The status of an Arava if its leaves fell off:
Bedieved-Letter of law: If majority of the Arava leaves fell off. the Arava is Pasul. One must be very careful regarding this matter, as it occurs that upon inserting the Aravos into the [binding of the] Lulav, and likewise upon shaking the Lulav, that leaves fall off. [It is thus advisable to check the Aravos daily to verify they still contain majority leaves. Likewise, it is advised to purchase a number of sets of Aravos on Erev Sukkos, which can be used in a case of need throughout Sukkos. When replacing the Aravos, one is not to stick the new Aravos into the knot of the Lulav, and rather one is to undo the knots and then place the new Aravos inside.] If only minority of the leaves fell off, the Arava is valid.
Lechtachila-Mitzvah Min Hamuvchar: It is a Mitzvah Min Hamuvchar to take an Arava which contains all its leaves. Thus, although a majority leaved Arava is valid, as explained above, nevertheless, Lechtachila one is not to take such an Arava if finding a fully leaved Arava of which none of its leaves fell off, is easily attainable.
Must majority of the Shiur [i.e. 24 cm] still contain leaves or majority of the entire branch?
Ø Example: If the Arava is seven Tefachim long and is majority/fully leaved within its Shiur of three Tefachim, but is not leaved for the remaining four Tefachim, what is the law?
We follow majority of the Shiur of 24 centemeters and not majority of the branch.
If the Arava was originally Kosher, and the leaves fell off as a result of the shaking, is the Arava now invalid?
The Arava is invalid if majority of its leaves fell off as a result of the shaking. However, some Poskim suggest that the invalidation of an Arava due to the falling of its leaves is only in the event that one did not yet fulfill the Mitzvah with it. If, however, one already fulfilled the Mitzvah of Daled Minim with this Arava, such as on the 1st day of Sukkos, then it remains Kosher throughout Chol Hameod even if the leaves fall off due to the shaking. Based on this suggestion, we can justify the custom of many to not bother to inspect the validity of the Aravos and its state of intact leaves throughout Chol Hamoed. Practically, one is to not rely on this approach, and is to perform a daily insepction of his Arava prior to doing the Mitzvah to verify its Kashrus state.
Directives for Mivtzaim:
Those fulfilling the great and holy Mitzvah, and directive of the Rebbe, to merit other Jews with the shaking of Daled Minim, must be very careful to periodically verify throughout the day that their Aravos remain intact with majority of leaves. This especially applies when people shake the Lulav in a very strong way. A number of sets of Aravos should be brought with the person so he can change the branches as deemed necessary.
 Admur 647:5
 Admur ibid; Michaber 647:2; Tur 647; Mishneh Sukkah 33b as understands Rosh Sukkah 3:13
The reason: As it is no longer considered Hadar. [ibid]
 Admur ibid; M”A 647:1; Elya Raba 651:6
 Bikureiy Yaakov 654:4; M”B 654:4
 Admur ibid; Michaber 647:3; Mishneh ibid
 Admur ibid; M”A 647:3 in implication of Michaber 647:2 that Lechatchila one should not use such an Arava; Machatzis Hashekel ibid; Kaf Hachaim 647:19; Alef Hamagen 647:13; Nitei Gavriel 22:2
Other opinions: Some Poskim rule the Arava is Kosher even initially if it is only missing minority of leaves. [Elya Raba 647:6, brought in Machatzis Hshekel ibid; Bikureiy Yaakov 647:9; Chaim Ubracha 223; See Nitei Gavriel 22:2]
 Zechor Leavraham 6 Lulav; Mishnas Yaakov 647; Nitei Gavriel 22:4; See Admur 646:3 regarding the Hadas that the majority Meshulash leaves are only required within their Shiur of 24 cm!
 Admur ibid and M”A ibid “And likewise through the shaking”; Setimas Kol Haposkim; See also Admur 649:18 and Michaber 649:5
 Hisorerus Teshuvah 3
 The reason: As it is considered “Shiyurei Mitzvah” or Mitzvah leftovers. Now, although we find no precedence in the Poskim regaridng such a concept regaridng the Hadas or Arava, we do find it regarding Tzitzs, that if it later tore to below the Shiur, it nonetheless remains valid under certain conditions. [Hisorerus Teshuvah ibid]
 So is clearly implied from Admur ibid who warns one to be very careful with the shaking that a) It is invalid for the rest of Sukkos if it falls off and we do not apply the rule pof Shiyurei Mitzvah; b) One has an obligation to verify this matter, hence the warning of “One must be very careful”