A Discussion of Selection and Breeding for Thrips and other Insect Predator Resistance

A Preliminary Discussion of Selection and Breeding for Resistance to Thrips and other Insect Predators in Daylilies

SUN DRAGON

My breeding program began in 2010, with the first major round of seed production happening in 2011. Even before I had started my program, as I planned the program I would pursue in 2008 and 2009, I planned to select for resistance to pests and pathogens, because that mirrored the work I had previously done with poultry (immunogenetics), and was an area of interest for me. Having grown daylilies since the mid 1970s, I had always noticed the frequent spottiness of flowers in anthocyanin pigmented types. I also noticed problem behaviors like bud drop, enations, spottiness and even fully blasted young scapes. Until I began to consider a daylily breeding program, I had not looked at the causes of all these problems, but I repeatedly found references to thrips issues. I knew there were thrips endemic to my very dry, south-facing property with sandy soil - thrips heaven! So I knew rust and thrips would be two major areas of selection for me, but I was not able to start selecting effectively for either within the first couple of years of my program. Rust didn’t not appear until the late summer of 2012, and I did not have a major thrips outbreak until the very dry spring of 2013. I knew that because rust is not endemic to my area, I would make my rust resistance selection time period coincide with the time when I would be brining in the most new plant material to build my daylily breeding program. I let the near-certainty of getting rust when buying a lot of daylilies from a lot of different sources become an asset rather than a problem. However, I set my rust resistance breeding program for five years, at which time I cut back on bringing in plants and the last few plants I brought in went through quarantine and never produced any rust. I haven’t brought in new plants since 2019.

SOLARIS SYMMETRY 

(photo by Theresa Maris of the cultivar in her garden)

The first fancy, near-white, lavender eye/edge tetraploid that I ordered to start my breeding program in 2010. I ordered it because it was a northern-bred dormant hailing from Nate Bremer's breeding program in Wisconsin. I received it in spring of 2011 and saw beautiful foliage and great performance from the get-go. By 2016 I had come to realize that Solaris Symmetry was the most thrips and late freeze resistant cultivar I grew, at both ploidy levels. I had been using it as a breeder every year since 2011, but in 2017, I used its pollen on 90% of my tet crosses, including backcrosses to its own best offspring - seedlings that would later become such introductions as Sun Dragon and Spice Addiction.

Before I proceed to speak about my own thrips resistance selection program, I want to mention that I know some people feel that Aphids are responsible for things attributed to thrips. In my garden, I only rarely see aphids, but I do see thrips, and issues like enations, bud drop, flower deformity and spotting of petal pigmentation all increase when thrips swarms occur here (generally in dry spring weather). I do not see as many issues with thrips in the late summer and fall. They are predominantly a spring issue here. With that said though, while I know thrips are what I am working with, I do believe that any sap-feeding insect with a proboscis can likely cause all of these same issues, so I am completely open to other insect pests such as aphids causing these same issues in other gardens. I do not believe this is an either/or proposition, but that the symptoms I have described above can be attributed to ‘insect predators’, generally, rather than having to say that these issues are diagnostic of one particular insect predator. I will use ‘thrips’ and ‘insect predators’ interchangeably, but acknowledge that the culprit may not be thrips in someone else's garden. I believe the methods I am using will work in selection for any insect predation, and I would not be surprised if the resistance factors I am observing for thrips may work for more than one genera of predator insects. But even if each type of insect has to have specific resistance to deter them, the manner of locating resistant individuals with breeding value and selecting for resistance would be pretty much the same. That allows this article to be useful to anyone having an insect issue who might want to breed for natural resistance to insect predators. My specific references here will apply to what I am selecting for, but you can use the process to select resistance to whatever you choose.

A tell-tale example of thrips damage. Note the curved, wavy lines of damage on this flower (above). That is something characteristic of thrips predation, though, with that said, it is not always present, or visibly obvious, on every flower with thrips damage. On some flowers there will be less damage and so one may not see these wavy lines, while in instances with more severe damage, the damage can be so extensive that is merges into larger non-pigmented areas, obscuring the wavy rows making up the area (below). I consider both of these images to show severe thrips damage, but it can get worse than this. Click either image for a larger example.

