Alfalfa (Medicago sativa L.) hosts several species of aphid, Acyrthosiphon pisum (Harris), Aphis craccivora Koch and Therioaphis trifolii (Monell). The preference of the aphids of alfalfa plants for dense assemblies or individual plants, as well as for healthy or infested plants, was investigated in the field as in the laboratory. Years of field research have revealed the specific preferences of all three species of aphid. A. pisum and T. trifolii are most commonly found in alfalfa crops, while A. craccivora is mostly found on alfalfa weeds. Also, a single species of aphid alone is usually present on a plant. In order to determine the reason for this clear preference and to establish whether at the very beginning, i.e. at the stage of choosing a host, aphid species distance themselves from each other, we tested the effect of the volatiles of healthy and infested plants on their attractiveness to aphids. A. craccivora is repelled by the volatiles of dense crops and plants previously infested with one of the other two species. A. pisum and T. trifolii choose a dense assembly of plants, repelled by the volatiles of plants previously infested with A. craccivora. A. pisum displays the weakest competitive traits, and A. craccivora the strongest. This research showed that competition between aphid species does not occur only when they find themselves on the same plant at the same time, fighting for resources, but also in the choice of plant, in order to avoid later competition.

The alarm behavior plays a key role in the ecology of aphids, but the site and molecular mechanism for the biosynthesis of aphid alarm pheromone are largely unknown. Farnesyl diphosphate synthase (FPPS) catalyzes the synthesis of FPP, providing the precursor for the alarm pheromone (E)-β-farnesene (EβF), and we speculate that FPPS is closely associated with the biosynthetic pathway of EβF. We firstly analyzed the spatiotemporal expression of FPPS genes by using quantitative reverse transcription-polymerase chain reaction, showing that they were expressed uninterruptedly from the embryonic stage to adult stage, with an obvious increasing trend from embryo to 4th-instar in the green peach aphid Myzus persicae, but FPPS1 had an overall significantly higher expression level than FPPS2; both FPPS1 and FPPS2 exhibited the highest expression in the cornicle area. This expression pattern was verified in Acyrthosiphon pisum, suggesting that FPPS1 may play a more important role in aphids and the cornicle area is most likely the site for EβF biosynthesis. We thus conducted a quantitative measurement of EβF in M. persicae by gas chromatography-mass spectrometry. The data obtained were used to perform an association analysis with the expression data, revealing that the content of EβF per aphid was significantly correlated with the mean weight per aphid (r = 0.8534, P = 0.0307) and the expression level of FPPS1 (r = 0.9134, P = 0.0109), but not with that of FPPS2 (r = 0.4113, P = 0.4179); the concentration of EβF per milligram of aphid was not correlated with the mean weight per aphid or the expression level of FPPS genes. These data suggest that FPPS1 may play a key role in the biosynthesis of aphid alarm pheromone.

Thiamine is a vitamin that has been shown to act as a trigger to activate plant defence and reduce pathogen and nematode infection as well as aphid settling and reproduction. We have here investigated whether thiamine treatments of seeds (i.e. seed dressing) would increase plant resistance against aphids and whether this would have different effects on a generalist than on specialist aphids. Seeds of wheat, barley, oat and pea were treated with thiamine alone or in combination with the biocontrol bacteria Pseudomonas chlororaphis MA 342 (MA 342). Plants were grown in climate chambers. The effects of seed treatment on fecundity, host acceptance and life span were studied on specialist aphids bird cherry-oat aphid (Rhopalosiphum padi L.) and pea aphid (Acyrthosiphon pisum Harris) and on the generalist green peach aphid (Myzus persicae, Sulzer). Thiamine seed treatments reduced reproduction and host acceptance of all three aphid species. The number of days to reproduction, the length of the reproductive life, the fecundity and the intrinsic rate of increase were found reduced for bird cherry-oat aphid after thiamine treatment of the cereal seeds. MA 342 did not have any effect in any of the plant-aphid combinations, except a weak decrease of pea aphid reproduction on pea. The results show that there are no differential effects of either thiamine or MA 342 seed treatments on specialist and generalist aphids and suggest that seed treatments with thiamine has a potential in aphid pest management.

