Varroa destructor is the main culprit for the death and reduced populations of overwintered honey bee (Apis mellifera) colonies in Ontario, Canada

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Table III

Effect of different fall factors on winter mortality for 408 honey bee colonies found alive or dead the following spring in Ontario, Canada. P based on Chi square tests.

Factor	Condition⁵	No. alive	No. dead	%Mort/Total⁶	P
Bee population¹	Low	122	77	69.4	< 0.0001
	High	175	34
Food reserves (kg)	Low	140	75	67.6	< 0.001
	High	157	36
Varroa mite	Positive	214	95	85.6	< 0.01
infestation²	Negative	83	16
Tracheal mite	Positive	13	12	10.8	0.0159
infestation³	Negative	284	99
Nosema disease⁴	Positive	76	38	34.2	0.0833
	Negative	221	73

Number of frames covered by bees per hive;

number of mites per 100 bees;

number of parasitized bees in 100;

⁴

number of spores per bee;

⁵

colonies were classified as having low or high food reserves or bee populations if the values for these variables were below (for low) or above (for high) the means obtained from all colonies used in the study (low < 23.6 kg; high > 23.6 kg for food reserves; low ≤ 7 frames with bees; high ≥ 8 frames with bees for bee population; n = 408);

⁶

percent mortality calculated from the total number of colonies that died during the winter (n = 111).