The only alfalfa weevils observed this week were relatively mature larvae (see fig. 1) and thus most feeding damage should be about finished. All alfalfa fields monitored had been treated, with one exception. This year’s overall alfalfa weevil infestations seemed considerably reduced compared to most of the past years. This, coupled with good growing conditions, allowed plants to keep up with alfalfa weevil feeding resulting in less defoliation then usual as seen in fig. 2 (The plant on the left was typical of an untreated field which was adjacent to a treated field from which the plant on the right was removed from).
Figure 1 Alfalfa weevil larvae (picture by Cody Wyckoff)
Figure 2 Alfalfa defoliation Left: untreated field Right treated field (Cody Wyckoff)
–by Anthony Zukoff–Southwest Research & Extension Center
March Flies (Bibio sp.) have begun emerging in large numbers in some areas of southwest Kansas. The synchronous nature of adult emergence in spring draws attention and concern as thousands of adults can show up almost overnight on structures and in cropland. These congregations of small (~6 mm), large-eyed flies are mating swarms and generally will be short lived as adults survive only a few days to a week. Another name for these flies is “Lovebug” since males and females will attach themselves at the end of their abdomens and remain like that at all times even while flying. After mating, females will excavate a small chamber in the soil, lay eggs and then die in the same chamber. Just as the adults congregate, so do the larvae. Dense pockets of hundreds of larvae can sometimes be unearthed. These larvae generally are detritivores, feeding on dead vegetation and other organic matter in the upper portion of the soil. However, some species are known to feed on living plant roots and are considered pests of various cereal, forage and vegetable crops. In 1891, entomologist Vernon L. Kellogg wrote of a new Bibionid fly emerging in large numbers in many wheat fields of seven western Kansas counties during the last week of April.
The large swarms of flies concerned farmers, but no evidence of damage by the flies was ever detected despite a large number of larvae being found in the soil weeks previous to adult emergence. Kellogg stated that the adults disappeared “suddenly and simultaneously”. Despite a large number of flies being detected in wheat and alfalfa this year, no unusual damage to these crops has been noted and the mating swarms should vanish in the coming week.
Alfalfa weevils have been actively feeding and developing quite rapidly throughout south central and north central Kansas during the last 7-10 days. However, infestation levels have not increased very much. The most significantly infested fields monitored during this period reached the 55% level, i.e. just over 1 larva/2 stems. Most fields were between 25-35% infested, with about 75% of the larvae at least in the late 1st- 3rd instar stage. Some fields have been sprayed and it seemed to provide good knockdown (see fig. 1 of dead larvae). All fields not treated had prepupae and pupae (see fig. 2). Alfalfa weevil larval sampling/monitoring should continue for at least another 10-14 days.
Pea aphid populations have decreased significantly in the last 7-10 days in the untreated fields.
Figure 1: Dead Larvae (photo by Cody Wyckoff)
Figure 2: Alfalfa Weevil Pupa (Photo by Cody Wyckoff)
None of the fields sampled over the past 10 days had been treated with an insecticide. Thus, pea aphids are really prevalent and increasing in population density. However, much like alfalfa weevils, none of the fields monitored had infestation levels anywhere close to a treatment threshold. These pea aphid populations are often utilized by beneficials (fig.2 of pea aphids and parasitized pea aphids, called “mummies”) early in the season to increase their populations, which often help against other pests in other crops.ie. greenbugs, corn leaf aphids, soybean aphids, etc. Again, monitoring pea aphids should also continue until swathing.
Alfalfa weevils and pea aphids are considered cool season pests and primarily affect alfalfa up to the 1st cutting. Most alfalfa weevil larvae detected (fig. 1) were late 1st/early 2nd instars and, thus will probably finish feeding in about 2 weeks at the temperatures predicted for that period of time (60-70’s for daytime temp’s). Thus, sampling for both alfalfa weevil larvae and pea aphids should continue until at least the 1st cutting.
Figure 2 Pea aphids and parasitized pea aphids (Cody Wyckoff)
Alfalfa weevil larvae continue to feed and thus increase in size (fig. 1). However, after monitoring several fields throughout north central Kansas over the last 10 days, there was not yet any field that had a 50% infestation level. (Infestation level determined by the stem count bucket method where individual stems are removed and quickly shaken into a 1 gallon white bucket to dislodge any weevil larvae that may be present. Then, count the number of larvae in the bucket and divide into the number of stems shaken into the bucket to get the percent infested stems). Alfalfa weevil monitoring should continue, however, as we are still relatively early and more larvae will probably be hatching.