The influence of solitary versus group rearing on the development and survival of the three-striped ladybird beetle Brumoides suturalis (Fabricius) was studied using two aphid species, Acyrthosiphon pisum (Harris) and Aphis gossypii Glover, as diet. Rearing pattern (solitary or group) had no significant influence on the success of rearing, but prey species did. While immature stages developed faster on A. pisum, the survival of early instars was higher on A. gossypii. Diet had no effect on pupal survival.

DL-β-aminobutyric acid (BABA) is a non-protein amino acid that is an effective inducer of resistance against a variety of plant pathogens. However, examples of BABA-induced resistance against insect herbivores have not been reported. We applied BABA as a soil drench to legumes and monitored its effects on the pea aphid Acyrthosiphon pisum (Harris). On tic bean (Vicia faba var. minor), BABA increased aphid mortality, caused a reduction in the mean relative growth rate of individual insects and lessened the intrinsic rate of population increase (rm). BABA also caused significant reductions in the growth rate of A. pisum on pea (Pisum sativa), broad bean (Vicia faba var. major), runner bean (Phaseolus coccineus), red clover (Trifolium pratense) and alfalfa (Medicago sativa). No direct toxic effects of BABA against A. pisum were found, and no phytotoxic effects that may have caused a reduction in aphid performance were detected. Possible mechanisms behind this BABA-induced inhibition of aphid performance are discussed.

Six field pea Pisum sativum (L.), lines (Holetta Local-90, 305PS210687, 061K-2P-2/9/2, 061K-2P-14/7/1, JI-898 and 304WA1101937) were screened in a greenhouse for their resistance to the pea aphid Acyrthosiphon pisum (Harris) and compared to NEP874UK line (control). The total number of progeny and the average and maximum number of nymphs produced per female during a 24-h period were significantly lower in the six lines compared to the control. The intrinsic rate of increase was also significantly higher in the control than in the other lines (except Holetta Local-90). The aphid preferred to feed on the control than on the tested lines. Line 061K-2P2/9/2 was the most resistant genotype as its height and weight were less affected by aphid attack. Based on antibiosis, antixenosis and tolerance effects, lines 061K-2P2/9/2, JI-896 and 304WA1101937T performed better against A. pisum.

Glasshouse studies were conducted to determine mechanisms of resistance of five genotypes of lentil Lens culinaris (Medikus) previously identified as resistant and moderately resistant to the pea aphid Acyrthosiphon pisum (Harris). A susceptible standard, ILL 8127, was also included. French Indigo had high levels of antibiosis, demonstrated by reduced reproductive period, fecundity and longevity, while Spanish Brown had high levels of antixenotic and tolerance components of resistance. Based on overall resistance, French Indigo was the most resistant genotype followed by Spanish Brown. These genotypes could be a source for resistance to A. pisum. All five genotypes had varying levels of antibiosis, antixenosis and tolerance components of resistance compared with ILL 8127.

