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The topical treatments of hopper bands with IGRs and botanicals carried out in 1993 caused the complete dispersion or elimination of the treated bands. However, due to the influences of predators there were considerable difficulties in the evaluation of these tests. These problems were overcome in the 1994 season by conducting improved control experiments. Here, after treatment, the insects (4 x 50 individuals for each treatment group) were transferred to field-cages (2x2x2 m) set up 0.5 to 2 km from the test areas. In some cases, the changes in the population densities on the treated and untreated plots were also determined (cf. 2.1.2.1).

The occurrence of hopper bands in 1995 was confined to several zones in the west and south-east of Mauritania. Those bands with numbers of individuals of up to 6 million per band were scattered across wide areas. Therefore, topical applications of IGRs and botanicals were carried out at sleeping and resting places.

In both 1993 and 1994 barrier treatments were carried out using triflumuron (50 g a.i./l). In both years mortality rates of up to 80 - 90% were obtained. However, the maximal effect was not reached until after 16 - 18 days and the type of application (wind drift or blower) was of crucial importance. The areas protected by these barriers was 40 - 60 ha. Fig. I shows a sketch of one of these test areas with the sampling plots used for estimating population densities. Untreated control sampling plots 3 - 5 km away from the test plots were included in order to avoid population mixing and contamination due to wind drift. The larvae in the test areas were 70 - 80 % solitary and transient forms. The daily migrations were seldom more than 200 m.

Fig. 1 Test area, with the barriers and the location of the sampling plots drawn in. Because of limited migration movements, a barrier width of 10 m and a spacing of 100 m were chosen. In the sampling areas A, B, and C, 10 plots each were examined daily (size 1 m², selected randomly), all larvae present were collected, and their number, status and stage determined.

The total numbers of larvae found in the sampling areas are shown in Fig. 2. While the number of larvae in the control areas remained relatively constant, in the treated areas it decreased by 70 - 80 %.

Fig. 2 The total number of larvae in the treated and control areas. The data are the means ąSD for 2 (controls) or 3 (treated) sampling areas with 10 daily determinations each.

Fig. 3 Change in the composition of the populations on the control and treated areas. The evaluation of the data was analogous to the description for Fig. 2.

The changes in the age distribution of the larvae from the treated and untreated areas were quite different. In the untreated areas the decrease in L5-larvae was accompanied by an increase in the L6 stage. Their decrease was in turn matched by an increased appearance of adults. A completely different situation was found on the treated areas. Although the decrease of L4 stages was initially compensated by an increase in the number of L5 animals, their subsequent decrease was not met by an increase in either other larvae or adults. In other words, as shown in Fig. 3, an increasing mortality rate can be inferred starting at the 8th day after treatment.

The measurements of mortality rates carried out in field-cages (Fig. 4) in parallel to the population measurements were highly consistent with the latter. An earlier onset and steeper rise for the mortality curve were found after application with the blower-sprayer (Soloport). This can be explained by the more effective contamination of the vegetation in comparison to the wind-drift procedure.

Fig. 4 Mortality of larvae from untreated and barrier treated test areas.

The data are the means ąSD for 4 populations in each case, containing 5 larvae each of stages L3 to L5. The animals were collected on the 4th day after the barriers had been established in the test areas.

Larvae were topically treated in their resting places with both IGRs and botanicals.

In 1993 the treatments were first performed with a ULV handsprayer. Because of the denser vegetation and the extreme proximity of the animals, some even standing on top of others, they were not adequately contaminated. For treatment with neem oil and neem oil with M. volkensii extracts, dramatic reductions in fitness were obtained, although this did lead to increased cannibalism and/or destruction of the bands by predators. In both 1993, 1994 and 1995 the larvae lost their walking and jumping behaviours after 24 h. They no longer left their sleeping and resting areas and were devoured by birds, notably Cursoris cursor, Passer luteus and P. simplex saharae (Figs. 5 and 6).

In 1994 and 1995, sleeping place treatments were carried out essentially using a motorised knapsack sprayer (Soloport 23). The IGR triflumuron (50 g a.i./l) as well as neem oil or a mixture consisting of neem oil as the carrier substance with M. volkensii extract In the case of triflumuron application, 100 % of the insects transferred to field cages to allow observation of the mortality rates were dead after 6 to 8 days (Fig. 7). This mortality was also confirmed in the field by our observers, who followed the treated bands for over 14 days. Application of neem oil or neem oil plus M. volkensii extract produced mortality rates in the field cages of 45 % and 95 %, respectively, after 14 days (Fig. 8).

Fig. 5 Migratory movement of a hopper band before and after treatment with M. volkensii extracts dissolved in neem oil

The hopper band was discovered on 22 October during moult to L2, and monitored until I4 November. The daily distances migrated were determined using a GPS. The original number of individuals of the band was around 200,000. The observations were discontinued 7 days after the treatment, on 14 November, since by that point only around 800 larvae had survived (dosage 0.7 l/ha, 10,000 ppm M. volkensii/l). The mortality of almost 100% was brought about both by the products themselves, and through attacks by birds (P. luteus).

Fig. 6 Migration of hopper bands before and after treatment with neem/M. volkensii formulations

The columns represent the mean values of the distances migrated by 4 bands during the stages L3 L5. The daily recording of migratory movements using a GPS was commenced after moult to L3. The treatments (0.6 - 0.8 l/ha, 10 000 ppm/l M. volkensii extract) were carried out 1 - 2 days after moult to L4.

Fig. 7 Mortality of S. gregaria larvae after treatment of their sleeping areas with Alsystin

The data are the means ąSD for 4 populations of 50 individuals each

Fig. 8 Mortality rate after sleeping place treatment with neem oil and M. volkensii extracts

The evaluation of the data was analogous to the description for Fig. 7.

All in all, treatment of sleeping areas and barrier applications appear to offer effective and - thanks to the high savings of insecticide - very economical alternatives (Tab. 3).
The saving of insecticide achieved in these cases is 63.5 - 86 %. Since paying for insecticides represents a substantial proportion of the total cost of a control measure (8 to 12 US $/ha), drastic reductions in cost can be achieved with these methods. Treatments of sleeping areas can also be carried out with conventional insecticides, but for barrier treatments they must show a good persistence of at least 20 days. This has been demonstrated to be the case with the IGR triflumuron (Fig. 9).

Fig. 9

Remanence of triflumuron. 30 3rd/4th instar hoppers were placed in the cages on the treated areas at the times indicated. The hoppers remained in the cages for 48 h and were then transferred to cages with untreated vegetation. The data represent the mean values from 4 experiments.

Tab. 3 Possible savings of insecticide with barrier and sleeping place treatments

The size of the protected area was calculated taking the distribution of the various bushes (roosting place) and the number of hoppers per band into consideration. For blanket treatments, normally carried out while the hopper bands are migrating, a basic value of 80 to 100 hoppers/ m² was assumed and a dosage of 1 l/ha.

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