The following are a number of points, especially emphasized with regard to the tomato plant, to ensure optimal utilization of the bumblebees in order to receive quality yield under high temperatures:

1) The tomato plant.
 The process of fruit-set will be divided into 5 different stages. In each stage, the effect of high temperatures (and sometimes relative humidity) will be stressed, based on results from various studies. It should be borne in mind, however, that the effects might vary between different tomato varieties.

1.1) Pollen Production and Quality
 Exposure of the plants to a temperature of 40°C, for 3 hours over two consecutive days, during 5-9 days prior to flowering, seriously harms the vitality of the pollen grains and hence the fruit-set. 
 Already at temperatures of 32°C and above, there is a decrease in the vitality of the pollen. 
 From 32°C and above, the quantity of pollen produced in the flower decreases.

1.2) Pollination  
 Higher temperatures (39°C during the day and 23°C at night) cause the pollen to loosen with difficulty. 
 Optimal relative humidity for pollination is about 70%, although there is no great difference between 50% to 90%.As to the female part of the flower: 
 At a temperature range of 33°C-38°C (differences exist between varieties), the style lengthens considerably, while the stamens do not lengthen enough. As a result, the stigma extends beyond the cone of stamens. This situation markedly decreases the chance for a proper pollination and fruit-set. 
 The stigma itself is receptive to pollen from 4 days before the flower opens until 4 days after its opening. The duration of its highest receptivity is also dependent on temperature.

1.3) Germination
 Temperature of 35°C for 5 or more consecutive hours adversely affects pollen germination. 
 An excess of nitrogen may cause the pollen to become sterile.

1.4) Growth of the pollen tube  
 At a temperature of 35°C, the pollen tube completes its growth within 7.5 hours. At 38°C, the tube lengthens slowly.  
 Relative humidity between 50% to 90% is good for the growth of the pollen tube. The higher the relative humidity, the quicker it grows.

1.5) Fertilization
 Exposure of the plant to a temperature of 40°C for only 4 hours, following normal pollination, harms the ovary and causes degeneration of the seed-buds up to 96 hours after pollination. Thus even if high temperatures have not harmed pollen formation and/or its vitality, nor the receptivity of the stigma, nor the capacity of pollen germination and/or growth of the pollen tube, the damage to the ovary and seed-buds will prevent fruit-set and will cause premature drop of flowers.

In general, it can be stated that at temperatures above 35°C, there is a risk of damage to at least one of the stages leading to normal fruit-set of tomatoes. This risk exists from 9 days before flowering until 4 days following flowering. Any means of lowering the temperature in the greenhouse, for example: overhead sprinkling, whitewashing the roof, intensive roof ventilation and dust-free netting, will reduce the harmful effect of high temperatures on the plant and will increase the chance of an appropriate fruit-set.

2) Care of the bumblebee hive.
 Once the bumblebee hive is placed in the greenhouse, every measure (see below) should be taken in order to allow the bumblebees to function at high temperatures. This is in addition to the special structure of the hive, with its multiple ventilation slits, which helps the bees to regulate temperature at the close proximity of the brood.

 Be aware of the proper location of the hive in the greenhouse. Place it in the coolest part of the greenhouse, under the shade of the plants. The height of the hive on its stand should be as low as possible, 10-20 cm. above the ground. At the beginning of the crop it is desirable to place a special shade over the hive, as the plants are too small to provide the appropriate shading. 
 Avoid introducing the hive into the greenhouse (either for first introduction or placement following chemical treatment in the crop) during the hottest period of the day. Introduce the hive either early in the morning or late in the afternoon (at least 2 hours before dawn) when temperatures are relatively mild. 
 During a continuous heat wave, remove the hive from the greenhouse from 10:00 in the morning until 4:00 in the afternoon. During these hours, keep the hive in a relatively cool place. The bumblebees will function sufficiently early in the morning and late in the afternoon. One should bear in mind that during the hottest hours, the bumblebees are busy cooling the hive anyway. Thus exercising this procedure will enable a proper pollination and at the same time will maintain a viable bumblebee colony. 
 Hormones  can be used to backup the bumblebees in the case of problems in pollination and fruit-set during the hot period. Since hormones do not affect pollen production, but rather the female parts of the flower, they do not interfere with activity of the bumblebees. It should be stressed that the first truss of flowers that develops on a young plant, which is incapable of self-cooling, is especially vulnerable to heat stress. Later trusses, like the 6th and 7th ones, may also express improper fruit-set but mainly due to over loading of fruits on lower parts of the plant or imbalanced growth of the plant.

In summary, it should be emphasized that the condition of the flower during the hot season could be such that the visit of the bumblebee, accompanied by the typical visitation mark on the cone of stamens, will not result in a proper fruit-set. In this case, do not automatically "blame" the bumblebee for lack of fruit-set, but take into account the constraints of the plant itself under these conditions.