Figure 1 Alfalfa weevil larvae and leaf damage (Cody Wyckoff)
–by Anthony Zukoff — Southwest Research Extension Center
In the southwest region near Garden City, several alfalfa fields checked on March 31 (344 accumulated degree days) had active 1st instar weevil larvae. Damage to the plants was not very apparent yet, but light pin-holing could be found. Warm weather in the high 80’s is forecast for the area this weekend which likely will give the weevils and alfalfa a boost. The alfalfa in the area is about 4 inches tall right now. Remember, for 3 to 7-inch-tall alfalfa, treat when feeding is evident on the top inch of growth (1-2 larvae per stem); retreatment may be necessary before cutting. Pea aphids were also present in these fields at low levels. Light populations of aphids in the field can help boost the number of beneficial insects, such as ladybugs, which will migrate from the alfalfa into other nearby crops later in the season. If predators and parasitoids are not apparent, treatment may be needed if there are 50 aphids per stem in 10 inch tall alfalfa.
Army cutworms are again relatively common this year, however, thus far, not nearly as destructive as last year (2020). This could be because infestations are much reduced compared to 2020 – or that growing conditions have been much better than last year for both alfalfa and wheat. However, birds (and/or skunks) feeding in alfalfa or wheat fields are still the easiest way to detect army cutworm larvae.
Alfalfa weevil larvae have been hatching throughout south central and north central Kansas for the last couple of weeks. First larvae/feeding reported from south central Ks. on 15 March and on 21 March from north central parts of the state. The easiest way to find new larvae is to note small, pinprick sized holes in leaves and/or brooming feeding damage on the tips of new terminals (see figure 1). Larval hatching and subsequent feeding will only increase as temperatures increase. It takes temperatures in the mid to low 20’s(F) for a couple hours to actually adversely affect young larvae (remember 2018- and last year, 2020, in mid-April). Also, please remember– utilize whatever treatment threshold that has been successful for you in the past (hopefully-not just spray because your neighbor is–we use a 33-50% infestation level using the “stem count bucket “sampling method). In addition, it is really important to spray with an adequate amount of carrier, i. e. water, to achieve good coverage. After application, and the reentry interval (REI) for the product used, continue monitoring as eggs may continue to hatch for another few weeks, or until swathing. Also, always check the preharvest interval (PHI) for your product of choice.
Figure 1: Leaf feeding plus 1 day old larva (photo by Cody Wyckoff)
Most soybeans have developed well past the point of being susceptible to soybean pests (excluding lodging due to Dectes stem girdling). However, there are still some late-planted fields that have pests feeding on the pods. See (fig 3) of grasshopper feeding bite site and bean leaf beetle chewed hole. However, grasshoppers should be dying soon and thus not causing much actual damage. The bean leaf beetles will soon be migrating from these soybean fields to overwintering sites.
Figure 3. Soybean pod damage by grasshopper and bean leaf beetle (Cayden Wyckoff)
Alfalfa should probably still be monitored for pests. Every alfalfa field sampled throughout south central and north central Kansas from 8-13 October had significant infestations of potato leafhoppers (see fig 1). The treatment threshold for potato leafhoppers is variable as the economics of hay production is so dynamic depending upon the end use of the hay. However, potato leafhoppers can stress alfalfa in 2 ways–removing plant fluids due to their feeding with the simultaneous injection of a toxin (see fig 2 of “hopper burn”). These two factors may stress alfalfa at any time of the year, but especially this time of year (especially during this dry period) because it may hamper the plant’s ability to prepare for overwintering. One positive note, however, all potato leafhoppers observed in the last 10 days were adults; so hopefully, they will be migrating out of Kansas for overwintering sites in the southern U.S. All fields sampled had not been swathed, so if they are cut soon this should also help eliminate any leafhopper problems.
Figure 1. Potato Leafhoppers (Cayden Wyckoff)
Figure 2. “Hopper Burn on alfalfa (Cayden Wyckoff)
