INFLUENCE OF SIMULTANEOUS INFESTATIONS OF AND SITOPHILUS ZEAMAIS ON THE REPRODUCTIVE PERFORMANCE AND MAIZE DAMAGE

CP Rugumamu Department of Zoology and Marine Biology, University of Dar es salaam, P.O. Box 35064, Dar es Salaam, Tanzania, [email protected]

ABSTRACT This study examined the combined effect of Prostephanus truncatus and Sitophilus zeamais infestations on their reproductive performance and on the damage they cause to maize variety ZM 621 with 12.5% moisture content. The specific objective was to determine the number of the pests at F1 and F2 and the grain weight losses caused by the simultaneous pests’ infestations of shelled maize in the search for a control strategy. The results showed a change in adult insect numbers from F1 to F2. During single infestation the change in P. truncatus was 47.54% while in simultaneous infestations it was 41.66%. The change in S. zeamais was 31.25% and 18.60% during single and simultaneous infestations respectively. The t-test indicated significant differences in the percentage change of adult insect numbers between single and simultaneous infestations at P< 0.01. Further, the grain weight loss in simultaneous infestations was 28.53% while in single infestation P. truncatus and S. zeamais caused loss of 26.40% and 6.21% respectively. The Kruskal-Wallis test showed a significant difference in weight losses among the infestation types at P < 0.01. The P. truncatus and S. zeamais numbers were positively correlated with the grain weight losses at P < 0.05. It was concluded that simultaneous infestations greatly reduced the reproductive performance of S. zeamais up to F2 while that of P. truncatus was less affected. These findings thus contribute to the components of Integrated Pest Management (IPM).

INTRODUCTION hazardous. There is therefore a need to In maize storage systems in Tanzania the synergize control strategies available in an newly introduced and most dangerous pest effort to reduce chemical applications in the of maize, Prostephanus truncatus (Horn) context of developing Integrated Pest (Coleoptera:) is often associated Management (IPM). with other insect pests especially the cosmopolitan Sitophilus zeamais (Motsch.) IPM is a system that, in the context of the (Coleoptera:Curculionidae). Both are associated environment and the population primary pests of maize, the most important dynamics of the pest species, utilizes all cereal crop grown in this country. The thrust suitable techniques and methods in as of this laboratory study was to investigate compatible a manner as possible and the numbers of adult produced at F1 maintains the pest population at levels and F2 as well as the maize weight losses as below those causing economic damage (Hill a basis for determining the insects’ 1987, van Emden 1999). It is reported by reproductive performance and damage levels Southwood and Henderson (2000) that during single and simultaneous infestations information about populations is of shelled maize. The results may have sought for a variety of purposes including implications on the design of the stored pest management since it guides the pests control strategies. Current control application of control measures after the methods for stored insect pests of maize assessments of incidences and or damage include applications of chemical pesticides levels. Thus a laboratory ecosystem is which are expensive and environmentally worthy studying as the results give the Rugumamu – Influence of simultaneous infestations of Prostephanus truncatus …

