Developing a Unit for Vacuuming Insects and Increasing Aerodynamic

Experimental models for insect vacuuming, inside greenhouses and in the open field
July 28, 2014

The Israeli climate encourages insect proliferation and insects cause considerabledamage to greenhouse vegetables, especially to herbs raised in net houses. Despite the use of fine-gage mesh in net houses and frequent pesticideapplications, herbs like chives and parsley are liable to become infested andsuffer extensive damage. The fact that herbs grown for the kosher market must bepest-free to comply with strict Jewish dietary restrictions is an addedconsideration.

 

This study was designed to develop and test experimental models for insect vacuuming, both tractor-mounted and hand-held, inside greenhouses and in the open field. Previous experiments with an insect-vacuuming unit had shown that it was fairly effective at reducing insect populations and removing insects from the plant canopy. To increase the number of insects removed from plants, especially in their flightless stages, two main types of vacuuming units were built for the current study. The first system resembled a previous model and included only suction. The second system incorporated both suction and a blowing jet. The typical suction velocity was 9 m/s (meters per second) at the inlet portal, and the blowing velocity was approximately 10 m/s. This combination of vacuuming/blowing exploits the principle of maintaining the velocity of blown air, compared with suction alone, and results in higher drag forces and greater numbers of insects removed from the plant. In the system based on the principle combining vacuuming with blowing, an average 2.9 times reduction in thrips numbers was achieved in chive greenhouses, as compared with thrips reduction in treatments without suction. A marked reduction in leaf damage from thrips was also achieved in chives, and averaged 1.8 times less than in crops treated without suction.

The number of pesticides and pest-control methods available to greenhouse vegetable growers is limited, particularly in organic crops. The problem is exacerbated because many insects develop resistance to pesticides. Added difficulties when combating pests are the propensity of small insects to penetrate even fine-mesh greenhouses, and demands to reduce toxic residues in market produce.

 

Chives (Allium schoenopransum), are harvested several times in a growing season and make up an important part of Israel's herb exports. In recent years, growers have encountered difficulties in exporting chives because of serious silver-spot leaf damage caused by Onion Thrips (Thrips tabaci) feeding on the plants. If silver-spot damage on chives is less than 30%, it is usually possible to select undamaged leaves for market. If rates are higher than this however, selection is not viable and either the crop is sold as second-class produce or it is completely non-marketable. Thrips damage in chives has become more problematic with the extension of the marketing period into the summer season, as well as the reduction in the number of authorized pesticides and the reduced efficacy of permitted pesticides as a result of resistance buildup. Once thrips have gained entry to greenhouses and become established in chive crops, it is very hard to eliminate them with chemical treatments. Thrips tend to hide in the narrowest parts of the plant, where they feed and multiply. Natural predators and pesticides have difficulty reaching these areas and are therefore largely ineffective. Consequently, alternative methods are required that do not rely on chemical pesticides to reduce the number of thrips.

 

The current working hypothesis is that insect vacuuming and removal can be used throughout the growing season at regular intervals, as an alternative form of pest control. In addition, with a blower-based insect-vacuuming unit a net bag can also be attached at the air outlet to collect the vacuumed insects. In growing tunnels where tractors can access the beds and move along them, the vacuum unit can be mounted on a tractor. For smaller tunnels, small hand-held machines can be developed.

It was found that in order to ensure that insects were dislodged from the plants even when flightless (for example in the larval stage) higher drag forces were required than when vacuuming flying insects. The study set out to develop an effective mechanism to reduce insects in herb and leaf-vegetable greenhouses, by dislodging and vacuuming the insects in order to achieve good pest-control results.

 

Materials and methods

The trials conducted in the study examined whether insect removal and control were improved by the use of blowing air in addition to vacuuming. Two prototype suction units were designed, built and tested. The first unit included only vacuuming. The second unit included both vacuuming and a blowing jet. The study specifically examined the premise that even when surrounded by foliage, and with a geometry of narrow airways, the drag-force advantage produced by the flow of blown air is maintained.

 

The influence of air velocity on dislodging force, and estimates of required intensity

To examine ways of increasing the dislodging force, the study examined the influence of air velocity on the drag forces; it also estimated the blowing velocity required to dislodge an insect from a plant, assuming that the dislodging velocity is greater than the hover velocity, or terminal velocity, of the bodies that have to be dislodged.

The hover velocity (terminal velocity) of a physical body can be calculated using the formula:

Vterminal = √(2mg/CρA)

Where:

A = cross-sectional area of body

m = body mass

g = gravitational pull

ρ = fluid (air) density

C = drag coefficient (for current geometric conditions, usually approximated to the value of 0.5.)

 

The body to be caused to hover:

Young thrips larva - length 0.6 mm, weight 0.01 g.

Adult thrips larva - length 0.8 mm, weight 0.02 g.

Adult thrips fly - length 1.2 mm, weight 0.04 g.

The drag force on an adult thrips fly is relatively higher and the hover velocity is lower, since the fly has appendages like wings. In order to find the maximum value of the hover velocity, therefore, we will calculate the drag force needed to cause a large larva to hover, because of its large weight-surface ratio. For a 0.4 mm-diameter larva in a horizontal position, the resulting terminal velocity will be 1.6 m/s. In a vertical position, the resulting hover velocity will be 2.5 m/s. This means that the dislodging velocity required must be greater than 2.5. m/s. From the above formula, it is also clear that the force applied is relative to the velocity squared, so that the velocity has a significant impact on the drag forces.

