Bulletin of Insectology 66 (1): 93-101, 2013 ISSN 1721-8861 Larval food plants can regulate the cabbage moth, Mamestra brassicae population Luule METSPALU, Eha KRUUS, Katrin JÕGAR, Aare KUUSIK, Ingrid H. WILLIAMS, Eve VEROMANN, Anne LUIK, Angela PLOOMI, Külli HIIESAAR, Irja KIVIMÄGI, Marika MÄND Institute of Agricultural and Environmental Sciences, Estonian University of Life Sciences, Tartu, Estonia Abstract The effect of different food plants on various parameters of development and hibernation in the cabbage moth, Mamestra brassi- cae L. (Lepidoptera Noctuidae), a serious polyphagous pest, was tested. Brassica oleracea, Brassica napus, Beta vulgaris, Allium cepa and Pisum sativum, differed in their influence on larval development rate, on body mass, mass loss and mortality and on the intensity of pupal diapause. When the larvae were fed on A. cepa, B. vulgaris and particularly on P. sativum, larval development was longer, mortality was higher and pupae had a smaller body mass with diapause not deeply engaged, leading to death during hibernation. Therefore, P. sativum may be used to exhaust the resources of M. brassicae until local outbreaks perish. This study provides strategic information for establishing integrated pest management for cropping systems and for predicting the population dynamics of the cabbage moth. Key words: Mamestra brassicae, food plants, development rate, body mass, pupal diapause, metabolic rate. Introduction cotton bollworm, Helicoverpa armigera (Hubner), es- tablished a direct correlation between larval food quality The cabbage moth, Mamestra brassicae L. (Lepidoptera and the duration of development, pupal mass as well as Noctuidae) is a serious pest throughout the world. In the the number of progeny. Furthermore, food quality may past, it was only a sporadic pest in the Baltic countries, interact with photoperiodic and temperature responses but it has gradually become more widespread and dam- to influence diapause induction, as demonstrated for pu- aging, probably due to climate warming (Bale et al., pal diapause in Hyphantria cunea (Drury) (Morris, 2002). In northern areas, it usually has one full genera- 1967), H. armigera (Liu et al., 2009; 2010) and larval tion, although, in more favourable years, a second gen- diapause in Choristoneura rosaceana (Harris) (Hunter eration may occur (Finch and Thompson, 1982). The and McNeil, 1997). cabbage moth is a polyphagous pest on over 70 host According to Harvey (1962), diapause is a state of de- plant species in 22 families (Rojas et al., 2000). In addi- velopmental arrest in insects, characterized by minima tion to feeding on wild plants and one of the main host in both endergonic biosynthetic activities such as pro- cabbage, Brassica oleracea (L.), the larvae may also tein synthesis and exergonic energy trapping activities cause substantial economic losses to a wide range of such as metabolic rate and gas exchange patterns. Dia- other vegetable crops (Turnock and Carl, 1995). pause is induced by various environmental cues and Protection of vegetables from the cabbage moth is pri- represents a complex dynamic process characterized by marily based on the use of chemical insecticides. several specific physiological and behavioural features Chemical control has several unwanted side effects, (Tauber et al., 1986; Danks, 1987; Denlinger, 1991). such as pesticide residues in consumer products and a The best known cues associated with diapause are pho- decrease of biodiversity in the cropping system. There- toperiod and temperature (Tauber et al., 1986); other fore, more environmentally-friendly methods should be factors, such as food quality of larvae (Hunter and developed for the control of this pest. For this purpose, McNeil, 1997; Liu et al., 2009; 2010), humidity (Lenga it is essential to clarify which factors influence its popu- et al., 1993), pathogens (Metspalu, 1976) and predation lation dynamics. (Kroon et al., 2008) have been found to influence the For polyphagous insects, the availability of different intensity of diapause of various insect and mite species. host plants plays an important role in triggering popula- Liu et al. (2009) showed that high quality larval food tion outbreaks (Singh and Parihar, 1988) and studying plants provide a better preparation for diapause, which the effects of food quality on the insect’s biology is im- appears to be a prerequisite for successful overwintering portant for understanding host plant suitability in infest- and increased survival of H. armigera. ing pest species (Xue et al., 2010). Plant quality is a Various parameters can be indicative of the intensity broad term that encompasses any physical, chemical or of diapause: greatly decreased metabolic rate (Withers, biological traits of plants (Zehnder, 2006). For normal 1992), reduced respiration, frequency of gas exchange growth and development of larvae the proportions of cycles (Kestler, 1991), transpiration (Jõgar et al., 2004), nutritional elements in the food plant are of primary im- heartbeats and circulation (Tartes et al., 2002) as well as portance (Awmack and Leather, 2002; Syed and Abro, cold hardiness (Denlinger, 1991). 