Relatively few insects have been reported to cause significant problems on soybeans in New York. Soybean stands can be reduced by seed corn maggot and yields can be reduced by infestations of soybean aphid and two-spotted spider mites.
SEED CORN MAGGOT
Adult seed corn maggots are medium-sized flies that are very similar in appearance to the common house fly. Adult flies are present throughout the growing season and locate egg-laying sites by flying alternately close to the ground’s surface or searching the moist soil cracks on the soil surface. The adult female flies search for egg-laying sites close to decaying plant material or germinating seeds to provide a food source for the newly hatched larvae. Adult flies lay eggs in moist soil cracks near these potential food sources, and typical-looking fly larvae (maggots) hatch from the eggs within a few days. After hatching, larvae move through the soil searching for decaying plant matter or germinating seeds to feed on.
Large-seeded crops like soybeans are very susceptible to seed corn maggot attack, resulting in stand losses. Germinating soybean seeds are often killed or severely injured, thereby reducing plant populations within an area of the field or throughout the entire field. Losses typically range from 3,000 to 8,000 plants per acre. Fields in which animal or green manure crops have been used have a greater potential for seed corn maggot attack than fields not using these manures. However, non-manured fields are also at risk from seed corn maggot damage.
Seed corn maggot is easily and inexpensively controlled with the regular use of insecticide added to the planter box as a seed treatment or seed which has been pretreated with a seed corn maggot active insecticide.
Soybean aphids, Aphis glycines Matsumura, were first observed in NY in 2001. These insects, native to Asia, have been present in the Midwestern US since 2000 causing economic losses.
Soybean aphids are small, yellowish-green, soft-bodied insects with 2 distinctive appendages (cornicles) on the tip of their abdomen. Soybean aphids are the only aphids that infest soybeans in the US. If present, aphids can easily be found on newly unfolding leaves and the under surface of the uppermost leaves. In high populations soybean aphids can also be found on stems, petioles, pods and the under surface of lower leaves. Indications of a soybean aphid infestation can include stunting of plants, yellowing and miss-shaped or contorted leaves, an obvious presence of natural enemies such as lady bird beetles or ants in the uppermost canopy, and a charcoal gray discoloration of leaves indicating presence of sooty mold.
Soybean aphids have a complicated lifecycle with as many as 15 generations possible during the growing season under ideal conditions. Soybean aphids are capable of overwintering as eggs on buckthorn (Rhamnus spp.) a common understory shrub. Fields become infested with soybean aphid in two waves of immigrants. The first wave of aphids arrive from buckthorn where overwintering eggs hatched and the first generation developed. These first aphids in the field usually arrive in early to mid-June. The second wave of aphids arrive in late-June to Mid-July aboard storm systems and the migrants originate in states north and west of NY where the cool weather and the wide spread occurrence of buckthorn encourage rapid spring population buildup.
Monitoring of fields for soybean aphid should begin in mid-June and weekly visits should be scheduled. Fields near buckthorn, the overwintering host, may be colonized earlier and require earlier scouting. In areas without buckthorn, winged aphids migrate in from other areas in mid-season. Check 20 to 30 plants per field. Examine the entire plant, particularly new growth. Scout late-planted fields closely.
Midwestern guidelines recommend management action if monitoring detects an average of 250 soybean aphids per plant if populations are actively increasing and soybean crop development is prior to early pod fill (R4). Spraying after R6 has not been documented to increase yield, especially if the crop has grown well through the vegetative stages.
Plants are likely to be considerably above threshold if stems or pods are covered with aphids and honeydew, sooty mold covers the bottom leaves, and plants are stunted. Insecticide treatment is probably still of value, but the optimal time for treatment (greatest economic return) is past.
Check for mummies (parasitized aphids) and for winged females. Do not spray if mummies are numerous, or if a majority of the aphids are winged or developing wings, an indication that the aphids will soon leave the field.
Producers are reminded that spraying at early reproductive stages poses a threat to bees. Communicate treatment plans to beekeepers and follow precautions to minimize bee kills.