Abstract

The effects of two neem insecticide formulations (an oil-free powder formulation and an oil formulation) on field parasitism of diamondback moth, Plutella xylostella (L.), larvae and, on the longevity and foraging behaviour of Diadegma mollipla (Holmgren), were evaluated in field and laboratory tests. Overall, larval parasitism in plots sprayed with the oil-free neem formulation was not significantly different from that in the water-sprayed plots over the entire observational period, but parasitism in plots receiving the oil formulation was significantly lower during two weeks. Field parasitism by two individual parasitoid species, D. mollipla and Oomyzus sokolowskii (Kurdjumov) showed substantial differences in their relative response to the neem treatments. In laboratory tests, longevity and foraging behaviour of D. mollipla was not affected by contact with the neem insecticide sprays. The results indicate that the neem products may be relatively safe to larval parasitoids of the diamondback moth.

Key words

Diadegma mollipla, neem, non-target effects, Oomyzus sokolowskii, Plutella xylostella

Introduction

Crucifers are important vegetables in Kenya, providing necessary micronutrients as well as income to small-scale farmers, the main producers. The diamondback moth (DBM), Plutella xylostella (L.) (Lepidoptera: Plutellidae) is the most serious pest on Crucifers worldwide and also the most difficult to control (Talekar & Shelton, 1993). Unilateral reliance on chemical insecticide control is no longer viable because of the pest's capacity to develop insecticide resistance and spiralling costs of newer, effective insecticides. Current control strategies for DBM place greater emphasis on the integrated use of biorational products such as neem-based insecticides (Schmutterer 1992) and larval parasitoids (Talekar 1992, Verkerk & Wright 1996). Several studies have demonstrated the efficacy of neem insecticides against DBM (Dreyer 1987, Schmutterer 1992, Javaid et al. 2000). Neem insecticides are also reported to be safer to natural enemies (Schmutterer 1990, 1995, 1997). In Kenya, Neemros^® (0.5% azadirachtin) and Neemroc EC^® (0.03% azadirachtin) are two neem insecticide formulations derived from the neem tree (Azadirachta indica A. Juss) that have been registered for use against pests in horticulture, including the DBM. Neemros^® is an oil-free neem kernel cake powder (NKCP) formulation while Neemroc EC^® is a neem seed oil (NSO) formulation. Formulation, among other factors, determines the extent to which a neem-based insecticide will affect organisms (Schmutterer 1995) and thus there is a need to test new formulations, both for their efficacy against the pest as well as for their side effects on natural enemies. The objective of this study was to evaluate the effect of the two neem products on field parasitism rates of DBM. Bioassays were also conducted to assess longevity and foraging behaviour in Diadegma mollipla (Holmgren) (Hymenoptera: Ichneumonidae), a common larval parasitoid of DBM in Kenya, following direct sprays with the neem products.

Materials and methods

Treatments

Neemroc EC^® (=NSO) and Neemros^® (=NKCP) were applied as foliar sprays at the recommended field dose rate of 15ml/ l and 25 g/ l of water, respectively (Varela 1998, Waiganjo 1998). Water alone was used as a check.

Insects

D. mollipla adults were obtained from an insectary colony that had been reared for no more than four generations on a DBM-cabbage (Brassica oleracea var. capitata cv. Copenhagen Market) system, at 22-25ºC and 14L:10D photoperiod. Parasitoid adults were maintained on a 20% honey solution and distilled water.

Effects of the neem insecticides on field parasitism rates

A field trial was conducted in May to July 1999 at Juja (Kenya), which lies at longitude 37°00' E, latitude 1°05' S and approximately 1525 m above sea level, with a bimodal pattern of rainfall (856 mm per year). Four-week old cabbage seedlings were transplanted to plots measuring 4.2 x 4.4 m, at a spacing of 60 cm between and 40 cm within rows. The treatments were compared in a randomised complete block design replicated six times. Weekly applications of the treatments were made by high volume (hand-operated knapsack calibrated to deliver 100l spray/ ha) spraying, starting from the second week after transplanting (WAT) when the DBM population established naturally until the ninth WAT. Furrow irrigation was adopted to minimise interference to DBM establishment. Sampling and counting of DBM was done prior to the neem applications each week. To assess larval parasitism, at least thirty IV instar or prepupal DBM were collected each week from the remaining plants in each plot, for recording DBM infestation. The samples were reared in the laboratory until DBM or parasitoid adults emerged. Unemerged pupae were dissected to determine whether or not they had been parasitised. Percent parasitism for each week was determined as a proportion of the hosts sampled that were parasitised.

Longevity of neem-sprayed D. mollipla

Two-day old parasitoids were sprayed with 4-6 ml of aqueous solutions of NKCP, NSO or water. Daily records were taken of the number of wasps dying in each treatment.

Foraging behaviour of neem-sprayed D. mollipla

Two-day old naïve, mated parasitoid females were given the same treatment as in the previous experiment. After 24-36 h, a single parasitoid was introduced into a Perspex cage containing a DBM-infested cabbage plant and its foraging behaviour on the plant continuously observed for 25 min using The Observer^® software (Noldus 1995). Records were made of the number of hosts parasitised and duration of bouts of searching, oviposition and grooming during the 25 min period.

Data analysis

Parasitism data were arcsine transformed prior to analysis using Repeated Measures Analysis of Variance (PROC GLM, SAS Institute 1990). One-way ANOVA was used to test for differences in mean longevity, number of successful attacks and mean duration of bouts of searching, oviposition and grooming among the treatments. Where significance was indicated, means were separated at P<0.05 using the Student-Newman-Keuls (SNK) multiple range test.

