The establishment of alfalfa into different maize residues by conservation-tillage and its effect on insect infestation

The effect of management of maize residues on the population of insects ini mical to the establishment of alfalfa, Medicago sativa L., was examined 1 m onth after sowing. Alfalfa was sown in early and late April, and late May f or 3 consecutive years in Centre County, Pennsylvania, USA. The sowings wer e made by conservation tillage (direct drilled into maize residue with mini mal disturbance of soil) into three different maize residues after (1) sila ge, (2) silage-plus-winter rye cover and (3) grain. A fourth sowing was by conventional tillage (spring ploughing and harrowing maize residue) after g rain. A pesticide, carbofuran (granular formulation) was applied at sowing to half of the plots, Methiocarb bait, a molluscicide, was applied as a spl it application to the same plots, one-half at sowing and the remainder 2 we eks later. Alfalfa plots in the silage-plus-rye maize residues were coloniz ed with fewer insects than the other residue treatments. Excessive growth o f rye in early spring that was not successfully suppressed by herbicide tre atment produced vigorous rye plants and fewer alfalfa seedlings. Consequent ly, silage plus rye had the lowest yield of alfalfa in early April sowings in 2 out of 3 years. Insects known to feed on alfalfa, such as tarnished pl ant bugs, Lygus lineolaris (Palsot de Beauvois), the plant bug, Plagiognath us politus Uhler, pea aphids, Acyrthosiphon pisum (Harris), and meadow spit tlebugs, Philaenus leucophthalmus spumarius (L.), were collected by sweep n et 1 month after sowing and less often in the silage-plus-rye treatment. Th ese species were present in greater numbers in the other maize residues tha t had significantly more alfalfa forage. The insidious newer bug, Orius insidious (Say), and a damsel bug, Nabis ame ricoferus Carayon, were collected in significantly greater numbers in the e arly April sowings, which corresponded with the peak populations of pea aph id. The potato leafhopper, Empoasca fabae (Harris), was most abundant in pl ots sowed in late April or late May. Pesticide treatment applied at the time of sowing had very little effect on numbers of insects collected by sweep net 1 month later in 1986 and 1988. However, pesticide treatment significantly increased yield by 280 kg ha(-1) in 1987, even though sweep net collections of insects were not reduced by the pesticide treatment. Therefore, the beneficial effect of the pesticide could not be explained on the basis of the insects collected. The highest yields of alfalfa were obtained from the early April sowing int o maize residues. This coincided with the time when the majority of alfalfa pests were less abundant than in later sowings; fewer pests were found on the sowings into silage-plus-rye residue. Also, when the rye forage yield w as combined with the alfalfa yield, this became the most productive system.