This problem usually usually occurs on strawberries that ripen in the mid and late season. Berries fail to size and color up properly and retain a hard "seedy" texture like a green/white strawberry. The most complete technical discussion of this problem is contained in an old web page by the New Brunswick (Canada) Department of Agriculture and Rural Development -- click here. This New Brunswick web page is particularly important because it mentioned "flower thrip" damage can occur even on immature fruit. Seedy Strawberries were a major problem for us in 1994, 1996, and again in 1999. In 2001 we "confirmed" that thrips were the problem.
In 1994 this problem occurred from Missouri to eastern Ohio in a narrow band roughly 50 to 100 miles north & south of I-70. After the fact analysis by Purdue and University of Illinois extension personnel "leaned" toward the explanation that a very small insect pest called the Eastern Flower Thrip was feeding on the Strawberry blossom and causing a deformity in the berry. Up to then, there had been no reports of thrip damage on strawberries for over 70 years.
In 1996 Lakeview Farms experienced another severe seedy berry problem but evidently very few other reports of this damage were made throughout the Midwest.
In 1998 I noticed relatively high populations (5 to 10 thrips per blossom) of these tiny insects feeding on our strawberry blossoms and ran a test spraying some strawberry blossoms and leaving some blossoms unsprayed. Neither the sprayed or unsprayed blossoms ever developed into seedy berries casting some doubt in my mind as to what the real culprit was. Soil moisture was relatively high.
In 1999 I noticed relatively low levels of thrips (2 to 4 ) feeding on the blossoms and elected not to spray based on the results I had seen the previous year. (see Purdue Newsletter) Seedy berry damage for 1999 unfortunately turned out to be especially severe! Soil moisture was relatively low. In 1999, Seedy berries were also a problem in Kentucky with low soil moisture and even more severe a problem in Iowa.
During the 2001 strawberry bloom we again started to see thrip levels building in the field so we decided to repeat our study of 1998 where some strawberry rows we sprayed and others were left unsprayed.
The variety I chose for the study was Allstar primarily because we (unfortunately) have lots of experience with how it looks in "seedy berry" years like 1999, 1996, and 1994. Allstar is a relatively late berry compared to varieties like Earliglo, Northeaster, and Honeyoe and generally prone to much more damage. Seedy berry damage symptoms on Allstar are slightly different from all the other varieties we have grown -- besides the seedy appearance, poor color, smaller size, and dull, bronze like finish Allstar (unlike other varieties) will usually have cracking like powdery mildew on west coast seedless grapes. I took digital pictures of the berries we counted in each observation.
Here is a picture of what a damaged Allstar berry looks like: See underlined links in Table 2 below for more pictures.
Thrip counts began April 27 roughly one week into the Allstar bloom period. Ten to twenty blossoms were placed into a sliding zip lock bag and three drops of ethyl acetate (nail polish remover) added to keep the thrips from moving around. It is my opinion that the ethyl acetate made counting easier and more accurate. The zip lock bag was put over a white sheet of paper and thrips counted with the aid of a 4X magnifying glass. While both adult and juvenile thrips were counted the juvenile population was never more than 20 to 30% of the total. All but three check rows (300 feet each side by side) were sprayed with a Endosulfan/Diazinon insecticide mixture on May 2. Thrip counts in the sprayed area on May 3 were zero.
I failed to check thrip counts in the sprayed area again until May 8 and was alarmed to see they had built up to 13.2 ( very close to the unsprayed control levels) so another application of Endosulfan (alone) was made on May 9. Once again, thrip counts in the sprayed area were essentially zero the day after spray. I made the mistake of putting the three check rows upwind of the sprayed controls so this might possibly have accounted for the rapid recovery of the thrips after the first insecticide application.
Click here to view a picture of the study layout. The small berry size of the 6/1/2001 and 6/5/2001 samples for both sprayed and unsprayed control plots is not representative of what is commercially saleable -- none of my U-Pick customers would pick normally pick all the ripe berries regardless of size. All this data is summarized in Table 2 below:
|No Spray West Control||pictures||pictures|
|% Seedy by count||55%||54%||13%|
|No Spray East Control||pictures||pictures|
|% Seedy by count||57%||57%||14%|
|% Seedy by count||0||0||0|
|% Seedy by count||0||0||0|
It is obvious from the data that application of an insecticide to kill thrips can totally eliminate seedy berry damage. There are some remaining, unanswered questions, however.
1. Why did the last counts in the no spray controls show dramatic improvement? I have never seen thrip damage improve as the season progressed. Even though we did not do a formal count on our last day of the season, I could find no seedy berries in any of the unsprayed rows! The only environmental difference between the mid bloom versus late bloom Allstar that I can recall is that a relatively dry soil condition changed rapidly to a relatively wet soil condition.
2. Why did berry damage become obvious so early. Since thrip levels did not reach critical levels ( >2) until May 3, I would not have expected to see damage until June 3. Could the thrips be feeding on young berries after the flower petals have dropped? I have noticed that thrip counts on "older" blossoms (ones that have some brown coloration) generally have more thrips than freshly opened blossoms.
I think the New Brunswick web page mentioned at the top of this page explains why thrip damage showed up so early in 2001 ( item 2 above) and possibly why we had heavy damage in 1999 at relatively low thrip blossom counts ( I stopped sampling too early ). Evidently, thrips continue to damage strawberries even after blossoms drop. It is my guess (for what it is worth) that strawberry plants whose roots are under any stress like: low moisture, black root rot, anthracnose crown rot, red Steele... are more susceptible to thrip feeding damage.
My advice to a strawberry grower seeing the first "orange cast" symptoms of thrip berry damage on immature fruit would be to spray immediately with a low PHI insecticide at high pressure and water volume even if no strawberry blossoms were present -- at a minimum you still might be able to save the later maturing berries since there is some evidence to indicate "flower" thrips also damage green fruit .
Of course, a better alternative would be to stop a problem before it started with good scouting. A very rough calculation over the last ten years showed that flower thrips cost me about $65, 000 in lost strawberry revenue so the cost of "information gained" in this manner can be painful.
I I would be remiss if I failed to mention the person who threw out the challenge to attendees at the Illinois Small Fruit and Vegetable Conference in 2000 that this problem could only be resolved by on-field grower trials was Dr. Rick Weinzierl, University of Illinois Entomology. We all appreciate the help Rick has provided to Illinois (and out of state) growers over the years.
Some later work (2002) by researches in Iowa showed that the Western Flower thrip ( Frankliniela. occidentalis ) can overwinter in Iowa and that higher thresholds of thrips on blossoms and fruit do not always translate to berry damage.
GETTING A GRIP ON THRIPS IN STRAWBERRIES M.Y. Steiner, S. Goodwin in NS Wales, Australia from the V International Strawberry Symposium(May 2006) ---the abstract seemed to suggest that the most severe damage to strawberry fruit occurred two weeks after bloom on young green fruit but I did not have access to the full text of the research. Initially, the primary thrip in NS Wales was western flower thrips, Frankliniella occidentalis .