Based on the above measurements determining the air velocity required for vacuuming and blowing, a unit was built that combined air suction with blowing. The unit was mounted on a tractor with a three-point hitch, and incorporated a blower adjusted to 1m-wide beds. The air velocity in the middle of the vacuum inlet was set at 8 m/s. The blowing jet aimed in the direction of the plants was located in front and blew an air-jet measuring 10 m/s in the center of the outlet portal.

 

Pest-control trials in parsley

Insect-vacuuming trials in parsley were conducted in tractor-accessible growing tunnels belonging to the Hasalat Company, combining air-stream suction and blowing - Model II. The tunnels were 50 m long and 6 m wide. Random samples of parsley plants were collected throughout the tunnel, before and after vacuuming, in six cycles. The trials were conducted using the maximum air-velocity settings on the unit - 9 m/s for suction and 10 m/s for blowing - as well as at half these maximum velocities. In each sample, the remaining insects were counted by skilled workers.

 

Onion Thrips pest-control trials in chives

Onion Thrips pest-control trials in chives were carried out at two locations - one in tunnels belonging to the Bar family at Moshav Kemehin and the other in experimental growing tunnels at Neve Yar farm. The size of these tunnels ranged from a third to half a dunam (1 dunam = 1000 m2); since the tunnels were relatively confined, small units of both types were used - one including only vacuuming and the other with suction and blowing. The unit tested at Neve Yar had better control of the air outlet jet. In addition to the suction system, either one or two blowing jets could be operated.


Results and discussion

Decline in air velocity with vacuuming and blowing

In conditions of access similar to those in which the second type of insect-vacuuming unit was used (including vacuuming and blowing), it was found that the air velocity declines at a far slower rate when blown than when vacuumed.

These results indicate that in order to dislodge the insect from the plant, it is best to first apply blowing forces to dislodge it and then, while the insect is hovering in the air, vacuum velocities greater than 2.5 m/s are required to propel it up into the net trap.

Results of insect vacuuming in parsley using a unit with vacuuming and blowing air jets: 1 - control without vacuum, 2 - low air velocity with a single vacuum jet, 3 - low air velocity with two vacuum jets, 4 - maximum air velocity with a single vacuum jet, 5 - maximum air velocity with two vacuum jets. Low air velocity - 5 m/s, maximum air velocity - 10 m/s.

 

Trials in dislodging and vacuuming Onion Thrips in chives

Table 1 - Results of Onion Thrips pest control in chives using a vacuum and blowing unit - Kemehin, March 2011

Treatment

Zero count - March 13

Count - March 30

Reduction compared with initial values - X

Number of thrips

SD

Frequency

Number of thrips

SD

Frequency

Pesticides

0.2

0.6

10

2.4

4.9

60

0.1

Daily vacuuming + pesticide application every 10 days

24.7

28.8

100

5.6

5.5

100

4.4

Daily vacuuming

7.5

17.6

50

2.2

3.3

50

3.4

 

SD - Standard Deviation

 

Table 2 - Results of Onion Thrips pest control in chives using an improved model of the vacuum and blowing unit - April 12

 

Plot

Treatment

Level of infestation

Rate of damage

Number of thrips

SD

% with damage

SD

1

Without vacuuming

43.2

25.7

81.0

10.8

With vacuuming

13.0

6.4

36.0

8.2

Reduction compared with control - X

3.3

2.3

Significance

1.7E-02

3.8E-05

2

Without vacuuming

46.2

19.8

78.0

15.7

With vacuuming

18

13.3

57.0

18.6

Reduction compared with control - X

2.6

1.4

Significance

1.5E-02

4.5E-02

 

SD - Standard Deviation.

 

Statistical significance: in the applied treatments, significance was tested using the t- (one-tail) test, in which significance = 4.5E-02 or p<0.045.

 

Summary and results

The results show that the rate of suction-velocity decline in a pneumatic insect vacuum is greater than the rate of blowing-velocity decline, even with narrow passage geometries and when vacuuming above dense foliage. It is therefore important to incorporate an air-blowing jet in the unit in order to amplify drag forces and dislodge larvae from the plant. The suction air velocity must exceed 2.5 m/s to ensure that insects such as Onion Thrips in chives are vacuumed once they are made to hover. Insect vacuuming and dislodging trials in parsley show that a unit capable of both suction and blowing is effective in significantly reducing insect populations found in plants.

Insect vacuuming and dislodging trials in chives show that suction combined with air-blowing jets results in 2.9 times the reduction in the number of thrips on average, compared with treatment without blowing jets.

 

 

Authors:

Rafi Regev1, Shmuel Gan-Mor1, Aaron Weisblum1, Yiftach Afgin1, Michael Chen2, David Ben-Yakir2

1The Agricultural Engineering Institute, the Agricultural Research Organization, Bet Dagan

2The Plant Protection Institute, the Agricultural Research Organization, Bet Dagan

 rafi@volcani.agri.gov.il  

 

From the publications of the Agricultural Research Organization, Beit Dagan, Article #732/11

(Published in ISRAEL AGRICULTURE, 2013)

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