahmoud, 2005 and Salem, 2007. In addition, El-Fatih, 2000; Salem, 2007 and Salem and Mahmoud, 2012, they recorded M. dirhodum on wheat plants at Giza, Beni-Suef and Assiut regions.

A-Dominance and abundance degrees of cereal aphids and associated primary and secondary parasitoid species:

* Cereal aphid species:

                Data in Table (2) show that, R. padi and S.gramimim seems to be the most important economic cereal aphids as indicated by the highest value of dominance (88.91 and 90.73%) as well as abundance degrees (100%) in 2011 and 2012, respectively, followed by R.maidis which had low dominance (3.77 & 2.48%) and abundance (69.23 & 76.92%) during 2011 and 2012 season respectively. Similar results were reported by Ali and Abdel-Rahman (2000) reported that R. padi and S. graminum were the most important economic pests by the highest value of dominance and abundance degrees followed by R. maidis. Salem (2007) reported that S. graminum was the most abundant cereal aphids species followed by R. padi .The corn leaf aphid was less abundant.

*Primary parasitoid species:

Data in Table (2) show that D. rapae seems to be the most important primary parasitoid species as indicated by the highest values of dominance (48.43 & 47.15%) and abundance (100.00%) during 2011 and 2012 seasons, respectively. However, the high abundance degrees (100.00%) of the parasitoid A. colemani and P. necans indicate that these species could be of economic importance if the environmental conditions charged in their favor. Meanwhile, the primary parasitoid species which had low values of dominance and abundance were
A. matricaiae, E. plagiator and Trioxys sp. represented (12.12 & 19.99%), (11.90 & 9.23%) and (3.42 & 3.53%) for the dominance degrees and (61.54 & 69.23%), (46.15 & 69.23%) and (30.77 & 53.85%) for the abundance degrees during 2011 and 2012 seasons, respectively. Salem and Mohmoud (2012) reported that D. rapae was considered the most dominate and abundance primary parasitoid species and may play an important role in controlling the cereal aphids in wheat fields.

*Secondary parasitoid species:

Data in the same Table (2) show that highest percentage of the secondary parasitoid species (34.00%) was recorded to D. carpenteri in the first season, while it was (47.43%) to A. australiae during the second season. Lowest dominance degrees were recoded to Chalcids sp. (19.95 & 16.42%) and Aphidencyrtus sp. (13.24 & 1.99%) during 2011 and 2012 seasons, respectively. Salem (2007) reported that Alloxysta sp. was the most common and abundant species among the secondary parasitoid species collected.

Table 2: Dominance and abundance degrees of cereal aphids and their parasitoid species collected by sweep-net technique inhabiting wheat plants, Assiut, 2011 and 2012 seasons

B-Population of cereal aphids and the associated primary parasitoid, D. rapae:

Data in Table (3) show that, in 2011 season, S. graminum was appeared on the plants during the 1^st wk of January (6.8 aphids/25 net strokes). The greenbug showed its peak of activity during the 4^th wk of January (230.2 aphids/25 net strokes). In spit of the bird cherry-oat aphid was first observed in the field in the 1^st wk of January (1.2 aphids/25 net strokes) and reached its peak (368.0 aphids/25 net strokes) during the 1^st wk of March. As regards, R. maidis was recovered in wheat plant during the 1^st wk of January (3.2 aphids/25 net strokes) and continued on wheat plant until the 1^st wk of March given the population peak during the 4^th wk of January (25.4 aphids/25 net strokes). On the other side, in 2012 season, the greenbug population followed nearly the same trend of the first season. Its population increased from 3.4 aphids/25 net strokes in the 1^st wk of January to reach a peak of 318.6 aphids/25 net strokes during the 1^st wk of March. Concerning the aphid, R. padi was recorded two weeks later than S. graminum (1.2 aphid/25 net strokes) and reached its peak during the 1^st wk of March (410.2 aphids/25 net strokes). On the other hand, R. maidis was noticed during the 1^st wk of January (5.2 aphids/25 net strokes) and reached a peak (16.2 aphids/25 net strokes) during the 4^th wk of January and decline from the wheat fields during the 3^rd wk of March. On the other side, the main peak of total aphid species was recorded during the 1^st wk of March (529.0 and 729.0/25 net strokes) during 2011 and 2012 seasons, respectively. Generally, cereal aphids were encountered on wheat plants during the 1^st wk of January and left wheat plants on the end of March throughout the study period. El-Heneidy et al., (2003) reported that the active period of cereal aphids in wheat fields in Egypt usually starts from the late tillering growth stage (during January) and continued through the stem elongation, booting, heading and ends during the ripening stage (during April).

Table 3: Population densities of cereal aphid species and associated parasitoid species collected by sweep-net inhabiting wheat plants, Assiut, 2011 and 2012 seasons

*S. graminum                                      ** R. padi                             ***R. maidis

Concerning the primary parasitoid, D. rapae (Table 3) was more common throughout the season. In 2011 season, two peaks of D. rapae were observed, the first peak recorded at the 3^rd wk of January (10.8 parasitoids/25 net strokes) and the second which was higher than the former, was observed in the 2^nd wk of March (408.0 parasitoids/25 net strokes) coincided with the peak decline of the aphid populations. Similar results was recorded in 2012 season except for the first peak was recorded during the 2^nd wk of February (102.0 parasitoids/25 net strokes). Generally, D. rapae was appeared on wheat plants during the 1^st wk of January and increased gradually until reached the maximum peak during the 2^nd wk of March throughout the two seasons coincided with aphid populations decline. Alichi et al. (2008) found that peak populations of the aphid parasitoids occurred after the peak of aphid populations by 1-3 weeks. Cai et al. (2009) stated that the density peak of parasitoid populations was 9-12 days behind the density peak of the aphid populations. On the other side, El-Heneidy (1994) found that highest population of the primary parasitoids was counted during February and March synchronizes the peak population of cereal aphids on wheat plants

Finally, the present work and the aforementioned literatures indicated that wheat fields are rich with aphid parasitoid species which of course play a significant role in suppressing aphid infestation. Thus, spraying insecticides particularly early in wheat growing season should be avoided to conserve parasitoids that are more efficient when aphid populations are low.

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