understanding and prediction of what could MATERIALS AND METHODS happen in natural situations. Maize samples An experimental maize variety ZM 621, was P. truncatus and S. zeamais are primary selected for the investigation of the pests attacking whole grains whose moisture reproduction of P. truncatus and S. zeamais content may be as low as 10.5% (Golob and and for the determination of the damage Hanks 1990, Meikle 1998) resulting in caused to the maize in terms of weight loss. severe damage and hence great weight losses The grains are dent due to the starchy in stored maize. Both insects can infest endosperm which extends to the apex of the standing maize in the field once the crop has grains. Shelled maize grains were used attained maturity and is drying in the field, since they are recommended to smallholder early before harvesting (Hodges 1994, de farmers as it has been proved that they are Pury 1968). The insects are known to less damaged by the serious pest, P. complete development over a wide range of truncatus compared to maize on cobs thus a temperatures, 270C to 320C and between component in the IPM (Golob and Hanks 70% and 80% relative humidity conditions 1990). The samples, free of infestation were (Hodges 1986, Throne 1994). The equilibrated for four weeks at favourable developmental period for both insects is conditions of temperature and humidity (29 about 30 days (Howard 1984). Both adults ± 2o C and R.H. of 70% - 80%). and larvae feed on the maize grains since they all have biting and chewing Rearing of the insect pests mouthparts. In this study the adult, being Initial insect stocks of P. truncatus and S. the final stage in the life cycles of the two zeamais were obtained from cultures at insect pests, is opted for in the assessment Ilonga Agricultural Research Centre, of both the amount of damage and egg Morogoro region. Adult insects were kept in oviposition. glass jars and raised on maize variety Staha, a susceptible variety and not used in the The intrinsic rate of increase of P. truncatus testing (Rugumamu 2000). For massive is reported by Cowley et al. (1980) to be production of the insects, S. zeamais were Lambda 1.399 at optimum conditions on reared on shelled maize while P. truncatus shelled maize grains and that this apparently were reared on maize on the cobs (Hodges low rate may reflect a feature of interspecific 1986, Kossou et al. 1992). competition with more fecund storage pests like S. zeamais. It is therefore important to Aging, sexing and conditioning P. examine the interaction of S. zeamais and P. truncatus and S. zeamais truncatus from this point of view. As noted Adults were removed from the jars after two by Chapman and Reiss (2002) organisms weeks - this was after egg oviposition. Then differ in their ability to compete with each the jars were left undisturbed and emerging other and hence varying impacts of one adults were collected for seven days. These species on another. Regarding interspecific are parent insects aged 1 to 7 days and were associations, Krebs (1978), Southwood and sexed according to Shires and McCarthy Henderson (2000), Weimerskirch et al. (1976) and Kossou et al. (1992). The parents (2003) observe that these can cause changes were conditioned to ZM 621 by infesting in reproduction since the effects of eight 60 g replicates with 18 adults i.e. the increasing intensity of competition among ratio of F : M = 2 : 1 (Dobie 1974). In the adult insects decreases fecundity or rate of case of simultaneous infestations the number reproduction after some generations. of parents of each species was halved. The set-ups were maintained at favourable conditions for 7 days. The R.H. was maintained using Calcium chloride solution

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(Solomon 1957). Experiments were then activity was carried out in both single and carried out for both single and simultaneous simultaneous infestation set-ups. infestations to assess (i) P. truncatus and S. Data analysis zeamais numbers at F1 and F2 (ii) weight The t- test was carried out to compare losses of the maize after the insects’ attack. changes in numbers of P. truncatus and S. zeamais at F1 and F2 during single and Assessment of F1 and F2 P. truncatus and simultaneous infestations of shelled maize. S. zeamais numbers Analysis of variance by the Kruskal-Wallis Six replicates of fresh 60 g maize grains and test was performed on the weight losses due the parent insects which were then between 7 to the insect infestations among the and 14 days old were set. For both F1 and infestation types at both F1 and F2 followed F2, the experimental design fell into the by Dunn’s multiple comparison test. The following sets: (i) single infestations of analyses were performed according to Gomez maize grains by parents, P. truncatus and S. and Gomez (1984), Fowler and Cohen zeamais; (ii) simultaneous infestations of (1999) using Instat software package. maize grains by both insects (iii) controls comprised of only maize grains without RESULTS insects. Each replicate was contained in a Numbers of P. truncatus at F1 and F2 in 130 x 60 mm glass bottle. The insects were the maize grains confined in the bottles by perforated The mean numbers of F1 P. truncatus Aluminium foils to allow adequate emerging from the maize samples during ventilation and the moisture content of the single infestation was 122 ±1.155 while in maize to equilibrate with the set conditions simultaneous infestation it was 60 ± 0.930. in incubators. After ten days the parents The mean number of P. truncatus at F2 was were removed from the test samples which 180 ± 0.930 and 85 ± 1.155 in single and were then undisturbed for 7 days. They were simultaneous infestations respectively. then inspected daily for emergence of F1 During single infestations the mean adults that were removed and numbers percentage changes of numbers were greater recorded. The exercise continued until no than during simultaneous infestations (Table any adult was emerging. In the separate set- 1). The t test indicated a significant ups for the second filial generation, F1 difference between the percentage change in adults were left to mature, mate and oviposit numbers of the insects from F1 to F2 in in the maize samples for two weeks and the single and simultaneous infestations, t 0.01 (2), F2 adults were then removed and recorded as 10 = 28.157. they emerged. Numbers of S. zeamais at F1 and F2 in the Assessment of weight losses of maize maize grains Each sample replicate was sieved using a 4 The mean number of S. zeamais was 160 ± mm aperture sieve to remove the created 0.894 and 86 ± 1.366 at F1 in single and maize dust/flour, insect frass and simultaneous infestations respectively. At F2 larval/pupal exuviae. The samples were then the adult insects were 210 ± 1.317 in single weighed to determine weight losses of the and 102 ± 1.155 in simultaneous maize at both F1 and F2. Moisture contents infestations. During single infestations the of the controls were measured before and percentage change in S. zeamais numbers after the experiments according to Pixton increased greatly from F1 to F2 while in (1982). The actual weight losses of the simultaneous infestations it was relatively maize grains due to insects' attack and small (Table 1). The t test showed a damage were calculated after the application significant difference in the percentage of the correction factor of percentage weight changes of insect numbers from F1 to F2 changes derived from the controls. This

