THE PEA APHID ON ALFALFA 
Prepared by Entomology Research Division, Agricultural Research 
Service 
In years of heavy outbreak, the pea 
aphid! is responsible for alfalfa crop 
losses of several million dollars. It 
also damages other forage legumes — 
sweetclover, trefoil, vetch, and sev- 
eral varieties of clover— garden peas, 
and sweetpeas. 
DAMAGE TO ALFALFA 
Enormous pea aphid populations 
often develop in the spring and, some- 
times, inthe fall. Boththe adults and 
the young (or nymphs) suck juice from 
alfalfa leaves, petioles, stems, and 
flower buds. They prefer young 
growth and congregate on the growing 
tips of the plants. 
Pea aphid feeding causes alfalfa to 
turn yellow and wilt. In heavy infes- 
' Acyrthosiphon pisum. 
tations, the tops die. Heavy infesta- 
tions in the spring can cause failure 
of the first crop, reduce the vigor of 
the succeeding crop, reduce yield of 
seed crops, and kill part of the stand. 
Pea aphid feeding also causes 
stunted plants with small leaves 
and spindly stems. When alfalfa 
growth is retarded, weeds often take 
over and crowd out the alfalfa. 
APPEARANCE AND 
DEVELOPMENT 
The adult pea aphid is soft bodied 
and ranges in color from light to 
dark green. It is about %/16-inch 
long and Y16-inch wide. The nymph 
is smaller, but otherwise resembles 
the adult. 
In most parts of the United States, 
the pea aphid is active and repro- 