At times in my life while growing daylilies as garden flowers, we would have dry years, and I would notice that flowers of dark colored and red cultivars often looked deformed and spotty. I just assumed it was drought related, and of course it was. To be specific, I would later learn that times of drought produce thrips swarms in my garden. I was graphically reminded of how bad that can get in the spring of 2013, and again in 2014 and 2015. We have actually had a series of ‘perfect storm’ springs throughout much of the past decade + that have a combination of late, hard freezes in April/May, drought in April/May/June, and then the resultant thrips swarm during the drought. I knew I would be selecting for “thrips resistance” from reading old articles about daylilies. I tried to find plants to purchase with resistance to thrips, but never found any that were known, proven quantities for such, though I did find a few rust resistant cultivars at the start. When those first drought years hit, that gave me a chance to begin to select for cultivars that performed the best in these conditions. At first I wasn’t specifically doing “thrips resistance selection”. I simply marked the plants that performed normally or close to normally. In those first years - 2013 through 2015 - I culled thousands of plants for a total absence of resistance - what I refer to as either a “thrips meltdown” or, depending on your pronouns, an “insect predation meltdown”. 

Such a meltdown can be characterized by the following behaviors - 50%+ bud drop, blasted scapes with no intact buds, extreme enation deformity of buds resulting in severally deformed flowers (especially sepals), and heavy spotting and pigment loss in petal pigmentation. During that period I also set out to confirm that I was seeing thrips instead of another insect predator, which I did to my satisfaction. One important thing that I observed is that the issue of thrips is most extreme in the early-early and early season, moderating some in midseason, and showing little impact in late season in my garden. Early-early and early flowering cultivars have consistently been the worst effected and the hardest area to find plants to breed from, to the extent that I almost gave up on the early season entirely. Luckily, I found a few plants in the early part of the season that showed exceptional resistance and have been able to work from them in establishing early season types that perform well in areas with late freezes and insect predation issues.

A fine example of a thrips "meltdown". I don't know about you, but I don't enjoy this presentation... Some plants will show damage like this on a few flowers at the beginning of the flowering period, but will then start to produce normal flowers and be normal through the rest of the season and into any subsequent rebloom, while other cultivars will show consistent problems with this level of damage throughout the season, when thrips are present. While I don't love the former, they aren't a total loss, and can certainly make good enough garden plants, and may be able to produce useable seedlings, especially when there are other valuable traits like rust resistance or advanced flower trait. The later, I think, are far less desirable and useful in the garden or in a breeding program.

From the year 2013 through 2015, I was not really breeding for thrips resistance, but was, first and foremost, confirming that there even was resistance to the thrips (and whatever other combination of factors were involved, such as late freeze tolerance and drought tolerance), and secondly, if any suspected resistance I might locate was genetic and heritable. What I found was surprising. First, no daylily I tested has ever shown immunity to thrips/thrip-related-issues. Second, those that do show high resistance are extremely rare, far rarer in fact than plants with high rust resistance. By 2016 I had identified a tiny handful of highly thrips resistant cultivars and seedlings and experimental breeding started in earnest. That was eight years ago as of this writing (2024). I feel that I am now in a position to make some preliminary assessments of the project. The most important thing is that there is a strong correlation between high resistance and breeding ability, implying that resistance is genetic and heritable. The key, then, becomes to identify plants with exceptional thrips resistance to begin with. That is easily done in gardens that become dry and also have endemic thrips populations. For other insects, under the right conditions for those insects, selection for resistant individuals should be possible.