67 Rugumamu – Influence of simultaneous infestations of Prostephanus truncatus …

between single and simultaneous (t 0.01 (2), 10 = 52.746). infestations Table 1: Mean number of insect pest species and percentage change in adult numbers from F1 to F2 in single and simultaneous infestations

Insect spps Infestation type Number of adults % change F1 F2 P. truncatus Single 122 ± 1.155 180 ± 0.930 47.54 ± 0.119 Simultaneous 60 ± 0.930 85 ± 1.155 41.66 ± 0.179 S. zeamais Single 160 ± 0.894 210 ± 1.317 31.25 ± 0.194 Simultaneous 86 ± 1.366 102 ± 1.155 18.60 ± 0.133

At F1 40 At F2 35

s 30 s o l

25 t h

g 20 i e

W 15

% 10 5 0 P S P+S Infestation types

Figure 1: Maize weight losses caused by P. truncatus and S. zeamais in two infestation types

2 2 Weight loss of the maize caused by P. χ = 15.174, P < 0.01, 5 at F1 and χ = truncatus and S. zeamais at F1 and F2 15.205, P < 0.01, 5 at F2. Multiple The maize grains lost different amounts of comparisons indicated significant differences weight after F1 and F2 P. truncatus have between means of the types of infestations at emerged. At F1 and F2 in single infestations P < 0.05. It was observed, however, that the the mean weight losses were 13.21 ±0.040% amount of weight loss is less in and 26.40 ± 0.120% respectively (Fig. 1). simultaneous infestation at F1 in relation to The mean weight losses of the maize caused loss in single infestation of P. truncatus but by S. zeamais was 3.40 ± 0.133% at F1 and greater than that in single infestations of S. at F2 the loss was 6.21 ± 0.082% during zeamais. Numbers of P. truncatus and S. single infestations. At F1 and F2 the maize zeamais were positively correlated with grains lost 10.81 ± 0.106% and 28.53 ± weight losses, r = 0.914 and 0.827 (P<0.05) 0.133% respectively during simultaneous respectively. infestations. Analysis of variance by the Kruskal-Wallis test indicated a significant difference among the types of infestations,

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DISCUSSION hence it fecundity. Further, S. zeamais The laboratory results of insect pests, P. females could no longer locate adequate truncatus and S. zeamais both in single and favourable sites to deposit their eggs or may simultaneous infestations of maize exhibit be even the larvae could not develop outside different levels of reproductive performance the grain since most grain contents were and varying weight losses caused to maize. eaten up by the serious pest, P. truncatus It was observed that the numbers of F1 P. save for the damaged grain sheaths (Hodges truncatus were fewer than S. zeamais in all 1986). Such a condition could not favour the tests. The effect of simultaneous the reproductive performance of S. zeamais infestations of the two insect pests was but affected P. truncatus only slightly up to detected at the F2 generation. the F2 generation.

