The Plague of Locusts was different from the other Makkos in a few ways. One difference is that it rested on Shabbos. The Baal Haturim learns this out from the two words וינח rested, once mentioned concerning Shabbos, and the other by the Locusts. This teaches that the Locusts did not destroy the crops on the day of Shabbos which is a detail not mentioned by any of the other nine plagues. What then is the special relationship that Locusts have to with Shabbos?
Secondly, the passuk says Hashem turned back a very strong wind and it carried the swarm of locusts and plunged them into the Sea of Reeds. Why is it that in the Plague of Locusts we are told the place where Hashem got rid of them? By the Makkah of ערוב it merely says that Hashem removed them from Mitzrayim. Thirdly, why was the destination of their death in the Yam Suf of all places?
One of the unique characteristics of the Locust species is that it has no king or queen leader which is in contrast to other species such as the bee, ant, wasp. One would therefore think that without a leader, the locust colony would be dysfunctional from the mere lack of direction. Yet Shlomo Hamelech tells us in Mishlei.מלך אין לארבה ויצא חוצץ כולו (30,27). Even though the colony lacks a king and organizer of unity and function, it nevertheless goes out to task with a sense of oneness and direction. Unlike the saying in numbers there is strength, for the locusts it is rather "in unity and achdus are where the strength lies. As the Bais Yaakov from Ishbitz writes (אות כב) that he message of Shlomo Hamelech is that every locust possesses within itself the koach of kingship and leadership qualities.
The root and source of anything contains everything. The more it spreads out and distances itself from the source, the more it becomes diluted, compromised, and loses its strength and vitality. It is in this area that the locust is unique. Hashem created every locust to possess the characteristic of a king who knows exactly what to do and is therefore connected to the source which is found in itself.
Now we can understand why the demise of the locusts was in the Yam Suf.
The Sfas Emes (בשלח תרנ"ד) asks why was it necessary to have the entire miracle of Yam Suf when they were already freed from complete servitude at their departure from Mitzrayim?
אבל הענין הוא כדכ' מצרים נוסע. שראו שרן של מצרים כו'. ויראו מזה. ומרע"ה ניחם אותם אשר ראיתם כו'. והענין הוא כי באמת כן עשה הקב"ה להיות למצרים אז עלי' ממש עד כל השורש שלהם. ואח"כ נאבדו הם עם השורש. וז"פ סוס ורוכבו. וזש"כ רמה וכ' ירה. כי נתעלו מקודם הירידה. שיפלו עם השורש. וזה גאה גאה שלא כדרך ב"ו כשמנצח מפיל השני. אבל הוא ית' הרים אותם ונעקרו עם השורש. וז"ש אחד מהם לא נותר. פי' אפילו שורש האחדות וראש בנין שלהם לא נותר. וכזה נתקיים בכל שבת. דבערב שבת הסט"א מתגברת לעלות. ויורדין עד נוקבא דתהום רבה. כמ"ש בפרוח רשעים
כמו עשב ויציצו כל פועלי און להשמדם עדי עד.
The Sfas Emes explains that even though they were freed from the servitude to Pharaoh, however from the servitude of Pharaoh's source, the heavenly angel in charge of Mitzrayim, they were still shackled and prosecuted. Many times we are successful in ridding ourselves from undesirable, unhealthy, manifestations of symptoms, but nevertheless we fall short of eradicating the cause and source of the sickness. At Kerias Yam Suf, not only did the Mitzrayim on earth meet their death in the sea, their angel in heaven the chief fault finding prosecutor to the Heavenly Tribunal against Yisrael also met its demise. This is hinted in the words ומצרים נוסע אחריהם the angel of Mitzrayim was chasing after them and that is why the Torah used the singular lashon of נוסע and not .נוסעים At the miracle of the Red Sea, Hashem stripped Mitzrayim from its power, all the way up to the source, to the angel appointed over the life energy of the Egyptian nation.
This interpretation is revealed in what seems to appear as the essential phrase in the Shiras Hayam סוס ורוכבו רמה בים. Rashi explains that the passuk is describing the death of the Mitzrayim whereupon the horse and the rider always remained attached to each other throughout their episode of death. Together they were lifted up high and then thrown down into the sea repeatedly. Why is their attachment together horse and rider such a noteworthy fact that even in Miriam's shira this was the only phase sung?
The profundity of this proclamation attests to the demise of the angel together with Mitzrayim. The horse is symbolic of the nation upon whom the rider, its angel who supplied them with a life force, also perished. Together they were destroyed, the earthly part of Mitzrayim with the spiritual sustainer of Mitzrayim its malach.