Grass pea Lathyrus sativus L. plays a major role in the nutrition of many poor people in Ethiopia. However, the crop suffers heavy damage by pea aphids Acyrthosiphon pisum (Hemiptera: Aphididae). To test potentially more sustainable and harmless alternatives to the commonly used insecticides, some botanicals (neem, garlic and hot pepper), soap, kerosene and the aphicide pirimicarb, were tested for their efficacy against A. pisum at Wondata village, close to the town of Bahir Dar, in northwestern Ethiopia, from 1999 to 2002. After pirimicarb, hot pepper was consistently the most effective botanical in controlling the pea aphid in terms of both pea aphid population as well as grain yield. Compared to the untreated control, there was up to 83 and 55% lower aphid population on pirimicarb- and hot pepper-treated plots in the 2000/2001 cropping season, respectively. There was also up to 93 and 54% higher grain yield on plots treated with pirimicarb and hot pepper extract, respectively. Soap was less effective, while kerosene caused phytotoxicity. Infestation levels increased with advancing maturity of the crop, rising exceptionally suddenly and dramatically near crop maturity. Combined ANOVA over years and crop growth stages indicated that aphid populations varied with years and growth stages. In 2001/2002, aphid populations were higher and grain yields were lower compared with the other years. Percentage parasitism of aphids was significantly higher on pirimicarb-treated plots (∼40%) than other treatments (<20%). Parasitism was essentially a late-season occurrence. Predator population, especially of ladybird beetles (Adonia spp.), was especially higher on hot pepper- and garlic-treated plots, and lowest on pirimicarb treated plots. Grain yield was positively correlated with level of aphid parasitism, ladybird beetle population and number of pods per plant, and strongly negatively correlated with aphid population. In conclusion, a fresh extract of hot pepper sprayed at the rate of 70 ml/4 m2 was selected for further research on rate, frequency and cost of application.

Once a minor pest, the pea aphid, Acyrthosiphon pisum (Harris) has recently become a major pest of field pea (Pisum sativum L.) in Ethiopia, particularly in the northwestern region. To determine the population dynamics of the pest, field experiments were carried out at Adet and Zema in northwestern Ethiopia in 1994, 1995 and 1996. Treatments were planting dates at 15-day intervals in May, June and July and sampling was carried out weekly until crop maturity. Combined ANOVA, over seasons and locations, showed significant effects of sowing dates, seasons and locations. Pea aphid populations increased and decreased with delay in planting at Adet and Zema, respectively. At Adet, peaks of weekly counts of aphid numbers were observed in August and seasonal totals of aphids on individual planting dates increased as planting was delayed. At Zema, peaks of weekly counts of aphid numbers were observed in mid-July (1996), late August and early September (1995) and mid-September (1994); peas planted on June 16 gave the lowest seasonal totals of aphids. Natural enemies (predators and parasitoids) appeared in considerable numbers in 1996 and these tended to increase as planting date was delayed. For most planting dates and years, weekly aphid populations showed positive correlation with maximum temperature, and negative correlation with minimum temperature and relative humidity. Increasing maximum temperature promoted the buildup of aphid populations while increased minimum temperature, rainfall and relative humidity suppressed it. Grain yield increased and decreased as planting date was delayed at Adet and Zema, respectively. From these results, it is recommended that planting of field peas should take place from mid- to late June at Adet and during the first week of June at Zema to obtain higher yields; by doing so a mean of 0.32 t/ha and 0.50 t/ha higher yield was obtained, respectively.

The biology of the pea aphid, Acyrthosiphon pisum (Harris) (Homoptera: Aphididae), on four legumes (faba bean, field pea, lentil and grass pea) was studied in a greenhouse during the 1999/2000 seasons. There was no significant difference in length of nymphal period and number of moults (instars) among the crop species tested. The aphids generally took 9 to 11 days to reach the adult stage with a maximum of 3 moults. Lifespan was significantly longer on lentils than on other crops. Significantly more aphids survived beyond 15 days on lentil than on other crops. Similarly, the reproductive and post-reproductive period and adult longevity were longer on lentil. The pre-reproductive period was significantly higher on field pea than on the other crops. The net reproductive rate (R0) was highest on lentil (103.56 females /female) arid the intrinsic rate of increase (r) was highest on faba bean due to a short generation time (14.27 days). Generally, lifetime fecundity was higher on lentil (115.10/female) due to a higher daily reproduction rate (6.7 nymphs/female/day) on this crop. Nymphal production per aphid was strongly and positively correlated with lifespan, reproductive period and adult longevity but not of the nymphal period. Daily nymph production was significantly correlated with minimum temperature on lentil alone (F = 4.61, P < 0.05). Knowledge of the crop species and varieties susceptible to pea aphid is useful for identifying diets for the mass rearing of aphids for use in resistant variety selection and as diet for artificial natural enemy production. The present experiment indicates that lentil is suitable for both purposes.