ABSTRACT

The pollen beetle, Meligethes aeneus F. (Coleoptera: Nitidulidae), is an important pest of oilseed rape crops throughout Europe. It poses a particular threat to crops of Synergy, a composite hybrid oilseed rape consisting of 20% pollinator plants and 80% male sterile hybrid plants, as adults and larvae consume the pollen necessary to pollinate the male sterile plants. The effects of pollen presence/absence on adult infestation, oviposition, and larval development were investigated. Adult infestation was greater on fertile plants than sterile ones. Although more sterile than fertile buds were devoid of eggs and larvae despite oviposition damage, there was no difference in the mean number of eggs laid per bud, indicating that females do not adjust clutch size in response to any host quality assessments. More second instar larvae were present on fertile than sterile plants, indicating a greater rate of survival, although fitness (weight) of larvae found on each plant type did not differ. Since more adults and second instar larvae were found on fertile plants than sterile, the need for pollen beetle control to ensure adequate pollination of the crop is highlighted.

INTRODUCTION

Synergy is a composite hybrid winter oilseed rape; a varietal association of 80% male sterile hybrid plants (cv. Falcon x Samouri) which produce no pollen, and 20% male fertile pollinator plants (cv. Falcon). In the UK, commercial crops of Synergy have regularly yielded over 5t/ha; an increased yield compared with the average of around 4t/ha from conventional winter cultivars such as Apex (CPB Twyford, 1996). This yield improvement results from the hybrid vigour of the sterile plants and their saving in energy otherwise spent in pollen production. The success of the crop, however, depends upon the pollinator plants producing enough pollen to fertilise the whole crop. Protection of the pollinator plants is therefore of great importance, and growers are advised to pay particular attention to control of the pollen beetle, Meligethes aeneus, at the green/yellow bud stage of crop growth. Treatment is recommended when there is an average of one pollen beetle found per every five plants inspected (CPB Twyford, 1996).

The pollen beetle is an important pest of oilseed rape crops throughout Europe (Winfield, 1992). There is one generation per year. Adults emerge from overwintering sites in the spring and migrate to oilseed rape crops where they mate. Females lay their eggs in the buds through an oviposition hole, which they chew into the base of the bud. The larvae develop within the bud, and at about the time the flower opens, they moult to the second and final instar. Second instar larvae complete their development in the flowers. On maturity, they drop to the ground and pupate in the soil. New generation adults emerge about one month later, feed, and in late summer seek overwintering sites. Both adults and larvae eat pollen from buds and flowers and it is this feeding which poses a threat to the Synergy crop. Pest status is traditionally earned due to bud abscission caused by oviposition injury, and feeding damage from adults and larvae (Winfield, 1992).

Here we report on pollen beetle responses to the presence or absence of pollen in buds and flowers of oilseed rape, using the composite hybrid rape, Synergy.

MATERIALS AND METHODS

Sampling

Pollen beetle assessments were made on four occasions; once at yellow bud and three times during flowering. On each occasion, five fertile and five sterile plants were selected at random from each of the four sides of the crop.

Adult incidence

To determine whether plants with pollen are more attractive to pollen beetles than plants without, adults from the top three racemes in bud/flower were counted. Numbers of adult beetles on fertile and sterile plants were transformed (log₁₀ +1) and compared by an analysis of variance.

Incidence of oviposition holes, eggs and larvae

Racemes sampled for adult incidence were cut, bagged, and the top 50 buds examined in the laboratory for oviposition holes. Buds with oviposition holes were dissected and the number of eggs and larvae recorded. To ascertain whether female beetles assess host quality at oviposition, the proportion of buds from fertile and sterile plants which had an oviposition hole but was devoid of eggs or larvae were compared using a logistic regression. To determine whether clutch size is altered according to host quality assessment, the average number of eggs laid in accepted buds were compared between sterile and fertile plants using a log-linear model. The model allowed for the differing numbers of accepted buds on each plant. During the flowering period, the top 20 flowers were examined for larvae, and the total number of 1^st and 2^nd instars recorded. These totals were transformed (log₁₀ +1) and compared between sterile and fertile plants by an analysis of variance.

Larval fitness

On the final sampling date, larvae were grouped according to their instar and its location on the plant: 1^st instar from buds, 1^st instar from flowers, 2^nd instar from buds and 2^nd instar from flowers. The mean weights of larvae for each group were compared between fertile and sterile plants using a weighted least squares regression where the weighting is equal to the number of larvae observed. As there were very few second instar larvae found within buds, no analysis was performed on this group.

RESULTS & DISCUSSION

Adult incidence

At the yellow bud stage there was no difference between the number of adult pollen beetles on sterile and fertile plants (p=0.124). However, significantly more beetles were found on fertile than sterile plants during flowering (p <0.001 for all three samples). This can be seen from Figure 1.

Figure 1. Mean number of adult pollen beetles found on fertile and sterile Synergy oilseed rape plants, with 95% confidence intervals, back-transformed from log scale

Incidence of oviposition holes, eggs and larvae

Number of buds with oviposition holes devoid of eggs and larvae. The proportion of buds with oviposition holes from sterile plants with no eggs or larvae present was significantly greater than that from fertile buds at every sampling occasion (P< 0.001 at yellow bud, flowering 1 and 2, and P <0.05 at flowering 3). The predicted proportions of buds with oviposition holes expected to be found with no eggs or larvae is shown in figure 2.