Results

Effects of the neem insecticides on field parasitism rates

Overall (= all parasitoid species combined) parasitism in the treatments varied significantly over the periods (WAT) (F= 3.09, d.f= 8, 36, P<0.05). Overall larval parasitism did not differ significantly between NKCP and control plots in any individual period, but significantly lower parasitism levels were recorded in the NSO plots during the fifth and eighth WAT (Table 1).

Table 1. Field parasitism of diamondback moth on cabbage receiving foliar sprays of two neem insecticide formulations at successive weeks after transplanting (WAT), Juja, Kenya, 1999

WAT	Percent larval parasitism (mean ± s.e)
	NKCP	NSO	Water
2	-	-	-
3	27.1 ±2.8a (n=41)	37.2 ±6.6a (n=54)	32.7 ±4.9a (n=53)
4	16.1 ±2.2a (n=23)	12.7 ±3.4a (n=18)	20.2 ±4.6a (n=41)
5	24.5 ±2.4ab (n=57)	6.7 ±6.7b (n=4*)	22.0 ±3.4a (n=73)
6	-	-	-
7	33.8 ±5.6a (n=98)	24.0 ±4.5a (n=26)	37.1 ±4.7a (n=107)
8	31.3 ±3.1a (n=83)	16.3 ±7.7b (n=15)	33.6 ±1.9a (n=65)

^a Within rows, means compare treatments and means with the same letter are not significantly different at P<0.05, SNK test. N= number of parasitised DBM collected. *some plots excluded in the analysis of data because no observations were made.

Larval parasitism of DBM during the season was contributed by three parasitoid species: D. mollipla, Oomyzus sokolowskii Kurdjumov (Hymenoptera: Eulophidae) and Cotesia plutellae Kurdjumov (Hymenoptera: Braconidae). Analysis of percent parasitism by the individual parasitoid species revealed no significant three-way interaction for periods, species and treatments (F= 1.63, d.f.= 16, 74, P>0.05) or between periods and treatment (F= 1.58, d.f.= 8, 48, P>0.05). However, a significant interaction was observed between periods and species (F= 14.44, d.f.= 8, 48, P<0.001). Furthermore, parasitism levels were also significantly different between the species (F= 60.03, d.f.= 2, 27, P<0.001) (Figure 1). Parasitism by C. plutellae was very low throughout the period, accounting for less than 2% of the parasitised hosts and so is not discussed further in this text. D. mollipla was the predominant parasitoid in the first week of observation in all treatments, but its rate of parasitism decreased thereafter to less than 5% from the fifth WAT. It did not show any significant differences between the treatments in any period. Parasitism by O. sokolowskii was initially low, but increased in subsequent periods. Parasitism level of this species did not differ significantly between NKCP and control plots. However it was significantly lower in NSO plots when compared with control plots after the fourth WAT.

Figure 1. Field parasitism of diamondback moth larvae by two parasitoid species in cabbage receiving foliar sprays of two neem insecticide formulations. Error bars represent standard errors.

Longevity of neem-sprayed D. mollipla

The mean longevity of D. mollipla was not significantly different among the treatments (F=0.74, d.f.=2, 272, P>0.05) (Table 2).

Table 2: Mean longevity of Diadegma mollipla adults sprayed with a neem insecticide formulation or water

Treatment	N	Longevity (days) (mean ± s.e.)
NKCP	82	13.4 ± 1.04a
NSO	91	12.6 ± 0.96a
Water	102	14.2 ± 0.92a

N= number of parasitoids tested

Foraging behaviour of neem-sprayed D. mollipla

The number of hosts parasitised within the observation period was not significantly different among the treatments (F= 0.44, d.f.= 2, 25, P>0.05) (Table 3). Similarly, mean duration of a bout of searching, oviposition or grooming was not significantly different among the treatments (P>0.05).

Table 3. Number of hosts parasitised and duration of a bout for three components of foraging behaviour by neem- and water-sprayed Diadegma mollipla during a 25 min period on an infested cabbage plant

Treatment	#hosts parasitised	Duration (sec) of bout (mean ± s.e.)
	(mean ± s.e.)	Searching	Oviposition	Grooming
NKCP (N = 22)	6.3 ± 1.0a	6.8 ± 0.65a	11.8 ± 1.6a	12.5 ± 1.59a
NSO (N = 20)	8.0 ± 1.3a	7.7 ± 0.50a	10.4 ± 1.0a	9.8 ± 1.31a
Water (N = 26)	7.5 ± 1.4a	6.8 ± 0.83a	9.8 ± 1.0a	9.5 ± 0.74a

N= number of parasitoids individually observed. Means within a column compare the same parameter among the treatments

Discussion

This is perhaps the first report on the non-target effects of the two neem insecticide formulations on DBM larval parasitoids. It was evident that the oil-free NKCP formulation did not have any significant effects on overall field parasitism or parasitism by individual species. In contrast, NSO had a more profound negative impact on overall field parasitism and parasitism by O. sokolowskii in some periods. This finding is consistent with Schmutterer's (1995) observation that neem products containing a high percentage of neem oil tend to show stronger adverse effects on non-target organisms than oil-free products. The laboratory bioassays showed no adverse effect of the neem products on the longevity or foraging behaviour of D. mollipla at the concentrations tested. These results indicate that at the recommended field doses, the two neem insecticide products may be relatively safe to D. mollipla. The differential response of the two predominant larval parasitoid species (D. mollipla and O. sokolowskii) to the individual neem formulations indicates the need for monitoring individual key parasitoid species when assessing the non-target safety of neem insecticides, rather than depending on overall larval parasitism as the parameter.

Acknowledgements

This study was conducted with the financial support of the Netherlands Ministry of Foreign Affairs and International Co-operation to the DSO project based at the International Centre of Insect Physiology and Ecology (ICIPE), Nairobi, Kenya.

References