2003). Variation in host plant quality influences insect A facultative pupal diapause in M. brassicae is in- herbivore survival and development time (Zehnder, duced in northern regions by the short photoperiod 2006). Studies by Liu et al. (2007; 2009; 2010) on the (critical photoperiodic threshold LD13:11) accompany- ing the low temperature of autumn. Pupae survive the University of Life Sciences near Tartu in 2009. To winter in the soil at a depth to 10 cm. Adult develop- avoid the systematic error by the time factor affecting ment can be initiated only after exposure for several the food plant quality, the clutches were all collected weeks at circa 5 °C (Goto and Hukushima, 1995). Some within one week. Only egg clutches containing at least information is available on induction and development 100 eggs were included in the experiment. Larvae of M. brassicae diapause, showing the major role of day hatched from each egg clutch were divided into 5 length and temperature (Goto and Hukushima, 1995; groups, each fed on one of the following food plants: Hodek, 1996). In spite of its economic importance, little white cabbage (Brassica oleracea L. var. capitata L., information exists on the nutritional value of different variety ‘Krautman’), pea (Pisum sativum L., variety food plants for M. brassicae. This information is essen- ‘Aamisepp’), red beet (Beta vulgaris L., variety ‘Bor- tial for developing a theoretical foundation for manag- doo’), onion (Allium cepa L., variety ‘Peipsiäärne’) and ing overwintering populations and forecasting their swede (Brassica napus L. var. napobrassica (L.) Rchb, population dynamics. In order to examine the interac- variety ‘Kõpu’). Each food plant treatment consisted of tions between the food plant and the cabbage moth, five at least 100 larvae (five treatments, each with five repli- annual crop plants commonly cultivated in Northern cations, each of them with at least 20 larvae, i.e. in all vegetable gardens were chosen, the effect of which on no fewer than 500 larvae). The food plants were se- the population dynamics of M. brassica is unclear. lected according to their importance as cash crops as The aim of this work was: 1) to study the influence of well as their known associations with M. brassicae. All food plants on certain biological parameters of M. bras- food plants were grown on the same experimental field sicae: the duration of the larval development, larval and under uniform agronomic conditions. Newly hatched pupal mortality, the pupal body mass and, sex ratio; 2) larvae were reared in Petri dishes (15 cm diameter and to investigate the possible effects of larval food plants 2 cm deep), at 20 larvae (representing one replicate) per on the intensity of pupal diapause. The following indi- dish, until the 3rd instar. The larvae were then placed in cators were assessed: pupal mass loss, standard meta- groups of five in 1 l breeding vessels covered with net bolic rate (SMR), discontinuous gas exchange (DGE) and layered with sheets of filter paper to absorb exces- and supercooling points (SCP). sive moisture. Larvae were reared on cut leaves of the food plant in environmental test chambers “Sanyo”. The leaves were replaced daily. The duration of the experi- Materials and methods ment with the larvae is indicated in figure 1. Larval mortality was recorded at 24 h intervals. Before pupa- Experimental design tion, a 10 cm deep layer of peat was placed on the bot- Egg clutches of M. brassicae were collected from tom of the vessels to allow the larvae to dig in and pu- white cabbage on an experimental field of the Estonian pate. To obtain winter diapausing pupae, larvae were 50 d 48 46 44 42 40 b 38 bc 36 34 c 32 30 a Average duration of larval stage (days) 28 26 Mean Mean±SE 24 Mean±SD 22 B. oleracea A. cepa B. napus B. vulgaris P. sativum Figure 1. Average duration of larval stage of M. brassicae reared different food plants. Different letters indicate sig- nificant differences (P < 0.05, ANOVA, LSD-test). 94 reared at 21 °C with a short photoperiod 12:12 DL and with six channels which enabled recording of the heat 75% RH. To ensure that the pupae reached a stable dia- flow simultaneously in six individuals. Each measure- pause state, they were then kept in the peat for one ment lasted 72 hours. The duration of DGE cyles was month, before collection by hand-sorting. Under these determined in ten pupae (five male and five female) conditions, 100% of the obtained pupae were in dia- from each treatment. Since the sexes produced identical pause. Each pupa was classified as alive or dead accord- results, the readings were combined for analysis. Calo- ing to the presence or absence, respectively, of abdomi- rimetry is the method for continuous recording of DGE nal movement in response to touch. Pupal gender was for weeks in individuals without evoking stress by han- determined according to external sexual characters on dling and adjusting the apparatus.