Figure 2. Predicted proportion of buds with pollen beetle oviposition holes expected to be found devoid of eggs and larvae, showing standard errors

Number of eggs laid per bud. At each sampling occasion, the number of eggs laid per accepted bud between fertile and sterile plants did not differ significantly (Table 1).

Table 1. Mean number of pollen beetle eggs laid per bud on fertile and sterile plants of Synergy oilseed rape

Sampling occasion	Transformed mean no. eggs laid per bud (gSE), [back-transformed mean]		Approx. ã2 Probability
	Fertile	Sterile
Yellow bud	0.6 (0.06) [1.9]	0.6 (0.08) [1.8]	0.365
Flowering 1	1.0 (0.04) [2.8]	1.0 (0.06) [2.8]	0.791
Flowering 2	1.1 (0.06) [3.1]	1.0 (0.06) [2.9]	0.283
Flowering 3	1.1 (0.06) [2.9]	0.9 (0.07) [2.5]	0.130

Number of larvae per plant. During the first two sampling occasions during flowering, more 2^nd instar larvae were found on fertile than on sterile flowers (Table 2).

Table 2. Total number of pollen beetle larvae present on fertile and sterile flowers of Synergy oilseed rape

Sampling occasion	Larval instar	Total number of larvae per 20 flowers [back-transformed totals]		SE of mean	P
		Fertile	Sterile
Flowering 1	1st	1.1 [11.7]	0.94 [7.7]	0.08	0.127
	2nd	0.8 [5.3]	0.51 [2.2]	0.09	<0.05
Flowering 2	1st	1.3 [19.8]	1.2 [15.6]	0.06	0.258
	2nd	1.1 [11.4]	0.8 [4.7]	0.08	<0.01
Flowering 3	1st	0.8 [5.8]	0.7 [4.3]	0.07	0.261
	2nd	0.9 [6.7]	0.8 [4.9]	0.09	0.344

Larval fitness

Table 3. Mean weight of pollen beetle larvae from fertile and sterile plants of Synergy rape

Larval group	Mean weight (mg) of larvae		F (1, 31)	P
	Fertile	Sterile
1st instars from buds	0.113	0.117	0.41	0.527
1st instars from flowers	0.178	0.160	2.20	0.147
2nd instars from flowers	0.854	0.748	0.78	0.385

The weights of larvae from the two plant types did not differ significantly (Table 3).

DISCUSSION & CONCLUSION

More adult beetles were found on fertile plants than sterile ones throughout flowering. This suggests that exposed pollen is attractive to pollen beetles. Indeed, there is evidence to suggest that pollen cues, in particular pollen odour, is used in resource location by pollen-seeking insects (Charpentier, 1985; Dobson, 1987).

More sterile than fertile buds with oviposition holes were devoid of eggs and larvae. This could indicate that females assess host quality when they place their abdomen over the oviposition hole, and reject buds of poor quality. Low host quality may reduce egg production by pollen beetles (Hopkins and Ekbom, 1996). However, once a female had accepted a bud for oviposition, there was no difference between the average number of eggs laid per bud on fertile and sterile plants. This indicates females did not adjust clutch size according to any host quality assessment.

There were generally more 2^nd instar larvae in flowers on fertile plants than sterile ones. This could indicate a greater survival of larvae on plants with better food resources. However, there was no difference in the weight of larvae from either plant. Weight is often taken as a measure of fitness; bigger/heavier insects tend to be fitter, and produce more offspring during adulthood (Dixon, 1985)

In conclusion, there are more adults and second instar pollen beetles present on fertile than sterile plants. These eat the pollen required to pollinate the crop. These findings support the need to protect the pollinator plants from pollen beetle attack to ensure good pod set and higher yield.

ACKNOWLEDGEMENTS

We thank Elspeth Bartlet, Francis Gilbert and Guy Poppy for useful discussions about this subject. This work was funded by the Biotechnology and Biological Sciences Research Council of the UK.

REFERENCES

1. Charpentier, R. (1985). Host plant selection by the pollen beetle, Meligethes aeneus. Entomologia experimentalis et applicata 38: 277-285

2. CPB Twyford. (1996). Break the barrier: Growers’ guide to Synergy hybrid winter oilseed rape. CPB Twyford Ltd., Church Street, Thriplow, Royston, Hertfordshire, SG8 7RE, England. 18pp.

3. Dixon, A. F. G. (1985). Aphid Ecology. Blackie & Son, London.

4. Dobson, H. E. M. (1987). Role of flower and pollen aromas in host-plant recognition by solitary bees. Oecologia 72: 618-623

5. Hopkins, R. J., and Ekbom, B. (1996). Low oviposition stimuli reduce egg production in the pollen beetle Meligethes aeneus. Physiological Entomology 21: 118-122

6. Winfield, A. L. (1992). Management of Oilseed rape pests in Europe. Agricultural Zoology Reviews 5: 51-95