The reproductive performance of P. According to Bell and Watters (1982), truncatus and S. zeamais Meikle et al. (1998), P. truncatus and S. The percentage change of the numbers of the zeamais exhibit different egg deposition two insects from F1 to F2 in both single and behaviour and this could probably explain simultaneous infestations showed differing their varying performance. It is reported that reproductive performance. In single P. truncatus females deposit their eggs infestations each insect exhibited relatively inside the grains and also could deposit higher mean percentage change of numbers them in the created flour and frass tunnels from F1 to F2 while during simultaneous and at the bottom of the container. Further, infestations; S. zeamais decreased greatly the larvae of P. truncatus are also able to but the reproduction of P. truncatus was develop in compacted flour after making less affected. The results reveal therefore, narrow tunnels against which they could that during simultaneous infestations the support themselves and push their two insect pests have a comparatively lower mandibles into the forward end of the tunnel performance compared to single infestation. for effective chewing. On the other hand, S. Krebs (1978) emphasized that in many zeamais females deposit their eggs only laboratory experiments a species can do well inside the grain and particularly in the when raised alone but can be driven to even endosperm other than in the germ or germ- extinction when raised in competition with endosperm interface and larvae development another species. It was further found out that is only completed inside the grain (Dobie S. zeamais produced more F1 individuals 1974, Urrelo and Wright 1989). The than P. truncatus during single infestations. survival of S. zeamais is dependent on its This therefore supports the revelation by ability to locate an appropriate site in the Cowley et al. (1980) that the intrinsic rate kernel to lay an egg and any misplacement of increase of S. zeamais is greater than that could result in retarded growth and of P. truncatus. However, the reproductive development or even death of the larvae performance of S. zeamais was more affected (Urrelo and Wright (1989). than that of P. truncatus in simultaneous infestations. Oviposition in insects is a dynamic process in which individual females respond to The above observations could probably be variation in host quality. This observation is due to inadequate food supply at the insect supported by Aluja et al. (2001) who points pest vicinity. It was pointed out by out that a female reproductive performance is Weimerskirch et al. (2003) that for most regarded as an important determinant of , the only way to obtain resources is ovipositional dynamics and whatever affects by expending energy while foraging and that oviposition affects reproductive output in the ability of individuals to adjust energy turn. In some instances insect species stop expenditure is likely to influence fitness and maturing oocytes when deprived of a

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favourable deposition site and this affects grains while in the case of adult S. zeamais performance because ovipositing females are mandibles on the prognathous type of head time limited (Fitt 1986). orientation are comparatively less adapted for grinding. A decrease in weight loss in Furthermore, van Emden (1999) reports that simultaneous infestation reflected a negative there are different responses to the crowding effect on the dual performance of S. zeamais. effect in individuals of different species. The The results show that, of the two pests, P. cited responses include the stimulation of truncatus caused greater weight loss of restlessness of individuals resulting in, maize in single infestations at F1 and F2 and among other things, a reduced fecundity or hence in simultaneous infestations. an arrest in reproduction. The findings of Chapman (1982) noted that the insect this study could be supported among other morphology relates to its behaviour. It is in things, the above stated responses. this context that the relationship of insect mouthparts and maize damage is Maize Damage caused by P. truncatus highlighted. and S. zeamais The maize damage reflected by weight loss CONCLUSION AND of the grains caused by the two insect pests RECOMMENDATIONS at F1 was such that most loss was caused by The effects of simultaneous infestations of P. truncatus while S. zeamais caused the P. truncatus and S. zeamais were indicated least. This could probably be explained by by the percentage changes of insect numbers two factors, namely, feeding behaviour and from F1 to F2. S. zeamais was more type of the mouthparts of the insect pests. negatively affected than P. truncatus. It With respect to feeding behaviour, of P. could therefore be deduced that there is a truncatus both the larvae and adults eat differential decrease in the reproductive voraciously creating a lot of dust and frass performance of the two insect pests. Further while it is more the larvae of S. zeamais the weight loss varies when maize grains are who do much of the feeding inside the attacked by different numbers of P. grains than the adult (Shires 1980; Dobie truncatus and S. zeamais. Following from 1988). This behaviour illustrates the this, it could be inferred that the amount of economic importance of P. truncatus with maize weight loss corresponds with the respect to maize storage and therefore P. numbers of a particular insect pest. truncatus is classified as a serious pest. Furthermore, the results reveal that In this study P. truncatus caused much knowledge about the varying insect weight loss at both F1 and F2 in relation to reproduction and shelled maize grain losses the estimated acceptable levels while losses sheds light to the influence of simultaneous caused by S. zeamais were lower than 9% at infestations. This could contribute in the F2. So far little effort has been made to design of control strategies of these pests apply the threshold concept to stored grain which normally infest maize prior to Economic Injury Level (EIL). simultaneously. These findings therefore McFarlane (1988), for instance, estimated a indicate that when P. truncatus and S. level of approximately 3% weight loss while zeamais are simultaneously infesting shelled Henckes (1992) estimated the maximum maize there is an increase in time before the weight loss accepted by the farmers to be onset of economic thresholds in terms of about 9%. insect numbers and weight losses is reached. Such a situation therefore, delays the EIL Regarding the mouthparts, the mandibles of that calls for chemical pesticides both larvae and adult P. truncatus are so applications. It is this important quality of adapted to effectively chew and grind maize the insects’ interactions that reduces the

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