This could explain the extra ordinary structure of the Shira which is written with unusual large spaces between pessukim and phrases אריח על גבי לבנה. This is in order to convey that our song of salvation is the victory over not only the physical visible enemy the Egyptian nation, but also over the spiritual energy force that is always concealed and hidden from the naked eye.
The Medrash says (Shmos Rabba 22,3) that the seventh day of Yetzias Mitzrayim the day of crossing the Yam Suf paralleled the seventh ,day of the week Shabbos. The Sfas Emes alludes to this connection between Shabbos and Kerias Yam Suf above. However the relationship between the two is even deeper. On Shabbos we receive the neshamah yesairah which connects us to our source above. Chazal tell us that an Am Haaretz if he is asked to whether he tithed his maaser on a particular food he is believed and trusted to have done so if he was questioned specifically on Shabbos but not during the week. Chazal say the fear of Shabbos us upon him and he would not say a lie. Here we are talking about an Am Haaretz an unlearned individual and yet on Shabbos he is transformed into an individual who is bonded with the emes!
In other words we can say that there is a סוס ורוכבו a horse and rider from the side of tumah, and there is a horse and rider from the side of kedusha. The neshama that is on earth inside of a Yid is like a סוס and the root neshamah in shamayim is the רוכב . On Shabbos the horse and the rider are bound together stripped from any existing mechitzos or chatzitos that would interfere or veil the purity of its deveikus to one another. At Kerias Yam Suf all Yisrael had the revelation of סוס ורוכבו and they even pointed with their fingers saying זה אלי ואנוהו which Rashi says (Shabbos 133) that the word ואנוהו is a combination of two words אני והוא that we are essentially bonded with Hashem by our two neshamos , one in our bodies and one in its source in shamayim.
On Shabbos, Hashem brings us to His treasure chamber. This conveys that on Shabbos we are gifted with a private showing of Hashem which is inaccessible during the week. In other words on Shabbos we are elevated to our roots where the purpose of our mission in this world is clear beyond any doubt. This relationship between Yam Suf and Shabbos is evident from the gematriah of כי גאה גאה סוס ורוכבו רמה בים which equals שבת and its three letters.
After understanding the above we can now return to answer how the plague of Locusts relates to Yam Suf and why they perished specifically inside the sea. The common denominator between Shabbos Yam Suf and the Locusts is that all three relate to the שורש roots. Shabbos connects a Yid to his roots to his neshemah yesairah in shamayim. The Zohar says that Shabbos is the source of all beracha in the world כי היא מקור הברכה. The miracle of Kerias Yam Suf related to destroying the root of Mitzrayim in shamayim which is their malach. The locusts was a makah in which there was an overt demonstration of the mitzvah of keeping Shabbos as the Baal Haturim wrote. It is because the locusts do not have a king to direct instructions in order to have a functioning colony. The fact that it possesses a highly sophisticated Locust society of achdus only shows that everyone in the colony has kingship within themselves and are the source and root of life energy to the colony. The Locusts being plunged into Yam Suf was a harbinger of the final fate destined to Mitzrayim, that of the Egyptians together with their angel will perish in the Yam Suf סוס ורורבו ירה בים .
Just like we find in all the other makkos that it was according to the seder נגוף למצרים ,ורפוא לישראל a plague to Mitzrayim and a refuah to Am Yisrael, so can be said of the makka administrated to Mitzrayim and their malach in the Final Blow. That Yisrael benefitted from the makka of סוס ורוכבו רמה בים of Mitzrayim, for Am Yisrael received the experience of .רוכב שמים בעזרך Rabbeinu Bachya Bamidbar 14,17 on this passuk
כי כשישראל מקיימין התורה והמצות הש"י רוכב שמים, וזהו (דברים לג) רוכב שמים בעזרך, כלומר בעזרת ישראל משפיע כח במדותיו, ובזה מוסיפין כח בגבורה של מעלה שנאמר (תהלים ס) באלהים נעשה חיל, וכשאין מקיימין התורה והמצות הוא מתרחק ממדותיו שנאמר (דברים לג) ובגאותו שחקים, כלומר שהוא מתגאה, מסתלק למרום מרומים ובזה משפיע כחו במדות ובזה מתישין כח של מעלה, שנאמר (שם לב) צור ילדך תשי.
When Am Yisrael fulfills mitzvos we thereby add strength to Hashem and he pours down shefa from His middos. Hashem is the rider who rides and is elevated from our Torah and Mitzvos and thereby bestows upon us berachos.
Tu Bishvat is the Rosh Hashanah for Trees. It is on this date that the sap begins to ascend the roots, start the revitalization process, which will end with the delivering of its fruits on the branches. It is fitting for Tu Bishvat to fall on the week of the Shirah which is read on Shabbos for all three deal with the tikkun olam which comes from the closer connection of Am Yisrael to their sources of divine spiritual life.