One cultivar that I identified with exceptional thrips resistance is Small World Hip-Hop Music. In the image above, from 2015, you can see the flower covered in thrips, but showing no damage. Click the image to see it larger, or blow it up to see it in detail, and you will note that the petals in this image are swarming with thrips. They are the tiny, white things that look like miniature rice grains all over the petals. I want to be clear that this cultivar is not immune, but is highly resistant. In the image below, taken the same day as the flower image above, you can see the buds. Note that none are shriveling to drop off and the buds, of all ages, show very little in the way of enations. For whatever reason, year-after-year, this cultivar showed this same level of resistance. It also can produce resistant seedlings when bred with other highly resistant cultivars, and passes its genes on in a seemingly-recessive form, as I have gotten very resistant seedlings with it as a grandparent, when its seedlings with less resistance have been bred back to other highly resistant plants. I would also stress that the daylilies surrounding this cultivar, in this picture and in the other years it was grown here, showed high thrips susceptibility, while at the same time this one showed little to no damage. 

The next most important thing is to consider the genetic makeup of the resistance trait. To begin, about 15% of daylilies in my garden showed some resistance to thrips. They were not immune, nor did they show exceptional resistance, but they did not suffer “thrips meltdown”. I found fewer than ten cultivars/seedlings that showed exceptional resistance to thrips, and oddly, most of the resistance was in tetraploids, with almost no diploids showing exceptional resistance. Further, I have located no species types that show exceptional resistance (or really, much resistance at all) to thrips. H. vespertina appears to have some resistance, but that may only because it flowers so late in the season. However, I believe that, much like exceptional rust resistance, exceptional thrips resistance is a concentration of multiple genetic factors. 

Because there is something of a bell-curve in observable thrips resistance, that further supports a quantitative trait. In breeding, we see mixed results. Of the resistant individual plants that I identified, breeding experiments were not immediately straightforward. In some instances, resistance appeared to be recessive, not showing up in the F1 seedlings, while the same cultivar, in another cross, would appear to be expressing the resistance trait as a dominant or partially dominant trait. I suspect this implies that the alternate partner in the later scenario was carrying enough recessive genes complimentary to the resistance genes of the resistant parent that resistance appeared in some of the seedlings as an apparent (observable) trait. This further implies that at least some, if not all, of the genes involved are recessive, though some may be dosage specific dominants that have low penetrance in heterozygotes, especially so in tetraploids. 

Extreme scape meltdown from spring 2015. This year saw a combination of issues, including an early spring that caused plants to emerge very early and very quickly, followed by late freezes in April and then again in mid-May, which did considerable damage. Following that last late freeze, we became very hot and dry and thrips swarmed, but I do suspect that this is not just an issue with thrips, but represents a combination of environmental factors - freeze damage and drought - combined with thrips and likely other insect predators, as well. In this garden, in 2015, there were some aphid problems, and so that is likely a major contributing factor. However, in other years with late freezes, but with no apparent aphid issues, I have still seen some scape blast like this, just not as severe as in 2015. I think that year was a perfect storm of multiple issues all colliding to make a jaw-droopingly ugly early-early season. Do note though, that the rebloom scapes emerging on this cultivar look normal, and they did, in fact, go ahead and flower, only showing a normal level of insect predation consistent with what I would normally see for the cultivar. However, there were a tiny handful of early-early and early flowering cultivars that did not show serious effects from this perfect storm, and they were those in the list below, which had shown resistance in previous year and have continued to do so.

So I think that thrips resistance, and perhaps insect predator resistance in general, may be a multigenic, recessive trait - a complex of multiple recessive genes. That makes a program hard to get going (which may explain why there is so much thrips susceptibility in Hemerocallis), but once the genes become concentrated into homozygosity they are very effective and appear with high regularity in the subsequent generations. Recessive traits are hard to work with initially, but once you concentrate your lines with those recessive genes, first as heterozygotes and then later as homozygotes, you see the trait with increasing frequency in subsequent generations of seedlings. However, what it does make more difficult is outcrosses. Once you have a line set for a complex of recessive traits, outcrossing will see the trait “vanish” in the F1 outcross to the unselected gene pool, and it will require either backcrossing to the resistant line or interbreeding siblings to start to bring the trait back out and visibly present. When working with heterozygotes for a multigenic recessive trait, backcrossing to a homozygote for the said trait complex will result in the maximum number of homozygotes (which will still be low, especially in tetraploids), while sibling mating will result in fewer homozygotes, but can produce a few.

