Kansas State University

–by Dr. Jeff Whitworth

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)

–by Dr. Jeff Whitworth

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 1^st 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.

Weevil pinhole

Weevil Pea aphids

–by Dr. Jeff Whitworth, Professor, Field Crops

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)

–by Dr. Jeff Whitworth

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)

–by Dr. Jeff Whitworth

Many alfalfa fields were sampled throughout north central Kansas over the last few days. Very few pea aphids were detected (fig 2.) – but those that remain are providing nutrition for many beneficials in alfalfa fields i.e., adult green lacewings and lady beetles (many) and lady beetle larvae (fig 3.). No alfalfa weevils were observed.

Figure 2 Pea aphids

Figure 3a Adult green lacewing

Figure 3b Adult ladybug

Figure 3c Lady beetle larva

However, potato leafhoppers, all adults, were collected in every field sampled (fig 4.). This indicates potato leafhoppers are immigrating into the state and will soon, if not already, be depositing eggs in stems, which soon hatch and thus increase populations. Potato leafhoppers remove plant juice and while so doing may inject a toxin into the plant. This feeding alone, may stress plants, especially in the often hot/dry conditions in July/August in Kansas. However, the introduction of a toxin may also negatively affect the plants. One characteristic symptom of potato leafhopper feeding starts out with yellowing from the tip of the leaves and travels through the leaf to the stem. This is often called “hopper burn” and can then negatively impact alfalfa production, both in quantity and quality by lowering the nutritional value.

Figure 4 Potato leafhopper

–by Sarah Zukoff

I wanted to update you on the alfalfa weevil situation I’ve been dealing with. I’ve gotten many different calls from farmers and agents across Kansas the last few weeks. After working with Romulo Lollato in Ag; Judy O’Mara and Erick DeWolf in Plant path; Frank Pairs from CSU and Kelly Seuhs from OSU I think we have an idea of what’s going on. 

I received calls of extensive damage in alfalfa fields and multiple field failures of alfalfa weevil sprays-(see pic’s) in May. After visiting over 30 fields across the state and doing lab assays on collected larvae we’ve determined pretty wide spread lambda-cyhalothrin resistance with suspected resistance to Indoxicarb and even Chlorpyrifos. We’re setting up assays to test these later this year. The weevil damage was extensive and after multiple weevil sprays had little mortality. The frost hit after this damage and these very stressed plants died back pretty severely. These plants then experienced soggy spongy roots with some rot that further damaged plants. Larvae seemingly occurred in multiple rounds as the frost interrupted normal development.  I haven’t determined if the different weevil strains play into this story yet. So lots and lots of larvae that are literally STILL active in fields now with many sprays not working. After ten days, the plants are slowly bouncing back with yellowed regrowth. Farmers are struggling to find anything that works in some of these fields, consultants are being blamed for not doing their job, and agents are being pressured to find SOMETHING to help the farmers with… Therefore, I now need to quantify resistance to the other AI’s and ponder this delayed expanded hatching over two months.

–by Dr. Jeff Whitworth

Pea aphids are still quite active throughout north central Kansas. However, most populations have been considerably diminished for several reasons, but now are mainly just a food source for various beneficial insects (fig, 1) as seen here, two species of lady beetles actively feeding on pea aphids, and (fig. 2) a syrphid fly larva that was also actively feeding on pea aphids.

Figure 1 Lady Beetle feeding on pea aphid  — (Cody Wyckoff)

Figure 2 Syrphid fly larva feeding on pea aphid (Cody Wyckoff)

Pea aphids are usually considered a pest when populations approach or exceed a treatment threshold, they can also be useful as a food source for beneficials until other aphid species increase, if they do, in other crops. Even though these aphids are still plentiful in most north central Kansas alfalfa fields, there was NONE that came anywhere close to a treatment threshold. Also, a very few alfalfa weevil larvae (fig. 3) can also be found, this is not unusual, or a cause for concern.

Figure 3 Alfalfa weevil larva (Cody Wyckoff)