In my experience, the most effective strategy is to cross any exceptionally resistant individuals you can find with each other, and cross your fingers while your at it.  They won’t always match up their genes perfectly, and so even in that instance, you may not get a crop of seedlings where all show resistance as high as the parents, but a few might, and in rare instances, you might get a full crop of very good to exceptionally resistant seedlings. The next best strategy (one I have used a lot, and that I think is the most practical) is to cross an exceptionally resistant individual with a very good to highly resistant individual. In more practical language, you cross the best with those that are just a bit less in regards to resistance. These crosses usually produce a crop where the majority are at the level of the less resistant parent, but a few may be more resistant or even exceptional, and all will carry the genes of the more resistant parent and can be backcrossed to that parent or another exceptionally resistant cultivar.

The third type of cross is the ‘salvage cross’ where you cross an exceptionally resistant individual with an individual showing poor or no resistance. This type of cross can be very frustrating, as the entire crop will tend to show poor resistance, but will carry resistance genes. In rare instance, the poorly resistant parent may have carried some of the resistance genes in a heterozygous state, and so a few seedlings within the cross may be more resistant than the poorly resistant parent. Pick those if they occur! However, my experience is that this is very rare, and even then, the “more resistant” seedling(s) rarely show resistance near that of the exceptional parent. The key with all the crosses is to not expect much in the F1 and to take the best of the F1 back to something of high or exceptional resistance, be patience, observe minutia and persist with selection and mating best to best, once you have a few exceptional resistant individuals to work with.

Selection for rust resistance is much easier than selection for thrips resistance, but I would never have been able to make rust resistance selection the center of my program simply due to my location, and there are a good number of daylilies that show rust resistance. I do not have endemic rust, but I absolutely have endemic thrips, and in my environment those “perfect storms” of late freezes and spring drought create frequent and recurring opportunities to make this type of selection, simply by doing nothing but making the observations, and then making breeding decisions based upon those observations. I don’t have to “bring in” thrips and bad weather. So since 2016, thrips resistance selection has become one of the central focuses within my program, building upon the initial observations and culling through the years of 2013 - 2015.

A trick image! Is it a broken patterned flower, or thrips damage? Well, it is thrips damage, visually mimicking the broken pattern, on an otherwise solid purple seedling. I had had people exclaim how lovely this flower is when I have shown them the image. I always think it is like telling a burn victim how nice their skin looks, but maybe that's just me. For me, while I love broken patterns, this is just damage and is not a desirable thing. Note in this image how the large areas of pigment damage are made up lot lots of wavy lines of typical thrips damage.

However, it is important to stress that just because this type of selection is a central focus, that does not mean all my introductions show exceptional thrips resistance. At this point, I have three introductions (listed below at the end of this article) that are truly exceptional in terms of their thrips resistance. All my introductions have been screened for thrips resistance, but because almost no daylilies show much resistance to thrips, and it is a slow breeding process due to the  nature of the genes involved, I am only now beginning to see lines forming where the majority of the seedlings are showing extremely high to truly exceptional thrips resistance. What I have done in my selection process is to eliminate all those cultivars and seedlings that were very susceptible, while retaining those that are above average, but above average doesn’t mean immune or exceptional, it just means better than average. Selection only works within the parameters of what is available. The majority of my introductions show some thrips susceptibility, but all have been selected to eliminate high susceptibility, and so all my introductions are the most thrips resistant/late spring freeze tolerant of their own seedling group. Such plants likely carry some genes for resistance, but are not highly concentrated for the trait, as in exceptional individuals. 

My breeding program is based in twenty-year increments, and I fully expect to have lines with exceptional thrips resistance, and introductions from such lines, by the time I reach the first twenty-year mark within my program. That is only about six years away at this point. With many of my introductions to date, I have been attempting to spread the best rust resistance plants/genes I was able to locate and concentrate within my breeding work, and so I felt it important to introduces such plants to be of benefit to Southern growers who may want to grow rust resistant plants or even breed for rust resistance, even if the thrips resistance was not at the level I would have preferred. I can say that that all of my introduction have been selected to remove high thrips susceptibility and so all are average or above average for the trait when compared to the general daylily gene pool. 

Sun Dragon is the most thrips resistant early flowering tetraploid I have produced and has shown both excellent resistance and breeding value for resistance. The pollen parent of Sun Dragon, Solaris Symmetry, turned out to be both exceptional resistant and a good breeder for thrips and late freeze resistance. It was pure, random chance - randomness, fate or whatever - that it did, as I had ordered it for its dormancy and flower phenotype. Sun Dragon is from Solaris Symmetry crossed to Ancient Elf, one of the four species/species-like base plant in my tetraploid program. Ancient Elf, though, shows moderate to poor late freeze/thrips resistance. Out of the cross between Ancient Elf and Solaris Symmetry, I raised thousands of seedlings and out of those thousands, most showed moderate to poor thrips resistance. A tiny handful showed high resistance, and out of them, the seedling that would become Sun Dragon showed exceptional resistance, as high or even a bit higher, than parent Solaris Symmetry. Through many years of breeding I have noted that Sun Dragon can produce seedlings with good to exceptional resistance when bred to partners showing some resistance. The genes are still not dominant and so outcrosses to poorly resistant individuals usually show moderate to poor resistance, but can produce better seedlings when backcrossed to highly resistant partners, underlining the potentially-recessive nature of the genes. It is my suspicion that plants that show moderate to good resistance are carrying multiple genes, some homozygous, but not enough to make the highly resistant phenotype, which would require a higher number of homozygous genes. Sun Dragon would suggest that Ancient Elf is carrying some resistance genes that are not, or only partially, expressed in the phenotype, but that when enough seedlings are produced, can produce a few examples where all the genes line up in a homozygous fashion, when Ancient Elf is crossed to an exceptionally resistant partner.

Over the last couple of years I have begun to consistently produce the level of thrips resistance I want to see within certain lines, and it is clear to me that I am both recognizing the best breeders for the trait and concentrating the genes in my tetraploid breeding lines. Complex traits such as thrips resistance cannot easily be merely selected for within a random population, because the genes in homozygous combinations are so rare and the genes are so hard to identify in heterozygotes, so they have to be selected for over time through an understanding of how the genes work, identification of exceptional breeding material and concentration of desired genes over multiple generations. In common language, “it’s not easy”, but it can be done with patience, observation and singular focus.

So in closing, I want to give you a list of the exceptional cultivars for thrips resistance that I have been able to identify, in case anyone wants to begin to integrate the trait into their programs. I suspect there should be a few more exceptional cultivars out there somewhere. To identify them, just don’t spray your plants in thrips swarms and watch what happens. Those that remain unaffected, even when thrips or whatever insect predator are swarming all over the plants and flowers, are likely to have resistance and are the place you want to start with selection. Keep observing such plants year after year and cross such plants with other plants showing apparent moderate to high thrips resistance. Observe the seedlings. Once you have identified a plant that is both showing exceptional resistance and can produce seedlings showing exceptional resistance (even in only in the F2, depending what you crossed it with), then you have something to work from, and your breeding and selection for this type of resistance can proceed.

Thrips Resistant Individuals

(These are cultivars that I have identified within my own program as showing exceptional thrips resistance/late spring freeze tolerance and having breeding value for the trait)

From other hybridizers:

Solaris Symmetry

Spider Man

Whooperee

Small World Hip-Hop Music

Tis Midnight

From my own introductions:

Samwise The Brave

Misty Mountains Cold

Sun Dragon

Samwise The Brave

(Photo by Bill Hurt of the cultivar in his garden)

Samwise The Brave and full sibling Misty Mountains Cold are the most thrips resistant diploids I have produced. They both show exceptional resistance, in spite of being very early flowering early-early season cultivars. Both show breeding value for the trait. When crossed to other plants showing some resistance, you will see some seedlings with resistance in the first generation. If crossed to poorly resistant seedlings, you will only see moderately to poorly resistant seedlings, at best, in the F1, but you will see high resistance seedlings in the next generation if the seedlings are sibling mated, crossed back to Samwise or other highly resistant cultivars, or when bred to moderately resistant plants.